The Tribological Evaluation of Compressor Contacts Lubricated

by Oil-Refrigerant Mixtures

ACRC TR-19

For additional information:

Air Conditioning and Refrigeration Center University of Illinois Mechanical & Industrial Engineering Dept. 1206 West Green Street Urbana, IL 61801

(217) 333-3115

B. Davis, C. Cusano

May 1992

Prepared as part of ACRC Project 04 Compressor--Lubrication, Friction, and Wear

C. Cusano, Principal Investigator

The Air Conditioning and Refrigeration Center was founded in 1988 with a grant from the estate of Richard W. Kritzer, the founder of Peerless of America Inc. A State of Illinois Technology Challenge Grant helped build the laboratory facilities. The ACRC receives continuing support from the Richard W. Kritzer Endowment and the National Science Foundation. Thefollowing organizations have also become sponsors of the Center.

Acustar Division of Chrysler Allied-Signal, Inc. Amana Refrigeration, Inc. Bergstrom Manufacturing Co. Caterpillar, Inc. E. I. du Pont de Nemours & Co. Electric Power Research Institute Ford Motor Company , General Electric Company Harrison Division of GM ICI Americas, Inc. Johnson Controls, Inc. Modine Manufacturing Co. Peerless of America, Inc. Environmental Protection Agency U. S. Anny CERL Whirlpool Corporation

For additional iriformation:

Air Conditioning & Refrigeration Center Mechanical & Industrial Engineering Dept. University of Illinois 1206 West Green Street Urbana IL 61801

2173333115

ABSTRACT

The tribological characteristics of the most common contact

geometries found in compressors of air conditioning and

refrigeration systems have been experimentally investigated by

means of a unique high pressure tribometer (HPT). The HPT has

been used to experimentally simulate the friction and wear

behavior of various metal contact pairs lubricated by oil­

refrigerant mixtures in environments found in compressors. The

refrigerants used in this program are CFC-12 to obtain baseline

data and its prime replacement candidate, HFC-134a. The CFC-12

has been tested with mineral oils and synthetic alkylbenzenes

while the HFC-134a has been tested with monoether polyalkylene

glycol (PAG's) and pentaerythritol polyolester oils. Since the

amount of refrigerant dissolved in the oil is a function of both

pressure and temperature, and the friction and wear of a given

contact can be significantly affected by the concentration of

refrigerant in the oil, the friction and wear data obtained from

this test program should be a good indicator of what can be

expected in compressors.

iii

TABLE OF CONTENTS

CHAPTER PAGE 1. INTRODUCTION........................................... 1

1.1 Overview......................................... 1 1.2 Oil-Refrigerant Mixtures ......................... 2 1.3 Tribological Background .......................... 3

1. 3.1 Lubricant Requirements..................... 4 1. 3.2 Specimen Testing................... . . . . . . . . 6 1.3.3 Pressurized Friction and Wear Machines ..... 7

1.4 Scope of Research ................................ 8

2. HIGH 2.1 2.2

2.3

PRESSURE TRIBOMETER (HPT) SYSTEM ................. . Overall Facility ................................ . High Pressure Tribometer ........................ . 2.2.1 Pressure Chamber .......................... . 2.2.2 Temperature Systems ....................... . 2.2.3 Tribometer Motion ......................... . 2.2.4 Tribometer Controls ....................... . External Equipment .............................. . 2.3.1 Purging Facility .......................... . 2.3.2 Charging Facility ......................... . 2.3.3 Sampling Facility ......................... . 2.3.4 Data Acquisition .......................... .

10 10 11 11 19 21 23 25 26 28 29 30

3. EXPERIMENTAL METHOD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 32 3. 1 Overview......................................... 32 3.2 Compressor Contacts .............................. 32

3.2.1 Counterformal Contact - Rolling Piston Compressor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

3.2.2 Area Contact - Swash Plate Compressor...... 36 3.2.3 Conformal Contact - Reciprocating Piston

Compressor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 38 3.3 Specimen Holders ................................. 41 3.4 Lubricants....................................... 45.

4. EXPERIMENTAL PROCEDURES...................... . . . . . . . . .. 51 4.1 Overview......................................... 51 4.2 Z-Axis Motion Control ............................ 51

4.2.1 Travel Mode ................................ 55 4.2.2 F·orce Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 56

4.3 a-Axis Motion Control ............................ 57 4.4 Temperature Control .............................. 59 4.5 Force & Torque Measurement ....................... 61 4.6 Installation of Specimens ........................ 66 4.7 Purging Procedure ................................ 67 4.8 Charging Procedure ............................... 71 4.9 Configuring the HPT Controls and Conducting

the Test ......................................... 75 4.10 Sampling ......................................... 79 4.11 Refrigerant Reclamation .......................... 81

5. RESULTS & DISCUSSION................................... 83 5.1 Measurement of Wear .............................. 83 5.2 Surface Analysis ................................. 86 5.3 Counterformal Contact Results .................... 87

5.4 Area Contact Results .............................. 90 5.5 Conformal Contact Results ........................ 92

6. CONCLUSIONS............................................ 95 6. 1 Research Summary ................................. 95 6.2 Recommendations for Future Research .............. 96

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 98

APPENDIX A - Equivalent Cylinders ......................... 100

APPENDIX B - Raw Data ...................................... 103

vi

1.1 Overview

CHAPTER 1

INTRODUCTION

Refrigeration and air conditioning systems require

lubrication to reduce friction and wear in the compressor, to act

as a coolant to remove heat from the compressor contacts, and to

help to seal the compressed gas between the high pressure and

suction ports. From a thermodynamic standpoint, the refrigerant

and lubricant should be completely separate to maximize

efficiency. Such an approach can be used in larger air

conditioning and refrigeration systems by means of oil separators.

For smaller systems, however, due to costs, the refrigerant and

oil should be miscible in order to aid in returning oil which

inevitably migrates from the compressor.

The decrease in production and use of dichlorofluoromethane

(CFC-12), as required by the Montreal Protocol, has forced the

refrigeration and air conditioning industries to develop

replacement refrigerants. The prime replacement for CFC-12 is

(1,1,1,2) tetrafluoroethane (HFC-134a). Although HFC-134a is

thermodynamically quite similar to CFC-12, it poses new problems

for compressor lubrication. The new refrigerant is not miscible

with the lubricants now used with CFC-12. Currently, lubricant

manufacturers are developing synthetic lubricants that are

compatible with HFC-134a and provide adequate lubrication for

critical contacts in compressors.

1

1.2 Oil-Refrigerant Mixtures

This section describes the basic relationship between

refrigerants and oils in compressor environments. Since a

lubricant in a compressor can be subjected to refrigerant vapor at

relatively high pressures and temperatures, and assuming that the

oil and refrigerant are at least partly miscible, the properties

of the mixture are very dependent on the environmental conditions.

Of prime interest to the lubrication of compressor contacts is the

amount of refrigerant in the oil and the tribological properties

of the resulting mixture. Much of these mixture data are taken

from previous research conducted to determine thermodynamic

properties of oil-refrigerant mixtures.

Data on the miscibility of oil-refrigerant mixtures as a

function of pressure and temperature have been investigated for

many oil-refrigerant combinations. Little [1], Parmelle [2],

Spauschus [3], Spauschus and Speaker [4], Pate et al [5,6,7], and

Grebner [8] have researched the effects of temperature and

pressure on the miscibility of specific oil-refrigerant mixtures.

In general, as long as the oil and refrigerant are at least partly

miscible, the amount of refrigerant in the oil depends on the

temperature and pressure. Figure 1.1 shows the saturation

properties of CFC-12 with a paraffinic mineral oil. Note that by

increasing refrigerant vapor pressure or decreasing the

temperature, the amount of refrigerant saturated into the oil

increases. The tribological properties of these resulting

mixtures are often less well defined.

Viscosities of oil-refrigerant mixtures are of prime concern

for compressor lubrication and the efficiency of the overall

2

system. Data for the viscosities of currently used oil-

refrigerant mixtures are well known in the literature

[1,2,4,5,6,7]. These evaluations have centered on CFC-12 with

various mineral oils and some synthetic oils. The data for

viscosities involving replacement refrigerants with the

appropriate lubricants are less complete. Some limited viscosity

data involving HFC-134a with PAGs has been obtained by Thomas [9]

but most other research with HFC-134a is currently in progress.

2.0,-----------------------------~~--~------~

1.5

-40

Figure 1.1

Oil Concentration

By Mass

-20

a 0%

• 20%

• 40% o 60%

• 80% 1:1 95%

o 20 40 60 80 100 120

Saturation Curves for CFC-12-Paraffinic Mineral Oil Mixtures at Various Concentrations

1.3 Tribological Background

This section provides general lubrication requirements for

compressor contacts, as well as some available tribological data,

in the open literature, for oil-refrigerant mixtures. These data

3

are a result of an extensive literature search involving lubricity

of oil refrigerant mixtures for compressor applications. Most of

what is included is a result of either actual compressor tests, or

specimen modeling tests. The last portion of this section will

outline the type of modeling necessary to more accurately simulate

conditions in compressors.

1.3.1 Lubricant Requirements

Lubricant requirements for air conditioning and refrigeration

compressors are discussed in chapter 8 of the ASHRAE Handbook of

Refrigeration [10]. Properties such as lubricity, chemical

stability and solubility between the refrigerant and lubricant all

contribute to the overall effectiveness of an oil-refrigerant

mixture. These requirements must be met for the lubricant to be

useful in compressor lubrication applications.

The main purpose of the lubricant in a compressor is to

decrease friction and wear and, therefore, prolong the life of

critical components. Ideally, the lubricant should completely

separate the contacting surfaces. These conditions can be

characterized by two lubrication regimes: hydrodynamic

lubrication, which occurs between converging surfaces, and

elastohydrodynamic lubrication, which occurs in counterformal

contacts which entail material deformation. Ideal conditions are

not always easily obtained, therefore, the conditions in the

compressor often are not capable of generating full fluid films

between the critical contact pairs.

Under start-up, shut-down, transient, rough surfaces, or

elevated temperature conditions, boundary or mixed lubrication can

4

be dominant. In boundary lubrication, there is insufficient oil

film thickness to separate the surfaces. This allows some

asperity interaction between the contacts which results in wear of

the surfaces. However, this wear can be minimized through the

formation of surface films. These films tend to reduce adhesion,

abrasion, surface fatigue, and corrosion, which are the four

prominent wear modes under boundary conditions. The lubricants

designed for use in compressor applications often require some

additives to aid in the formation of films to reduce wear under

boundary lubrication conditions.

Other than lubricity, the properties of the oil should also

encompass several additional properties. The stability of the oil

in the presence of refrigerants and compressor materials is an

extremely important property of the lubricant. Among these

stability requirements are: oxidative stability, primarily for

storage considerations, thermal stability, to avoid lubricant

polymerization/depolymerization at elevated temperatures, and

chemical stability, to resist reaction with refrigerant and

materials in the compressor. In addition, the oil should be

miscible with the refrigerant. The refrigerant vapor transfers

some of the lubricant from the compressor to the evaporator and

condenser. This oil must have sufficient low temperature

miscibility to allow the refrigerant to carry it back to the

compressor within a reasonable amount of time. If the refrigerant

fails to return the oil, the compressor can rapidly fail under

starved lubrication conditions. In addition, the oil should

remain miscible with the refrigerant to permit good heat transfer

while it is migrated to the evaporator.

5

1.3.2 Specimen Testing

If the oil and refrigerant are miscible, subjecting the oil

to the refrigerant under pressure tends to saturate the

refrigerant into the oil. The lubricative properties of the

resulting oil-refrigerant mixtur.e can be greatly affected by its

composition. Friction and wear tests must be capable of

accurately modeling these mixtures. The initial approach for

modeling the compressor environment has historically been to use

standard test equipment, Falex tests or pin on disk tests, and

bubble refrigerant through the. lubricant to achieve a mixture.

Huttenbacher [11] used just such a setup to determine .oil-CFC-12

wear characteristics using a Falex machine. Although this method

seems to have gained popularity with equipment manufacturers and

oil suppliers to screen prospective lubricants, the validity of

these tests in modeling actual compressor conditions has not been

established.

The limitations of this type of testing are mainly with its

inability to accurately model the actual environmental conditions

existing in compressors. Although by bubbling refrigerant through

the oil some refrigerant does tend to saturate into the oil, the

environment found in compressors is not simulated. In addition,

since the test occurs at atmospheric pressure, there is the

possibility of other gases saturating into the oil. Among these

is oxygen, which would show adverse effects on wear. The other

major drawback from these types of tests is the inability of the

apparatus to accurately model the actual contact geometries and

operating conditions of tribo-contacts in compressors.

6

The use of actual compressors in accelerated testing programs

is an alternative to specimen testing. Sundaresan [12,13] has

completed many long term tests in which various PAGs were used

with HFC-134a in a reciprocating piston compressor. The results

of these data are extremely useful in lubricant screening for that

one compressor, but results for other types of compressors cannot

be accurately inferred. Another drawback with this type of

testing is that it requires long term tests and is therefore not

particularly cost effective. For these reasons, the trend in the

tribological evaluation for tribo-contacts in compressors has been

towards the use of pressurized friction and wear machines.

1.3.3 Pressurized Friction and Wear Machines

To better simulate the environments found in compressors, the

use of pressurized friction and wear machines is gaining

popularity. Sanvordenker [14] utilized a modified Falex machine

to test CFC-12-mineral oil mixtures under pressure. Although this

work did not model the wide range of conditions found in actual

compressors, it was capable of reasonably modeling the

refrigerant-oil mixture at low pressures. More recent innovations

and advances in pressurized testing equipment for tribological

evaluation have lead to the development of high pressure friction

and wear machines. Such a machine has been used by Komatsuzaki et

al [15], Komatsuzaki & Homma [16], and Komatsuzaki et al [17] for

tribological evaluation of CFC-12-mineral oil, CFC-12-

alkylbenzene, and HFC-134a-PAG combinations, respectively. The

apparatus used in their investigation was a modified four ball

tester that had been equipped with a pressure chamber surrounding

7

the contact. However, the pressure capability of their machine is

not capable of modeling the pressures found in some compressors.

Since the pressure directly affects the amount of refrigerant

saturated into the oil, it is critical that the pressure existing

in compressors be duplicated in any testing program. In order to

accurately model environments in compressors, any testing

apparatus should have the following capabilities: the tests

should occur within a pressure chamber to allow for high pressure

refrigerant vapor testing, the apparatus should be capable of

modeling a wide variety of contact geometries for a wide range of

operating conditions, and the apparatus should have full

temperature control of the contact as well as the chamber itself.

Such a high pressure tribometer (HPT) was developed over the past

two years, as part of the Air Conditioning and Refrigeration

Center (ACRC) at the University of Illinois. This HPT is capable

of simulating environments found in almost all compressors.

1.4 Scope of Research

This project has been part of the ACRC, an industry­

university cooperative, to decrease development time of ozone safe

air-conditioning and refrigeration compressors. As a result, the

main thrust of this research has been governed by specific

compressors and lubricants of interest to the sponsors. Other

projects within the ACRC have provided data on oil-refrigerant

miscibility and saturation pressure curves that have been used in

this test program. It is felt that an effective means of

simulating refrigeration and mobile air conditioning compressor

contacts and conditions have been developed.

8

The accuracy of the results of this research are contingent

upon the extent to which the actual conditions of the compressor

are modeled. With the assistance of the manufacturers of

compressors, critical contacts are identified, as are their

appropriate operating conditions. Included among these conditions

are refrigerant vapor pressure, contact temperature, load, speed,

materials, geometry, surface finish, and type of lubricant. Since

the lubricant is one of the main factors affecting wear, a concise

discussion of the lubricants tested is warranted. The

capabilities and design of the HPT is examined as are the test

procedures used in this research program. Next, the friction and

wear results obtained from the HPT for baseline CFC-12 tests as

well as initial tests with HFC-134a, are presented. Finally,

recommendations for future investigations are included.

9

CHAPTER 2

HIGH PRESSURE TRIBOMETER (HPT) SYSTEM

2.1 Overall Facility

The design .of the facility for the tribological evaluation of

critical contacts in compressors, in pressurized refrigerant

environments, centers on the development of a tribometer enclosed

in a pressure chamber. Because of the relative complexity of the

HPT, it was decided not to build it in-house. The remainder of

the system, though, was developed and constructed, at the

University of Illinois, using commercially available materials and

components.

Figure 2.1 Photograph of High Pressure Tribometer System

10

The completed HPT system, shown in Figure 2.1, is located in

the Tribology Laboratory in the Mechanical Engineering Building on

the Urbana-Champaign campus of the University of Illinois. The

design of the HPT system began in the Spring of 1990 and consists

of five sections: the HPT, a purging facility, a charging

facility, a sampling facility, and a data acquisition facility. A

thorough description of the design and operation of the complete

high pressure tribometer system follows.

2.2 High Pressure Tribometer

The tribometer was designed and manufactured by Advanced

Mechanical Technology Inc. (AMTI) of Newton Massachusetts.

Central to its design is a special pressure/vacuum housing, which

surrounds the test, capable of testing any inflammable non­

corrosive gas. Multiple thermal control loops are included to

permit testing of any temperature typically found in compressors.

Two separate servo motors provide motion and loading capabilities.

The rotational, 9-axis, motor is capable of unidirectional

rotation and oscillatory motion. The load, Z-axis, servo motor

provides either static or oscillatory loads for the contact. A

complex transducer measures applied load, frictional forces, and

moments during a friction and wear test. The feedback from this

transducer, as well as other sensors, provide the HPT with an

excellent control system.

2.2.1 Pressure Chamber

In order to allow for a pressurized refrigerant environment,

the test must occur within the confines of a pressure chamber.

11

Figure 2.2 Cross-Sectional Assembly Drawing of HPT Chamber

12

The chamber of the HPT was designed using the ASME code and is

rated for a 1.725 MFa operating pressure. The chamber consists of

two separate units that come together to form the pressure

housing. The upper half of the chamber is stationary, while the

lower half can be raised or lowered by the Z-axis servo motor.

When the lower half telescopes into the upper half, it engages a

seal and subsequently can be pressurized with refrigerant.

The upper half of the chamber is an assembly of fastened

pieces sealed together by o-rings. Figure 2.2 shows a cross­

section view of the entire pressure chamber. The telescopic seal

is held securely in the upper half by an aluminum ring bolted to

the upper housing. There is also a pair of seals used to seal the

joint where the shaft of the 9-axis motor enters the chamber.

The pressure chamber is sealed at the joint where the two

halves meet by a custom made six-inch telescopic seal. The seal

is kept lubricated by Braycote 803RP, a perfluoronated polyester

grease. The telescopic surfaces must be kept clean in order to

prevent abrasion of the seal and the surfaces. The 9-axis motor

shaft is sealed by a custom high pressure seal, designed for dry

sealing in refrigerant environments for pressures of up to

1.725 MFa and speeds of up to 2000 rpm. The design of this seal

is such that when a positive pressure is generated in the chamber,

the lip of the seal is held securely, by the pressure, against the

wear sleeve on the shaft as shown in Figure 2.3a. The higher the

pressure, the tighter the seal. In a vacuum, however, the lip is

actually pulled away from the shaft. For this reason, there is

also a V-ring seal, shown in Figure 2.3b, used in conjunction with

the rotary seal to maintain vacuum integrity. This seal is a

13

Custom-Made Rotary Seal

Upper Bearing Housing

Figure 2.3

(a)

(b)

(a) Custom-Made Rotary Seal Cross-Section (b) Vacuum Seal Cross-Section

14

standard Forshida V-ring seal, part #400500, which is lubricateq

with a PAG ACME screw grease from Nook Industries. When the

chamber is subjected to a vacuum, the lip of the V-ring seal is

pulled down and forms a tight seal between the shaft and the upper

bearing housing.

10-32 THD Self Locking Inserts 5 Places

Figure 2.4 Specimen Mounting Hole Arrangement on Spindle Face

The spindle serves as the mounting face to accept the upper

specimen holder. Figure 2.4 shows the specimen mounting hole

pattern in the spindle face. It has four 10-32 UNF self-locking

inserts on a 63.5 rom pitch diameter and a fifth center insert for

mounting specimens. Thus, the spindle face can be used to mount a

disk with a center hole or a specimen holder by up to four 10-32

screws. The vertical runout of the face, and th~refore the

specimen, can be adjusted by appropriately tightening or loosening

the three runout screws, shown in Figure 2.2, on the spindle.

15

Through the use of a dial indicator, the vertical runout of this

face can be easily adjusted to less than 0.0013 rnm.

The spindle also has a unique internal passage. This passage

serves to allow a heat transfer fluid to be pumped through the

spindle. This fluid, along with the high thermal conductivity of

the aluminum spindle, basically allow the spindle to remain at a

constant temperature throughout the test. The temperature of both

the upper specimen and the oil, therefore, are also maintained at

a constant temperature throughout a test.

The main bearings of the tribometer are intentionally located

outside the pressure chamber. Since CFCs act as solvents for most

oil-based materials, allowing the refrigerant to come in contact

with the grease in the bearings would degrade the grease and could

cause their premature failure. Recall that the V-ring seal for

vacuum integrity is located above the lower bearing. This

subjects the lower bearing to a vacuum every time the chamber is

purged. To minimize the effects of outgassing of the grease, a

synthetic grease with extremely low vapor pressure was selected.

A PAG, ACME screw grease from Nook Industries, was selected on

this basis as well as its ability to provide adequate bearing

lubrication. The bearings are lightly packed with this grease

prior to installation.

The lower half of the chamber is telescoped into the upper

half by the Z-axis servo motor. This motion is also what provides

loading during the test. The lower half consists of several

critical components shown in Figure 2.2. Also apparent is the

complex suspension system which allows for the accurate

application of load while the chamber is pressurized. These and

16

other features are described in greater detail in the sections to

follow.

10-32 THD Self Locking Inserts 7 Places

Sampling Hole

63.5 mm 0

Figure 2.5 Specimen Mounting Hole Arrangement on Cup

The tribo-contact occurs inside the cup. The cup is a

removeable aluminium piece which serves two important functions.

First, it serves as the mounting surface for the lower specimen

holder. The hole pattern, as shown in Figure 2.5, permits the

holder to be securely mounted by up to seven 10-32 UNF self-

locking inserts. Secondly, the cup serves as the lubricant

reservoir during the test. A Pyrex sleeve, sealed at the bottom

by an O-ring, surrounds the cup. This permits the cup to be

filled with a lubricant, completely submerging the contact to be

tested. Since the Pyrex sleeve is entirely within the pressure

chamber, there is no pressure difference across it, and only the

hydrostatic pressure of the oil requires sealing. Three sight

ports, machined in the wall of the cup, corresponding to sight

17

ports in the chamber walls, allow for viewing of the contact

during testing. Also shown in Figure 2.5 is a small hole labeled

sampling hole. This hole communicates with the sampling port on

the outside of the chamber and allows an oil-refrigerant mixture

sample to be drawn off during a test.

The removeable cup is bolted to a complex force transducer

module. The transducer is outfitted with an intricate array of

strain gages, which are used to measure the forces during a test.

Frictional forces (Fx , Fy), load force (F z ), as well as moment (Mz )

are of interest and are relayed to the control box outside the

pressure chamber. There is also a thermal sensor installed in the

transducer that is used to monitor the temperature of ~he lower

specimen. In general, it is preferred to maintain the transducer

at ambient temperatures, but the thermal design considerations

prevented this option for the tribometer. Accordingly, the strain

gages and adhesives were selected for their ability to function at

elevated temperatures and high refrigerant pressures.

The transducer module is firmly mounted to an internal

suspension system. This consists of a pair of diaphragm springs

which provide compliance in the Z-direction while maintaining high

stiffness in the x, y, and e directions. These diaphragm springs,

in conjunction with a Belleville spring, are used to permit

accurate loading while the chamber is pressurized. When the

chamber is pressurized to 1.725 MPa, it takes approximately

31,000 N to hold the two halves closed. Most of this force is

taken up by the suspension system, so that with proper strain gage

amplifier configuration, test loads as low as 4 N can be

accurately applied and monitored.

18

The lower half of the chamber also houses several safety

devices. There is an adjustable pressure relief valve, preset to

1.89 MPa, to prevent accidental over pressurization of the

chamber. A second pressure safety device is a rupture disk rated

for 2.41 MPa at 1200 C and 2.59 MFa at 210 C. This unit contains

a thin metal disk that will break at the rated pressure and

temperature to relieve the chamber. The other safety device,

shown in Figure 2.2, is an overload stop pin. This pin ensures

that, in the event of an overload, the transducer will bottom out

against the pin and will not be damaged due to buckling.

2.2.2 Temperature Systems

Thermally, the test chamber has several important features

that are described in this section. Virtually all internal

surfaces of the chamber can be heated. This is required to

prevent condensation of refrigerant on these surfaces at the high

test pressures. At the upper pressure limit of the pressure

chamber, 1.723 MPa, the temperature required to prevent

condensation is approximately 71 0 C. Both halves of the pressure

chamber are outfitted with cartridge heaters that are used to heat

the chamber walls above the condensation temperature. The upper

half is outfitted with a 400 W cartridge and the lower half

contains two 500 W cartridges. The temperature of each half can

be controlled from the main control panel.

The temperature of the rotary spindle, and therefore the

upper specimen, is controlled from -300 C to 1500 C by an external

recirculating unit A NESLAB RTE-110 Refrigerated Bath pumps a

heat transfer fluid, through a rotary union, down into the

19

spindle. Due to the high value of the heat transfer coefficient,

and the unique design of the passages machined in the spindle, the

upper specimen can be maintained within a couple of degrees of the

fluid temperature. A mixture of Union Carbide Heat Transfer Fluid

(UCAR) and distilled water serves as the recirculating fluid. By

using the appropriate concentrations of this fluid, different

temperature ranges can be tested. For higher temperature tests, a

ratio of 90% UCAR and 10% distilled water is recommended by the

manufacturer in order to maintain a high boiling point of the

mixture. Testing at temperatures below ambient temperatures is

accomplished solely through the use of the recirculator set to

deliver chilled fluid. A mixture of 50% UCAR and 50% distilled

water should be used to keep the viscosity of the fluid low and

permit easy pumping through the spindle. The controls for setting

temperature are located on the NESLAB unit and allow for setting

the desired temperature as well as providing a safety shutoff for

over-heating. The user should maintain the level of the fluid in

the RTE 110 above the fill line in the reservoir.

The last thermal system to be discussed is the chiller.

Similar to the recirculator, the chiller is an independent unit

with its own controls. A NESLAB CFT-33 Recirculating Chiller

pumps a 50/50 mixture of laboratory grade ethylene

glycol/distilled water through passages machined into portions of

the HPT. This unit is set at ambient temperature and is used to

cool critical parts of the tribometer. The chiller should be

turned on whenever the tribometer is operated.

20

2.2.3 Tribometer Motion

Motion in the tribometer is generated by two independent dc

servo motors. A large 9-axis servo motor provides rotational

motion for the upper specimen; while a second, somewhat smaller,

Z-axis servo motor provides axial motion and loading during the

test. These motors are thoroughly described in this section.

The 9-axis dc servo motor (3 kW) is controlled through a

pulse width modulated (PWM) amplifier. The low inertia motor

coupled with the high performance amplifier provides excellent

response and permits complex motion. The shaft of the motor is

attached through a flexible helical coupling to the shaft entering

the chamber. The position of the 9-axis is monitored by a

differential optical encoder that is used in controlling spindle

motion.

The Z-axis dc servo motor is controlled separately through

its own PWM amplifier. This fast response motor amplifier

combination supplies both Z-axis motion, up to 1.69 mm/sec, and

test loads, up to 4500 N. This motion is transmitted by a lead

screw which is driven through a backlash-free 100:1 harmonic

drive. An encoder feedback loop supplies a means to monitor the

location of the lower half of the chamber; while the transducer,

described in Section 2.2.1, acts as the force feedback loop that

controls the applied axial load. A pair of limit switches ensure

that the Z-axis cannot travel beyond preset positions. The lower

switch protects recirculator lines, while the upper switch is

adjustable and prevents the lower half of the chamber from

telescoping too far into the upper half.

21

N N

~r----. ~L-

Temperature Feedback

Al + RS- 2321 rOisPlayl Temp Control ~ 4 a Board , , r

-Z-Axis . Motor

MotherBoard ----(Microprocessor) -11. .,

-- Motor Amps .. a-Axis - Tribometer - -.. Panel Controls ~ -;7 .. Motor (Mechanical) • Z-Axis i---

• Transducer • a-Axis -. • Temp Sensors ... • Temp

Strain Gage Amplifiers

• Fx ~ ~ ~ • Fy "7 ~ • Fz

• Mz

Z Encoder Feedback , a Encoder Feedback 'r Force Feedback 'r

Figure 2.6 - Tribometer Control System Schematic

2.2.4 TribometerControls

Figure 2.6 shows a schematic of the units that serve as the

tribometer control system. There are four strain gage amplifiers:

Fx , Fy , Fz , and Mz . Each consist of an amplifier board that plugs

into the control box and interfaces with panel controls and

switches as shown in Figure 2.7. Also shown in Figure 2.7 are the

front panels of the two motor control circuit boards, and the

front panel of the temperature control board. The motherboard,

with a microprocessor, interfaces with the control circuits and

provides a wide variety of functions. It interfaces with a four

line LCD display and eight panel switches. In addition to

interfacing with the front panel, the microprocessor can perform

control, limit, and alarm features. It can also communicate with

a personal computer, via an RS-232c port, to allow for external

configuration and data acquisition.

Feedback for the loads is provided by the transducer in the

form of Fx , Fy , Fz , and Mz . Each of the force (torque) directions

has its own independent amplifier that excites the strain gages,

and is used to condition the load to equivalent engineering units.

This allows for each direction to be set to the appropriate

sensitivity so that the accuracy of the force (torque) reading can

be improved. This conditioning of the strain gages is permitted

by proper adjustment of gain and excitation voltage on each

amplifier control panel. A detailed explanation on how to

determine the proper settings is given in Section 4.5.

The Z-axis motor control board permits axial loads to be

applied during the test as well as providing motion to open and

close the chamber. The force can be static, up to 4500 N, or

23

oscillatory from O-~500 N at frequencies up to 5 II~. In addition,

this oscillation can be synchronized with the 8-axis oscillation

so that the load and 8-motion can be in phase. These controls can

also be used to set the speed at which the lower half of the

chamber can be raised or lowered.

Figure 2.7 Photograph of HPT Main Control Panel

The 8-axis motor control loop, with optical encoder feedback,

permits the 8-axis servo motor to be precisely controlled. The

motor is capable of simple unidirectional rotation (0-2000 rpm)

and oscillatory motion with amplitudes of up to 180 0 and

frequencies of up to 5 Hz. Presently, the tribometer controls

permit oscillatory motion with either sinusoidal or triangular

waveforms. The controls also permit a const~nt torque to be

applied to the test. In this mode, Mz is used as the feedback

24

signal and the speed will automatically adjust to whatever is

necessary to produce the required torque.

The on-board temperature controllers are used to control the

temperatures of the cartridge heaters. The upper and lower

heaters can be independently set from ambient to 95° C. Thermal

sensor feedbacks from the chamber allow the temperatures to be

accurately controlled to ±1° C. The recirculator and chiller can

be turned on from the front panel, but their controls are housed

within their independent units and their functions will be

described in Section 4.4

The last tribometer control is the microprocessor. The Intel

80C188EB 16-bit microprocessor, located on the motherboard, is

capable of independently controlling nearly all tribometer

functions. It can be used to control the two cartridge heaters as

well as 9-axis and Z-axis motion. The interface to the

microprocessor consists of a front panel keypad with eight

switches and a 4 line ~ 40 character LCD display. This display is

used to set test parameters and monitor data values. The

microprocessor can also interface with a personal computer to

permit for more complicated functions.

2.3 External Equipment

The HPT has also been outfitted with apparatuses for purging,

charging, and sampling, as shown in Figure 2.1, as well as a data

acquisition system. Two vacuum pumps work in tandem to purge the

system. The larger of the two pumps is used to purge the chamber

and external lines; while the smaller pump removes any vapor

outgassed from the grease in the main bearings. An external

25

pressure vessel is used to charge the chamber with refrigerant. A

silicone heating blanket around the vessel is used to generate the

required refrigerant pressure to charge the chamber. A 13.6 kg

refrigerant tank, attached to the pressure vessel, is used to

supply refrigerant to the vessel. The chamber is also outfitted

with a 50cc sampling cylinder which can be used to siphon off an

oil-refrigerant mixture sample during a test. There is also a

separate 6.8 kg drain tank which collects used refrigerant so that

it can be recycled. Data acquisition is possible through the use

of a PC linked to the motherboard by an RS-232c connector.

2.3.1 Purging Facility

In order to purge the system prior to initiating a test, a

facility to evacuate the chamber was developed. This facility is

shown in Figure 2.8. A Welch Scientific Duo-Seal Vacuum pump,

model 1402, is connected to the pressure chamber at a Whitey Model

60 ball valve by a 25.4 mm ID vacuum hose. This large pump is

able to rapidly evacuate the chamber and external lines, of the

charging facility and drain tank, to a very high vacuum. Recall

that the V-ring Forshida seal, placed above the lower bearing, was

installed for vacuum integrity. This places the lower bearing,

and its grease, in a high vacuum environment each time the chamber

is purged. To remove any vapor that is outgassed from the grease,

a second vacuum pump is used. The Thermal Engineering Co, model

1825Z, vacuum pump is connected by a (9.525 mm 0 by 0.9 m) Yellow

Jacket charging hose to a port located between the V-ring and

bearing. This serves to effectively remove any outg~ssed vapor.

26

Note: Yellow Jacket hose connects to the Lower Bearing Housing

Whitey Model Ball Valve (B-635S12-B)

25.4 rom 0

25.4 Hose

Welch Scientific "Duo-Seal" Vacuum Pump Model 1402

HPT Pressure Chamber

1/4" Whitey Needle Valve (B-14F4)

Thermal Vak-Check Model 4501

Yellow Jacket 9.525 rom 0 Charging Hose (0.9 m)

Thermal Eng. Vacuum Pump Model 1825Z

Ashcroft Vacuum Gage (SS-1279SS-0-30Hg)

Figure 2.8 HPT Purging Facility

The level of the vacuum is monitored by two separate vacuum

gages. The first is an Ashcroft model SS-1279SS-0-30v gage,

accurate to 0.5%, which is used to give a rough vacuum level in

inches of mercury. The second gage, a Thermal Vak-Check model

4501, is capable of extremely accurate readings, in microns, down

to an absolute vacuum. Both gages are isolated from the chamber

by a 1/4" Whitey needle valve, model B-NF4-B. This valve is used

to protect the gages from damaging positive pressures.

27

2.3.2 Charging Facility

In order to supply the chamber with refrigerant vapor at

pressures up to 1.725 MFa, a charging facility was developed.

Figure 2.9 shows the relevant components of this facility. The

3.63 kg pressure vessel, manufactured by E.F. Britten & Co, is

used to hold the refrigerant being evaluated. The vessel is

supplied with refrigerant from a standard DOT-39NRC 260/325, M1073

E. I. du Pont de Nemour & Co container with 13.6 kg capacity. A

720 W silicone heating blanket, manufactured by Conrad Corp, is

wrapped around the pressure vessel and supplies heat to generate

pressure within the vessel. A standard 5 amp 0-120 volt variac

supplies power to the heating blanket and is used to control the

temperature of the pressure vessel. The pressure is monitored by

an Ashcroft SS-1279SS02c30-300 dial pressure gage with an accuracy

of 0.5%. There is also an adjustable pressure relief valve,

Swagelok model SS-4CPA2-150, which is set at 2 MFa to relieve the

pressure vessel for safety purposes. Once sufficient pressure is

generated, a series of valves connecting the vessel to the chamber

can be opened, allowing pressurized refrigerant vapor to be

transferred to the chamber. This valve assembly consists of a

Swage 10k SS-43F2 valve at the pressure vessel, which connects to

Nupro Union Tee, SS-400-3, at a quick-connect (not shown in Figure

2.9). From the tee, a 1/4" Nupro plug valve connects to a 1/4" 0

Nupro hose, which connects to the pressure chamber at a Whitey

1/4" needle valve. The quick connect allows the pressure vessel

to be disconnected and weighed to determine the approximate amount

of refrigerant used in a test.

28

1/4" 0 Nupro Hose SS-7R4TA4TA4-48

1/4" Nupro Plug Valve (SS-4P4T-TB)

Nupro Union (SS-400-3) .

1/4" Nupro Plug Valve (SS-4P4T-TB)

1/4" 0 Nupro Hose SS-7R4TA4TA4-36

DuPont 13.6 kg Supply

Tank

Whitey 1/4" Needle Valve (B-1RF4)

HPT Pressure Chamber

Ashcroft Pressure Gage (SS-1279SS02C30-300)

Nupro Cross (SS-200-4)

Conrad Corp 720 W Silicone Heating Blanket

Nupro Relief Valve (SS-4CPA4-150)

Variac 5 Amp (0-120 V)

EF Britten & Co 3.63 kg DOT-4B Pressure Vessel

Figure 2.9 HPT Charging Facility

2.3.3 Sampling Facility

The design of the HPT permits easy sampling of the oil-

refrigerant mixture during a test. The sample is taken while the

chamber is pressurized, so the amount of refrigerant saturated

into the oil can be determined by weight analysis. Figure 2.10

shows the sampling facility of the HPT. A sampling port on the

chamber communicates with the cup by a 1.275 mm 0 capillary tube.

A 1/4" Nupro plug valve, model SS-2P4T-TB, is connected to the

sampling port on the chamber. This valve permits the chamber to

be sealed off from the outside. The valve is connected to a

29

Swagelok quick-connect, model SS-QF4, so that the sample cylinder

can be removed from the chamber while the chamber is still under

pressure. The sample cylinder, a 50 cc Swagelok SS-4CS-TW-50, has

a 1/4" Nupro plug valve, model SS-6P4T-TB, which is used to seal

the sample cylinder. This setup allows the oil-refrigerant to be

weighed on a Sartorius model LC1200S balance, with an accuracy of

0.001 g. The refrigerant is then evaporated and the weight percent

of refrigerant saturated into the oil may be determined.

Quick Connect (Swagelok: SS-QF4)

1/4" Plug Valve (Nupro: SS-6P4T-TB)

50 cc Sample Cylinder (Whitey: SS-4CS-TW-50)

/1/4" Plug Valve ~ (Nupro: SS-2P4T-TB)

-'Connects to Sample Port On HPT

Figure 2.10 HPT Sampling Facility

2.3.4 Data Acquisition

Data acquisition is accomplished by an IBM compatible

personal computer with an 80386SX processor running at 20 MHz.

The PC is connected by ribbon cable directly to the microprocessor

on the motherboard of the HPT by an RS-232c connector. The PC and

microprocessor use SECS-I communication protocol to communicate

with each other.

The interface program is written in Magic/L, a powerful yet

simple language which only requires 64 k to run. This low memory

requirement makes it an ideal program for use with the

30

microprocessor. The interface is command-line based and actions

are implemented by entering key words or commands on the PC

keyboard. The PC can be used to configure the strain gage

amplifiers, set the cartridge heater controls, and monitor the

output from the test. The maximum sampling rate is determined by

the number of parameters being sampled. In general, by sampling

all forces and all temperatures, samples can be taken every

40 msec. These data are read directly into a Ramdisk and then

transferred to the hard drive. They may later be imported as a

numeric file into Lotus 1-2-3 for further analysis and data

smoothing.

31

3.1 Overview

CHAPTER 3

EXPERIMENTAL METHOD

The goal of this research project is to use specimen testing

to accurately model the critical tribo-contacts of compressors in

a high pressure tribometer. The specimens should be able to

accurately simulate the geometries and materials found in the

actual critical tribo-contacts. In addition, the operating and

environmental conditions such as load, speed, pressure,

temperature, and lubricant need to be accurately simulated with

the HPT.

This chapter will describe the tribo-contacts under

evaluation. This includes examination of specific compressor

types, determination of their critical contact geometries, and

determination of actual material pairs used in the contacts. The

environmental operating conditions under which these contacts

operate will be described. Finally, lubricants will be identified

and described on a general bulk material property basis.

3.2 Compressor Contacts

Three critical contacts from mobile air conditioning and

refrigeration compressors were chosen to be modeled in the HPT

test program. After reviewing the requirements of the sponsors

with the function and use of many compressors, it was decided that

the rolling piston, swash plate, and reciprocating piston

compressors should be the focus of this research. The swash plate

compressor is primarily used in automotive air conditioning

32

systems, while the other two are normally used in refrigeration.

systems. The contacts chosen were the most critical based on

input from the sponsors. The actual compressor contacts and their

equivalent specimen models are described in the sections to

follow.

Piston: Gray Cast Iron Tool Steel

Suction Port Discharge Port

Cylinder

Figure 3.1 Schematic of Rolling Piston Compressor Section

3.2.1 Counterformal Contact - Rolling Piston Compressor

The critical contact for the rolling piston compressor is

shown in Figure 3.1. The piston is driven by an eccentrically

mounted shaft which produces oscillatory relative motion between

the vane and piston [18]. This contact serves as the primary seal

between the low pressure refrigerant on the suction side and the

33

high pressure refrigerant on the discharge side of the compressor.

Therefore, since the oil is subjected to relatively high pressure

refrigerant, more refrigerant is saturated into the lubricant, and

the need to model the high pressures is crucial.

Figure 3.2

Equivalent Radius (R=3.175 mm)

Section A-A

Upper Specimen: 76.2 mm 0 Flat Disk (Piston)

19.05 mm

Lower Specimen: 6.35 mm 0 Pin (Vane)

tLoad

A

.. . • I • ~

A

Equivalent Geometry of Counterformal Contact­Rolling Piston Compressor. Note that the lower specimen is secured in place by a specimen holder.

The vane is made from a hardened tool steel, while the piston

is a hardened gray cast iron. The geometry of this contact pair

is counterformal, and is modeled by using the concept of

34

equivalent cylinders that is developed in Appendix A. The

equivalent geometry, shown in Figure 3.2, is modeled as a

cylindrical pin rubbing against a flat plate. The relevant

geometrical and material data for the model are shown in

Table 3.1. The geometry, materials, and surface treatments for

this contact are all accurately modeled in the HPT.

a e . :ipec~men T bl 3 1 S P ropert~es

Counter-Area Description formal Conformal

Contact Contact Contact

Geometry 76.2 mm0 76.2 mm0 25.4 mm • Upper Flat Disk Flat Disk Square Pad

• Lower 6.35 mm0 5.08 mm0 6.35 mm 0 Pin Pin Flat Shoe with 1220 mm 0

L=9.52 mm L=9.52 mm Materials

• Upper Gray C.l. Ductile C.l Die Cast Al

• Lower Tool Steel Bronze Mild Steel Hardness (±1 Rc)

• Upper 50 42 ---• Lower 65 --- 63

Surface Topography Ground Ground Ground • Upper

• Lower Ground Lapped Ground Surface Finish (Ra)

• Upper (±O. 025 J.Lm) 0.127 0.127 0.254

• Lower 0.127 0.203 0.101

The operating conditions used in the specimen testing program

are also representative of those found in,typical rolling piston

compressors. These conditions are shown in Table 3.2. The type

of motion, speed, pressure, and temperature are all consistent

with the actual compressor operation; while the load was increased

to obtain measurable wear in a reasonable test duration of one

hour. The contact load, shown in Table 3.2, for the counterformal

35

contact is based on the Hertzian contact pressure between the

specimens [19].

Table 3.2

Operating Conditions

Conditions

Area Contact

3.2.2 Area Contact - Swash Plate Compressor

The critical contact examined for the swash plate compressor,

the shoe-plate, is shown in Figure 3.3. The swash-plate drive

mechanism features an i~clined plate which is rigidly attached to

the rotating shaft. The unidirectional rotation of the shaft

causes a nutation of the plate, which is used to transform the

shaft rotation to simple reciprocal motion at the piston. This

contact occurs within the crankcase and, therefore, the oil is not

normally subjected to refrigerant at as high a pressure as is

found in the rolling piston compressor.

The shoe is made from a bronze alloy that has a lapped

surface, and the plate is a hardened ductile cast iron with a

ground surface. The shoe-plate pair form an area contact. The

flat shoe rubs, under simple sliding conditions, against the flat

. plate. The load is transferred from the piston through a steel

ball onto the bronze shoe. This assures that the shoe can self

align with the plate during operation. The equivalent geometry,

36

shown in Figure 3.4, is essentially the same as that found in the

compressor. The area contact is made up of a bronze shoe, loaded

by a steel ball to permit the same degree of freedom found in the

actual compressor, sliding against a flat ductile cast iron plate.

Table 3.1 shows the geometry, material, and treatments used in

modeling the area contact. These data correspond identically to

those found in the actual compressor.

Shoe: Bronze Ductile Cast Iron

Figure 3.3 Schematic of Swash Plate Compressor Section

The modeled operating conditions for the area contact, shown

in Table 3.2, are essentially the same conditions of the swash

plate compressor with the exception of load and speed. When the

area contact was run at loads and speeds equivalent to the

compressor, hydrodynamic liftoff was observed. In order to avoid

liftoff and generate measurable wear, it was necessary to run the

area contact at higher loads and slower speeds than what is

37

typically encountered in the swash plate compressor. The contact

load, shown in Table 3.2, is given as a contact pressure. This

pressure is the test load (P), divided by the contact area (A) of

the bronze shoe. The temperature and refrigerant pressure modeled

in this program are typical of those found in the actual

compressor.

Upper Specimen: 76.2 rom 0 Flat Disk (Plate)

•

Lower speCimen:~t-5.08 rom 0 Pin (Shoe) Load

A

A

Section A-A

Ball to avoid edge loading

Figure 3.4 Equivalent Geometry of Area Contact-Swash Plate Compressor. Note that the lower specimen is secured in place by a specimen holder.

3.2.3 Conformal Contact - Reciprocating Piston Compressor

The critical contact chosen for the reciprocating piston

compressor is that of the wrist pin-bearing as shown in

Figure 3.5. The compression of refrigerant vapor is accomplished

by a piston which is driven through a connecting rod by a

crankshaft. The critical contact of this compressor occurs

between the wrist pin and connecting rod. This contact pair is a

conformal contact which essentially is a journal bearing. The

motion observed at this contact is sinusoidal. As with the area

38

contact, the conformal contact occurs within the crankcase and,

therefore, is not normally exposed to the high refrigerant

pressures found in the counterformal contact.

Wrist Pin: Mild Steel

~~~ischarge

~~~ort ~~~~~~~~~

Connecting Rod: Die Cast Aluminum

Figure 3.5 Reciprocating Piston Compressor Section

The wrist pin is made from case hardened mild steel, which

experiences sliding motion with a die cast aluminum connecting

rod. The geometry of this contact is also modeled by using the

concept of equivalent cylinders that is developed in Appendix A.

The equivalent geometry, shown in Figure 3.6, is modeled as an

approximate equivalent radius, R=1220 mm, rubbing against a flat

plate. The radius is approximate because of machining

limitations. The remaining contact conditions, shown in Table

3.1, are identical to those found in the actual compressor.

39

Upper Specimen Holder

Upper Specimen 25.4 rom Square Pad (Con. Rod)

19.05 rom

Connecting Rod

A

Lower Specimen: 6.35 rom Square Pin ·(Wrist Pin)

tLoad

A

Approximate Equivalent Radius (R=1219 rom)

Section A-A

Figure 3.6 Conformal Contact-Equivalent Geometry of Reciprocating Piston Compressor. Note that both the upper and lower specimens are mounted in specimen holders in the HPT.

Table 3.2 shows the operating conditions for the conformal

contact. All of these conditions accurately represent the

conditions found in the actual compressor. The motion is

approximately sinusoidal and the resulting maximum speed is

representative of the actual compressor. The only exception is

the load. The load is increased to obtain measurable wear within

40

a reasonable test duration. The load, shown in Table 3.2, is

given in terms of a P/LD pressure. This pressure, commonly used

in journal bearing theory, is found by dividing the applied load

by the projected area of the pin. The remaining conditions,

refrigerant pressure and temperature, are identical to those

typically found in the actual compressor.

3.3 Specimen Holders

In order to effectively be able to test the equivalent

geometries of the critical compressor contacts, specimen holders

were designed and made. The upper specimens for both the

counterformal contact and the area contact are 76.2 mm 0 flat

disks that are mounted directly to the spindle. They, therefore,

do not require any specimen holders. The remainder of the

specimens are held in place with various specimen holders.

The upper specimen for the conformal contact is made of a die

cast aluminum alloy. Due to difficulties in obtaining this

material cast in 76.2 mm 0 flat disks, and the fact that the

conformal contact is subjected to oscillatory motion with only

± 20° amplitude, a smaller specimen was used. The square aluminum

pad, shown in Figure 3.6, is a 25.4 mm square pad with a thickness

of 6.35 mm. This pad is sufficiently large to simulate the motion

of interest, but it is too small to mount directly to the spindle.

The required specimen holder is shown in Figure 3.7. The specimen

holder is made from a mild steel disk with a 25.4 mm slot to

accept the aluminum pad. The pad is held securely in the holder

by a pair of 4-40 UNC set screws. The specimen holder, with

mounted aluminum pad, can then be secured directly to the spindle

41

by four 10-32 machine screws. This configuration provides secure

mounting of the aluminum pad, while allowing the contact to

experience the appropriate motion to model the wrist pin-bearing

contact in the compressor.

Drilled and C'Bored for 10~32 socket head machine screws 4 @ 900 on 63.5 mm 0 circle (Ref)

/////// //////// //////// //////// //////// //////// ////////

- - - - i - .. ! - ::: !: !-.- - - - -- - - - ~~.:r = ......:;.....:;...:~~....:...~- i:.::-.r - - --

• • • • • •

6.35 mm

76.2 mm 0

Aluminum Pad (Lower Specimen)

Drilled and Tapped for 4-40 set screws 4 Places

Figure 3.7 Upper Specimen Holder for Conformal Contact

42

Specimen

6-32 UNC Set Screws

A

1

I

..-l A

Section A-A

Figure 3.8 Lower Specimen Holder for Counterformal and Conformal Contacts

All the lower specimens require specimen holders. The lower

specimens for the counterformal and conformal contacts utilize the

same specimen holder, while a different holder secures the

specimen used for testing the area contact. The lower specimen of

the counterformal contact, the tool steel pin, has a 2.38 mm 0

hole drilled radially through it. Similarly, the steel pin of the

conformal contact also has a 2.38 mm 0 hole drilled through it.

43

The aluminum specimen holder, shown in Figure 3.8, has a series of

2.38 mm holes drilled through it and a 6.35 mm slot milled into

the face. The specimens are held in this slot by a 2.38 mm 0

steel dowel that is held in place by two 6-32 UNC set screws.

This setup allows the specimen to self align in the direction

along the milled slot while retaining rigidity transverse to the

slot. The lower specimen holder is then bolted directly to the

cup with two 10-32 UNF machine screws.

Bronze Shoe (Specimen) 5.556 mm 0 Ball

Section A-A

o

A

I ~

A

00 00 ~

Figure 3.9 Lower Specimen Holder for Area Contact

44

The only other specimen holder required is for the bronze

shoe of the area contact. In order to accurately represent the

degree of freedom that the contact in the actual compressor

experiences, the specimen holder, shown in Figure 3.9, has a

5.556 mm 0 spherical joint, with a 52100 steel ball placed between

the specimen holder and the bronze shoe. The degree of pivoting

is restrained by the clea~ance between the shoe and the specimen

holder. This ensures that the shoe cannot be dislodged from the

specimen holder during a test.

3.4 Lubricants

The lubricants that have been evaluated are classified into

four types: mineral oils, alkylbenzenes, polyalkylene glycols

(PAGs), and polyolesters. The first two are used with CFC-12 for

obtaining baseline friction and wear data; while the latter two

are the more promising oil types for use with HFC-134a. The

mineral oils are currently used in the swash plate and

reciprocating piston compressors, and the alkylbenzene is used in

the ro~ling piston compressor. The PAGs tested are possible

replacements for swash plate compressors, while the rolling piston

and reciprocating piston compressors will possibly be lubricated

by polyolesters. Regardless of the intended application, both the

PAGs and esters were cross tested with different contact pairs.

The relevant lubricant properties for all the oils tested are

shown in Table 3.3.

The mineral oils used with CFC-12 are classified according to

their molecular structure as either paraffinic, naphthenic, or

aromatic. The paraffins are characterized by containing only

45

linear or branched carbon chains and are saturated hydrocarbons.

Naphthenic mineral oils are also saturated hydrocarbons, but they

are made up of ring structures. Aromatics are unsaturated

hydrocarbons that have a ring structure with double bonds

alternating between rings. Mineral oils that are classified as

paraffinic or naphthenic, also contain aromatic hydrocarbons but

the concentration can vary from 10% to 30%. The classification of

the oil always refers to the saturated hydrocarbon structure,

either naphthenic or paraffinic. The synthetic alkylbenzene oils

that were tested are classified as 100% aromatic hydrocarbons.

Due to problems of insolubility of HFC-134a with mineral or

alkylbenzene oils, other synthetic replacements have been

developed for use with HFC-134a. The two most promising types

that are used in this program are PAGs and esters. PAGs are

classified into families by their free hydroxyl groups. All of

the PAGs tested are known as monoethers, having one hydroxyl

group. The ester lubricants are classified depending on the

functionality of the alcohol or acids used in their manufacture.

The esters used are all of the pentaerythritol (PE) variety, a

polyfunctional alcohol.

Where possible, both base (B) and formulated (F) versions of

each lubricant were tested. Although proprietary in nature, the

formulated oils are versions of the respective base oil with an

additive package to improve lubricative properties. These

additives can include extreme pressure (EP) agents, oxidation

inhibitors, foam reducers, and other types of agents to improve

lubrication characteristics. The exact nature of the additive

46

packages for the lubricants tested were not supplied by the

manufacturer because of proprietary reasons.

Oil-refrigerant mixtures are classified as completely

miscible, partially miscible, or immiscible according to their

mutual solubility with each other. Completely, or fully, miscible

oil-refrigerant mixtures are mutually soluble at any temperature.

Therefore, with the appropriate refrigerant pressure, the oil­

refrigerant mixture can exist anywhere from zero percent

refrigerant to zero percent oil as a single phase at any

temperature. Partially miscible oil-refrigerant systems are

mutually soluble to a limited extent. Above a critical solution

temperature, the oil-refrigerant mixture is completely miscible.

Below this critical temperature, however, the liquid may separate

into two phases. This phase separation does not imply that the

oil and refrigerant are insoluble in each other, rather that each

phase is a separate solution. One may be oil-rich and the other

refrigerant rich and these two mixtures are immiscible with each

other. Table 3.4 shows the relevant miscibility data as well as

the weight percent of refrigerant in the oil as tested. All of

the lubricants tested, with the exception of the PAGs, are

completely miscible with their respective refrigerants. The PAGs

are only partly miscible and exhibit inverse miscibility with HFC-

134a. This type of immiscibility is characterized by a lower

critical solution temperature (LCST). At temperatures below the

LCST, there is complete miscibility; while at temperatures higher

than the LCST, the oil and refrigerant are immiscible. For the

conditions tested in this program, however, all of the PAGs are in

the fully miscible regime.

47

.J::> co

Table 3.3 Lubricant Properties

Oil Oil 1 Number Type Family Additives

Min1 Mineral Oil Para No Min2 Mineral Oil Naph No Alkbenz-B Alkylbenzene Arom No Alkbenz-F Alkylbenzene Arom Yes PAG1-B Polyalkylene glycol Mono No PAG1-F Polyalkylene glycol Mono Yes PAG2-B POlyalkylene glycol Mono No PAG2-F Polyalkylene glycol Mono Yes Est1-B Polyolester PE No Est1-F Polyolester PE Yes Est2-B POlyolester PE No Est2-F Polyolester PE Yes Est3-B Polyolester PE No Est3-F Polyolester PE Yes

1 Arom- Aromatic Para- Paraffinic Mono- Monoether Naph- Naphthenic PE- Pentaerythritol

Viscosity Il (cS)

@40o C @100o C

10? 11 12 12 2.6 57 5.8 57 5.8

135 25 135 25 100 20 100 20

23.94 4.88 23.9 4.87 91.37 10.19 91. 4 10.18 11.5 2.8 11.5 2.8

.t:. ~

Oil Number

Min1 Min2 Alkbenz-B Alkbenz-F PAG1-B PAG1-F PAG2-B PAG2-F PAG2-B PAG2-F Est1-B Est1-F Est2-B Est2-F Est2-B Est2-F Est3-B Est3-F

Ref Type

R12 R12 R12 R12

R134a R134a R134a R134a R134a R134a R134a R134a R134a R134a R134a R134a R134a R134a

Table 3.4 Lubricant Test Conditions

Temp Press. Weight % Miscibility Contact Type (oC) (MPa) Ref in Oil

Full Area 73.9 0.172 4.9 Full Conformal 100 0.172 1.7 Full Counterformal 80.6 1. 550 42.5 Full Counterformal 80.6 1.550 41.1

Partial Area 73.9 0.172 3.1 Partial Area 73.9 0.172 2.5 Partial Conformal 100 0.172 1.8 Partial Conformal 100 0.172 ---Partial Counterformal 80.6 1.550 17.1 Partial Counterformal 80.6 1. 550 21.2

Full Counterformal 80.6 1. 550 32.9 Full Counterformal 80.6 1. 550 28.5 Full Area 73.9 0.172 0.3 Full Area 73.9 0.172 0.5 Full Conformal 100 0.172 1.4 Full Conformal 100 0.172 1.9 Full Conformal 100 0.172 1.3 Full Conformal 100 0.172 0.9

Table 3.4 also shows the weight percent of refrigerant

saturated into the oil. These data were obtained by using the

sampling facility described in Section 2.3.3. The sampling

procedure is based on ASHRAE standards [20]. In this procedure, a

sample of the oil-refrigerant mixture is taken during the test and

then weighed. The refrigerant is then carefully removed from the

oil, and the final weight of the oil is measured. The amount of

refrigerant saturated into the oil can then be determined.

50

4.1 Overview

CHAPTER 4

EXPERIMENTAL PROCEDURES

In order to assure repeatability of results, the same testing

procedure was used for all tests. The specimens are cleaned to

remove surface residues prior to installing them into the chamber.

The appropriate lubricant is then added to the cup and allowed to

reach thermal equilibrium before continuing. The chamber is

purged to remove atmosphere and then charged with the required

refrigerant. Then, the system is allowed to reach pressure and

thermal equilibrium before initiating the test. After the test is

completed, an oil-refrigerant sample is taken, and the specimens

are analyzed for wear and surface films. The specific details of

these procedures are examined in this chapter.

4.2 z-Axis Motion Control

Before the HPT can have specimens installed or be charged

with ~ refrigerant, a basic understanding of how to operate the

HPT Z-axis motion is required. Figure 4.1a shows the motion

control panel of the main control box. This panel controls the Z­

axis servo motor, which provides motion for the lower half of the

chamber. The panel is divided into four separate sections. These

sections, along with their functions will be described in brief.

The upper section consists of a digital display linked to a

six-position rotary switch. This display is used to monitor any

of the six values for the switch. The display reads out in volts

from -10.0 v to 10.0 v. The equivalent engineering units to the

51

voltage reading can be determined by knowing the appropriate

conversion factors. Speed is displayed in volts, which converts

to SI units of 0.169 mm/sec/volt. The force reading depends on

the strain gage amplifier settings for the Z-axis, which will be

described in detail in Section 4.5. The Position setting

displays the axial position relative to some initial platen

position. The Position setting is not needed for basic HPT

functioning. The Setpoint reading displays the voltage that has

been set on the potentiometer in the Setpoint section of the

panel. Likewise, Frequency and Amp1itude readings display the

volt equivalents of the values set on the potentiometers in the

Osci11ation section of the control panel. Frequency reads out in

volts which has 0.5 Hz/volt equivalent, and amplitude depends on

the strain gage configuration.

The Contro1 Mode, located at the bottom of the panel, has

three toggle switches. The Amp1ifier switch is used to power up

the amplifier for the Z-axis servo motor. The Motor switch is

used to Enab1e/Disab1e the output stage of the motor amplifier

and thereby causes motion to start/stop. The Amp1ifier On/Off

switch is analogous to the ignition of an automobile and the

Enab1e/Disab1e switch is analogous to the clutch. The

CPU/Pane1/MCC switch allows the user to select among

microprocessor, panel, or personal computer control by a motor

control card. The initial positions for these three switches when

the main power is turned on for the tribometer is: the

Enab1e/Disab1e should be in the Disab1e position, the

Amp1ifier should be Off, and the CPU/Pane1/MCC switch should be

in the Pane1 position.

52

Vertical Axls-Z Rotation Axls- 9

-19.99 -19.99

Poaltion ~ Setpoint Force Frequency

Speed 0 Amplitude

PosItion ~ Setpoint Force Fntquency

Speed 0 Amplitude

Display· Display

Setpolnt Setpolnt

Travel Rele

o _Force_ @ PosItion Offset

Speed Rele

o _Force- @ Position Offset

LP ON

o o [XWII OON

r Oscillation r Oscillation """ + @ @ -0 Off

Spindle Sync. Fl1Iquency Fl1Iquency

Sine @ Sine @ 0 0

Triangle Triangle

A""I.ude A""I.ude

Waveform "- Waveform ~ ~

r """

r '" Enable 01 Enable 01

0 0 0 0 Disable Off Disable Off

Molor Ampllf. Molor AmpHf.

CPU CPU o Panel o Panel

MX MX

"- Control Mode Control Mode

AMn AMn

(a) (b)

Figure 4.1 (a) Z-Axis Motion Control Panel

(b) 9-Axis Motion Control Panel

The next section of the panel, the Oscillation section, is

only of use if oscillatory loads are applied. Although this

function is not used for any tests in this program, its functions

53

are included here for reference. Two potentiometers can be used

to set the frequency and the amplitude at which the load is

applied. It should be noted that only positive values of

amplitude are possible since a contact cannot support tension.

The Amplitude setting is used in conjunction with the Setpoint

section. The load will oscillate about the setpoint value with an

amplitude set with the Amplitude potentiometer. The waveform may

be either triangular or sinusoidal and is set with a separate

toggle switch. The Spindle Sync. switch is incorrectly marked

with the (-) and (Off) positions reversed. The Spindle Sync.

function synchronizes the motion of the vertical axis to the

oscillation of the rotational axis. In the activated. (+/-)

position, the frequency control of oscillation is transferred to

the value set with the 8-axis control. This synchronizes the

maximum and minimum load application with the maximum and minimum

8-position during rotational oscillation.

The Setpoint section of the Z-axis panel is the main control

point for the Z-axis motor. This section has a three position

toggle switch which is linked to a potentiometer. Either the

speed of travel, force applied, or relative position is set by

putting the toggle switch in the appropriate position and then

adjusting the potentiometer. Only one of these modes, Travel,

Force, or Position can be set and controlled at anyone time. It

is not possible, therefore, to simultaneously set both the speed

at which the platen travels and the force applied during the test.

The resulting value will be shown as a voltage on the display so

long as the six-position rotary switch is in the Setpoint

position. The speed can be set from 0-10 volts

54

(0.169 mm/sec per volt), the force is set from 0 to Fmax

(determined by strain gage amplifier configuration), and position

is relative from some initial position. The direction, either up

or down, can be set by proper position of the Up/Down toggle

switch.

Now that a basic understanding of the layout of the Z-axis

motor control section has been presented, the motor can be powered

up. The amplifier should not be turned on under load, so before

doing so, it is wise practice to zero all the potentiometers.

This is done by turning them fully counterclockwise until they hit

a stop. Having done this, the Amplifier switch can now be turned

On. The Motor switch should, however, remain in the Disable

position until motor motion is required.

4.2.1 Travel Mode

With the switch in the Setpoint section in the Travel

position, the potentiometer can be used to set the speed at which

the chamber opens or closes. The speed will be a function of the

setpoint of the Rate potentiometer. The speed can be read by

turning the rotary switch to the Speed position. The rate shown

is given as a voltage·from 0 to 10 volts. This corresponds to an

axial speed of 0 to 1.69 mm/sec. Once the rate and direction is

set, the motion can be started by turning the Motor switch to the

Enable position. The Rate potentiometer can be turned clockwise

to speed up the motion, or counterclockwise, to slow it down.

Even though limit switches provide good protection against over­

travel, the platen should always be monitored while in motion.

This machine is capable of applying very large forces, which could

55

potentially damage the machine or injure the operator. Once the

desired position of the lower half of the chamber is reached, the

Motor switch should be turned to the Disab1e position, and the

potentiometers should be turned fully counterclockwise.

4.2.2 Force Mode

This mode is used to apply a constant load during a test. To

place the HPT in this mode, the toggle switch in the Setpoint

section should be in the Force position. The potentiometer, also

in this section, can now be used to set the applied force from 0

to 10 volts. This voltage is read on the display when the six­

position rotary switch is in the Setpoint position. The actual

force equivalent to the voltage reading depends on the settings of

the Z-axis strain gage amplifier panel. The instructions for

configuring the strain gages are given in Section 4.5. From this

calibration setting, the 0 v reading corresponds to 0 N while a

reading of 10 volts corresponds to 4445 N. The force signal from

the transducer is read by the strain gage amplifier and compared

to the Setpoint value. The motor control then adjusts the

position of the lower half of the chamber until the desired force

is attained. This loop permits the force to be accurately applied

and controlled during a test.

Before the tribometer is used in force mode, several

conditions should be met. The amplifier should be turned on for

at least one hour prior to use. This allows the circuit boards to

warm up so that accuracy is improved. The tribometer should also

be at thermal and pressure equilibrium at the desired test

conditions. Pressure can slightly compress the lead screw of the

56

Z-axis motor and therefore cause slight changes in applied load.

If pressure equilibrium is not attained, the force readings can be

in error. Temperature has a very significant affect on the strain

gages of the transducer. Therefore, thermal equilibrium is

required before starting force mode control to ensure accurate

readings from the strain gages.

4.3 9-Axis Motion Control

The form of the 9-axis control panel, shown in Figure4.1b,

is similar to the Z-axis panel. It is divided into four sections:

Display, Setpoint, Oscillation, and Control Mode. By proper

manipulation of these settings, the 9-axis dc servo motor can be

controlled in a variety of ways. The description of this panel is

included in this section.

The Control Mode section of this panel contains three toggle

switches: Motor Enable/Disable, Amplifier On/Off, and

CPU/Panel/MCC. The functions of these switches are identical to

those in the Z-axis control panel so the reader is referred to

that section. As with the Z-axis, the CPU/Panel/MCC switch

should remain in the Panel position to allow the motor to be

controlled by panel settings. Also, all the potentiometers should

be zeroed before the amplifier is turned on. The Oscillation

section is also similar to the Z-axis panel. Waveform can be set

to either sinusoidal (Sine) or triangular (Triangle), and the

Frequency and Amplitude potentiometers function identically to

those in the Z-axis panel.

The Setpoint section, although identical in form, functions

slightly differently than the Z-axis panel. The most common mode

57

for this axis is the Speed position. With the toggle switch in

this position, the Rate potentiometer can be used to adjust the

speed from 0 to 10 volts. This corresponds to a rotational speed

of 0 to 2000 rpm, so the equivalence factor is 200 rpm/volt. The

voltage value for the speed can be read in the Disp1ay section

when the six-position rotary switch is in the Speed position. The

direction of rotation in set by using the ON/CON switch. The

convention used for clockwise/counterclockwise rotation is taken

from a position facing the shaft coming out of the motor.

The Force position of the Setpoint section has not been

used in this research, but its function may be useful in some

cases. The required torque is set by the potentiometer in the

Setpoint section. Like the Z-axis force control, the value of

torque depends on the settings of the Mz strain gage amplifier.

The torque feedback from the Mz strain gages is compared to the

Setpoint value. The speed of rotation is then adjusted to

whatever is needed to produce the desired torque.

The other mode of the Setpoint group is the Position mode.

This mode is used to set oscillatory motion of the 9-axis. The

Offset potentiometer is used to offset the 0° position. In this

case 10 v corresponds to 180° of offset, so a reading of 5 v will

offset the oscillation by 90°. The amplitude and frequency of

oscillation are set in the Oscillation section. Frequency can

be set from 0 to 10 volts (0 to 5 Hz) while the Amplitude

potentiometer can be set from 0 to 10 v (0° to 180°) amplitude.

58

4.4 Temperature Control

The temperature control panel has control of the cartridge

heaters in the chamber walls and can be used to turn on the

recirculator. This panel also can display any of the tribometer

temperatures. The panel, shown in Figure 4.2, has simple

controls and operation. The CPO/Pane1 switch should remain in

Pane1 position so that control of the cartridge heaters is done by

the panel switches. A separate switch allows the Recircu1ator to

be turned On/Off but the controls for setting the recirculator

temperature are housed in the separate NESLAB unit. The display

portion of the, temperature panel reads out in of and is used to

monitor the actual temperatures as well as the setpoint

temperatures. The six-position rotary switch is used to select

which temperature is displayed. The Lower Setpoint is the

desired temperature of the lower half of the chamber, while Lower

Temp. is the actual temperature. The two cartridge heaters in the

lower half are controlled by this panel to achieve the desired

temperature. The Opper Setpoint and Opper Temp. are controlled

in the same way and use the upper cartridge heater to obtain the

desired temperature. The Spind1e Temp. position reads the

temperature of the spindle at a point just before the shaft enters

the chamber. The P1aten Temp. position displays the temperature

of the cup.

The upper and lower cartridge heaters can be controlled by

this temperature panel. Both the upper and lower heaters have the

same type of controls. Either can be turned On or Off with a

toggle switch, and the desired temperature is set by adjusting the

59

appropriate potentiometer until the temperature is read in the

display.

r Temperature

-19.99 UpperS8lpOint~ ~rTamp. Lower Temp. Spindle Temp.

Lower SIIIpOint O. Plalen Temp.

r Upper Healer

@ 01

o Off

CPU 01 o 0 Panel 011

Reclrcullltor

AMTI ...

Figure 4.2 Temperature Control Panel

The recirculator temperature is controlled from the NESLAB

RTE-II0 temperature bath itself. A digital controller, shown in

Figure 4.3, displays the bath temperature in °c and permits

control of the temperature to ±O.l°C. The desired temperature is

set by depressing a Display button and then turning the Coarse

control dial until the desired temperature is displayed. The Fine

control dial can be used to aid in accurate setting of the desired

temperature. Once the operating temperature is set, the Display

button may be released and the LED display will read the actual

bath temperature.

60

1100.O°C 1

o Heat NESLAB oAtr,ess

RTE-110 D aT OJ

OPERATING TEMPERATURE

D Display

0 0 Fine Coarse

Figure 4.3 NESLAB RTE-II0 Recirculator Control Unit

4.5 Force & Torque Measurement

The three forces of interest are Fx , Fy , and F z , and the

torque of interest, in the Z-direction, is Mz . These values are

measured by a unique strain gage transducer that has low inter-

channel cross talk. In order to configure the transducer signals,

four identical strain gage amplifiers are used. They may either

be controlled by the panels, shown in Figure 4.4, or by the

microprocessor. The Panel mode, since it was easier to

understand, was was used in all tests for this research project.

As with the motor control panels, there is a CPO/Panel

switch in the Force panel. In CPO mode, the configuration of the

strain gage amplifiers is accomplished by the microprocessor

connected to a PC, while in Panel mode the switches on the panel

set the configuration. The display reads out the actual signal

from the strain gage in volts. In order to determine the

equivalent engineering units, the strain gages must be properly

configured with the appropriate gain and excitation voltage.

61

Friction Force-Fx '" Friction Force-Fy """

-19.99 -19.99

100 lk 5 10 10@lOk ~@20

.... (Hz) Exclbdlon (V)

1110 lk 5 10 10@10k 2.5@20

FI .... (Hz) Excitation IV)

2k 4k 8k xl-x2

lk@18k @ GUI GUlAdjuel

2k 4k 8k xl-x2

lk@l8k @ Gain Gain Adjuat

Enable Enable

0 0 Auto-ZMo Disable

0 0 Auto-Zaro Disable

CPU 60ft 0 Panel -

CPU 60ft 0 Panel -

ShuntCaJ Shunt Cal

\.. AMTI ... .) AMTI", ~

(a) (b)

r Axial Force-Fz '" I' Moment-Mz ~

-19.99 -19.99

100 lk 5 10

10@lOk 2.5@20

Filtar (Hz) Exclbdlon (V)

100 lk 5 10 10@10k 2.5@20 .0 F ..... (Hz) Excitation (V)

2k 4k 8k xl··x2

lk@16k @ Gain GUlAdjuet

2k 4k 8k xl-x2

lk@l8k @ Gain Gain Adjuat

Enable Enable

0 0 Auto-Zero Disable

0 0 Auto-Zero Disable

CPU 0011 0 Panel -

CPU o Oft 0 Panel -

Shunt Cal Shunt Cal

AMTI ... AMTI ...

( C) (d)

Figure 4.4 Strain Gage Amplifier Control Panels (a) Friction Force Fx (b) Friction Force Fy (c) Axial Load Fz (d) Moment Mz

62

The excitation voltage is set from 2.5 v to 20 v by a four

position rotary switch. It is recommended that the strain gages

be operated at 10 volts excitation voltage. This will maximize

the signal from the transducer and permit the use of the lowest

gain. Although 20 v will not damage the strain gages, the thermal

stability may be compromised. The gain section of the force panel

consists of three elements: a five-position Gain switch, a Gain

Adjust potentiometer, and Enable/Disable toggle switch. The

Gain is set from 1 k to 16 k by a five-position rotary switch.

The gain can then be further adjusted from 1 to 2 times the

reading of the five position switch by enabling the Gain Adjust

potentiometer. When the toggle switch is in the Enable position,

the Gain Adjust potentiometer allows for continuous variation of

gain between adjacent readings on the Gain switch. This allows

the gain to be set to any value from 1 k to 16 k. For instance,

if a gain of 6000 were required, the Gain switch should be set to

4 k, the Gain Adjust should be enabled and set to the point 1.Sx.

This setting will give a gain of 6000 (1.5 x 4 k = 6000). In this

way, the gain can be adjusted for calibrating the voltage output

into equivalent engineering units.

Appropriate engineering units can be set by proper gain

adjustment.· The equation for calculating the appropriate gain to

obtain an equivalent force/volt is given by:

where:

and

Fv Greq Vexc

Equivalent Force/Volt Required Gain Excitation voltage (normally 10 v)

63

(4 . 1)

Cl = Calibration constant for the appropriate axis = 8.78x10-7 l/N for Fx

8.90x10-7 l/N for Fy = 2.25x10-7 l/N for Fz = 1.63x10-S l/(N-m) for Mz ·

So, for example, to obtain a force/volt equivalent of 44.5 N/v for

Fx, the gain should be set to 2557. Now the difficulty is how to

set the Gain Adjust potentiometer so that a gain of 2557 is

obtained. Since a gain of 2557 is between 2 k and 4 k, the Gain

Adjust potentiometer must be used to obtain the required gain.

In order to set a known gain with the Gain Adjust

potentiometer, it is necessary to use the Shunt Cal switch. This

switch places a precision calibration resistor across one arm of

the appropriate Wheatstone bridge in the force transducer. The

+/- switch selects either of two adjacent arms which provides a

positive or negative output. The value of these outputs should

only vary by the sign proceeding the voltage, therefore, it will

not matter which is used to set the gain adjust. For convenience,

the + setting is used in this description. Before switching the

Shunt Cal switch to the + position, the Auto-Zero button should

be pressed. This will automatically balance, or zero, the output

of the transducer signal. To set the Gain Adjust potentiometer,

the output voltage is found from:

Vout Greq x Vexe x 1000

4 x Gswiteh

64

1 J Real Rbridge (4.2)

where: Vout

Greq Vexe Gswiteh

Real Rbridge

=

= = =

= = = =

amplifier output voltage upon switching Shunt Cal. to + position required gain from Eq. (4.1) excitation voltage (normally 10 v) required Gain Switch setting ego if Greq=6000 then Gswiteh = 4 k Greq=2100 then Gswiteh = 2 k 499,000 0 (calibration resistor) resistance of shunted arm of bridge 350 0 for Fx , Fy , and Mz 700 0 for F z •

The procedure for setting the gain adjust is somewhat

confusing and is best described by an example. Assume that a gain

of 2557 is required for Fx , found in the previous section to yield

44.5 N/v equivalent. This would require the Gain switch to be set

to 2 k so Gswiteh=2000. For Fx , Rbridge=35 00, and assuming an

excitation voltage of 10 volts, Eq. (4.2) reduces to:

Vout = (2557) x (10v) x (1000) [ 1 ] (4) x (2000) 1 + 499,0000

3500

Vout = 2.24 volts

To set this output voltage, the Gain switch should be in the 1 k

position, the Gain Adjust should be enabled, excitation voltage

should be set to 10 volts, and the Shunt Cal. switch should be in

the + position. Now by turning the Gain Adjust potentiometer

clockwise until 2.24 v is read in the display, the appropriate

gain adjust factor is set. The Shunt Cal. switch may now be

turned off, and the Gain switch turned to the 2 k position. The

Gain Adjust is always set while the Gain switch is in the 1 k

position because the display can only show Vout values less than

10.0 volts. After the Gain Adjust is set, the Gain switch

should returned to the Gswiteh position. The other strain gage

65

amplifiers are configured by the same method as Fx except that the

appropriate values for Rbridge and Cl should be used.

4.6 Installation of Specimens

Now that the method for configuring the HPT controls for a

test has been presented, the specimens are nearly ready to be

installed. Before the specimens can be installed in the chamber,

they must be thoroughly cleaned. The specimens, specimen holders,

screws, and cup must all be ultrasonically cleaned for 10 minutes

in a solution of mineral spirits. Then they are rinsed clean with

2-propanol to remove any residues. Care should be taken to avoid

touching the specimens since contaminants transferred ,from the

skin can affect the test, especially for tests conducted without

lubricants. The specimens should only be handled with clean

forceps during the installation process. The specimens are

mounted into the specimen holders, as was shown in Section 3.3,

and then mounted into the cup and onto the spindle. The upper

specimen/specimen holder is held directly to the spindle by four

10-32 machine screws, while the lower specimen holder is secured

to the cup by two 10-32 machine screws. The hole pattern in the

cup permits the specimen holder to be mounted in a variety of

orientations, but the most convenient orientation is that in which

the specimen is near one of the sight ports.

The cup can now be assembled with the glass sleeve, and the

sampling hole should be plugged with the threaded dowel. Now the

required oil can be added to the cup. The oil level in the cup

should reach the middle of the sight port to maintain equivalent

amounts of lubricant for all tests. Now the cup is ready to be

66

installed in the chamber. Care must be taken to align the

sampling hole in the cup with the sampling line in the top of the

transducer. The six steel locator pins on the top of the

transducer assure alignment as well as providing resistance to

torsional buckling. The cup is secured to the transducer by

tightening three 10-32 machine screws. Now the threaded dowel can

be removed and the chamber can be closed. Following the procedure

outlined in Section 4.2.1, the chamber should be closed to the

point just before the upper and lower specimens contact each

other. The gap between the specimens should be approximately 3 rom

to 6 rom.

4.7 Purging Procedure

Now that the specimens are in close proximity and the desired

lubricant, if any, has been installed, the system can be taken to

the test temperature. This is accomplished by setting the various

heaters and external units of the HPT. The recirculator should be

set to the desired test temperature, as should the cartridge

heaters. Although the cartridge heaters are primarily used to

prevent condensation of refrigerant, they should also be used when

testing without refrigerant. They help to heat the oil sample and

promote a uniform chamber temperature, thus minimizing thermal

gradients. The cartridge heaters are set by the temperature

control panel described in Section 4.4. The HPT should then be

allowed to come to thermal equilibrium. This should take

approximately 60 minutes, but can vary with the actual temperature

required. After the cup temperature, labeled Platen Temp on the

67

temperature control panel, and the Spindle Temp are within 5° F

of the test temperature, the chamber can be purged.

Before purging, both the charging facility and the drain

facility should be connected to the HPT so that their lines may be

evacuated at· the same time. The sample cylinder does not need to

be attached at this time, but will be evacuated during the

sampling procedure. The Welch and Thermal vacuum pumps are

attached to the HPT as described in Section 2.3.1. The pressure

transducer for the Thermal Vak-Check vacuum gage should also be

connected to the HPT, as shown in Figure 4.5, but valve #6 should

remain closed until after the vacuum pumps are running.

The system is now ready to be purged. First, valves #1, #7,

#8, and #10 should be opened and all other valves should remain

closed. Now the Thermal vacuum pump can be turned on, after which

the Welch pump can be turned on. Valve #6 can now be opened to

permit the vacuum gages to monitor the pressure in the HPT. Now

the valve to the Welch vacuum pump, valve #5, is opened until the

desired vacuum is read on the Thermal gage. With the two pumps

working in tandem, the entire chamber can be evacuated in less

than a couple of minutes.

For tests involving lubricants, the chamber should be

evacuated down to 500 microns; while for tests without lubricants,

the chamber can be purged to below 100 microns. The difference

between these purge levels arises from the fact that the lubricant

begins to boil violently at vacuums better than 500 microns. The

boiling limit, though, does depend upon the type of lubricant

being tested and its temperature.

68

V Iv 1 2 3 4 5 6 7 8 9

10 11

Legend

Drain Valve Drain Tank Valve Sample Port Valve Sample Cylinder Valve Welch Pump Valve Vacuum Gage Valve Throttle Valve Supply Valve Pressure Vessel Valve Filling Valve Supply Tank Valve

-..- To Welch Vacuum

Servo Motor

1-----1f--+TO The rmal .... Vacuum Pump

6

50cc Sample Cylinder

Pressure Chamber

13.6 kg Supply Tank

tnrl ~ 0)

Ul '-DUl .0)

(Y»

Silicone Heating Blanket

6.8 kg Drain Tank

Z-Axis Servo Motor (Load Cell)

Figure 4.5 HPT Mechanical System Schematic

69

The most efficient method of purging and charging the HPT is

to have the charging facility ready to charge the chamber before

initiating the purging procedure. In this way, as soon as the

chamber is evacuated, and the valves are closed, it may be charged

with refrigerant. This alleviates any problems with possible

vacuum leakages.

Once the desired vacuum is reached, all valves should be

closed. After valve #6 has been closed, promptly remove the

Thermal vacuum gage pressure transducer to avoid damaging it. The

Welch vacuum pump can now be vented to atmosphere with the small

venting screw on the valve #5, as shown in Figure 4.6. This must

be done to avoid damaging the pump seals the next time it is

restarted. The Thermal vacuum pump should continue pumping the

chamber until the chamber is charged with refrigerant and the test

begins. The Thermal pump must also be vented before it is turned

off to avoid seal damage. After the chamber is charged, the

Thermal pump should first be disconnected from the HPT and then

turned off.

Venting Screw

Figure 4.6 Venting Screw on Valve to Welch Vacuum Pump

70

4.8 Charging Procedure

Before the HPT can receive a charge of vapor refrigerant, the

3.64 kg pressure vessel must contain a sufficient amount of the

desired refrigerant. Filling the pxessure vessel will need to be

accomplished -every 8 to 10 high pressure tests, as well as any

time a new refrigerant is to be tested. This filling procedure,

therefore, is provided prior to the actual HPT charging procedure.

The refrigerant in the pressure vessel can then be transferred to

the HPT chamber by proper temperature control of the vessel to

generate sufficient pressure.

The pressure vessel should be filled while the entire system

is being purged. After the vacuum pumps have evacuated the system

to at least 500 microns, valves #7, #8, #9, #10, and #11 are

closed. The pressure vessel should now be placed in an ice bath

to aid in transferring liquid refrigerant from the 13.6 kg supply

tank to the pressure vessel. The supply tank should also be

placed, inverted, above the pressure vessel. This will supply

liquid refrigerant to the vessel. The supply tank should be

placed on a scale to monitor the weight of refrigerant transferred

to the vessel. Now valves #9, #10, and #11 may be opened until

the desired amount of refrigerant has been transferred. In

general, it takes approximately 5 to 10 minutes to fill the

pressure vessel, but this depends on ambient temperature of the

supply tank. After the pressure vessel is full, valve #9 should

be closed, the pressure vessel should be removed from the ice

bath, and the supply tank should be placed upright on the floor.

This will allow the liquid refrigerant trapped in the lines to

drain back into the supply tank. This draining should take less

71

than one minute. After this draining, valves #10 and #11 may be

closed. Now the pressure vessel can be used to charge the HPT.

Assuming that the chamber has just been purged and valve #5

has just been closed, the chamber is now ready to be charged with

refrigerant vapor. Depending on what test pressure is required,

the pressure vessel may need to be heated. For pressures below

0.414 MPa, the pressure vessel does not need to be heated, so the

refrigerant can be directly transferred to the HPT. By opening

valves #8 and #9, and then using valve #7 to throttle the flow of

refrigerant, the desired pressure can easily be attained.

For pressures greater than 0.414 MPa, however, the pressure

vessel must be heated. The heat is provided by the heating

blanket wrapped around the pressure vessel. A thermometer is

placed between the blanket and vessel to monitor the temperature.

The temperature that will generate the required pressure can be

determined from the appropriate refrigerant vapor pressure curve

shown in Figure 4.7. For example, Figure 4.7a shows that in order

to generate a vapor pressure of 1.5 MPa for HFC-134a, a

temper~ture of 58 °c would be required. The temperature of the

refrigerant, therefore would be raised to this temperature. It

should be noted that the temperature measured by the thermometer

is not the temperature of the actual refrigerant but that of the

outside of the pressure vessel. The actual internal temperature

of the refrigerant is generally 50 C lower due to convection,

radiation, and other heat losses from the pressure vessel. It is

therefore necessary to set the variac to generate a temperature of

approximately 630 C to obtain HFC-134a vapor at 1.5 MPa, for this

example.

72

2.0,-------------------------------------------n

cO 1.5

~ (l) 1-1 ::s [/) 1.0 [/) (l) 1-1 ~

(l) 0'1 cO t!J

0.5

O.O;,"TT~~~~TT~~MM~TT~~~~TTTr~~~TTi

-30 -20 -10 o 10 20 30 40 50 60 70

Temperature (oC)

Figure 4.7a HFC-134a Vapor Pressure vs Temperature

2.0,-------------------------------------------~

1.5 ttl 0.. ~

<Ii ~ ::l fIl 1.0 fIl <Ii ~ 0..

<Ii 01 ttl ~

0.5

-30 -20 -10 o 10 20 30 40 50 60 70

Temperature (oC)

Figure 4.7b CFC-12 Vapor Pressure vs Temperature

73

The variac should initially be set to approximately 50% of

its maximum voltage output and then adjusted to obtain the desired

pressure vessel temperature. At 50%, the variac will cause the

pressure vessel to reach a temperature of 77° C and a

corresponding pressure of over 2.0 MPa for either CFC-12 or HFC-

134a, so the pressure and variac setting should be closely

monitored to avoid over pressure conditions. Under no

circumstances should the temperature of the pressure vessel be

allowed to exceed 85° C. If a temperature of 85° C is not capa-ble

of generating the required test pressure, the charging process

should be discontinued. Either there is not enough refrigerant in

the pressure vessel to permit charging, in which case the vessel

will need to be refilled, or the refrigerant is condensing on the

chamber walls, which could be caused by a malfunction or incorrect

setting of the cartridge heaters. After the pressure vessel

reaches the desired pressure, or is slightly above it, the chamber

is ready to be charged.

Before continuing with the charging process, valves #5 and #6

must be closed. Charging is accomplished by opening valves #7,

#8, and #9 and allowing the chamber to reach the desired test

pressure. Once this pressure is reached, valve #7 should be

closed and the chamber should be kept at the test pressure for one

hour to allow the oil-refrigerant mixture to reach an equilibrium.

One hour has proven to be a sufficient amount of time to allow the

refrigerant to saturate into the oil [16]. Valve #7 may have to

periodically opened to maintain a constant refrigerant pressure in

the chamber as the refrigerant saturates into the oil. After one

hour, the variac should be turned off and the pressure vessel

74

should be allowed to return to ambient temperature before valves

.8 and .9 are closed. The HPT is now ready to be configured for

controlling and conducting the tests.

4.9 Configuring the HPT Controls and Conducting the Test

Now that the HPT chamber is·at thermal and pressure

equilibrium, the controls must be configured to run the test. The

strain gage amplifiers must be individually configured for each

test. Since the axial force is the applied test load, the F z

strain gage amplifier configuration is determined by the magnitude

of the applied test load. In order to achieve high accuracy, the

equivalent test load, in volts, should be between 7.0 v and

10.0 v; but for convenience in monitoring loads, it is desirable

to keep a simple force/volt equivalent. Suppose an axial applied

load of 561 N is required for a certain test, a convenient

force/volt equivalent might be 60 N/v. This would mean that the

applied voltage would be 9.35 v, which will satisfy the accuracy

condition as well as providing a convenient conversion from volts

to Newtons. From Eq. (4.1), the required gain to have 60 N/v is

Greq=7416. This gain can be obtained by using Eq. (4.2) and the

procedure of Section 4.5 to set the Gain Adjust potentiometer to

yield Vout=6.49 v.

The two friction forces, Fx and Fy , are set by a similar

method. As a general rule, the coefficient of friction will be

less than 0.20, so the magnitude of the friction forces will be

20% of the applied load. This requires that Fx and Fy be set to a

lower force/volt equivalent than the applied load Fz . For example,

if F z is set to 60 N/v, both Fx and Fy should be set to 12 N/v.

75

Then, from Eq. (4.1), the required gains are 9484 for Fx and 9364

for Fy . These gains correspond to output voltages of Vout=2.08 v

and Vout=2.05 v for Fx and Fy , respectively. These settings will

provide accuracy of the actual reading and convenience in

calculating the force equivalent of the Fx and Fy voltages

displayed on the control panels.

Similarly, the moment Mz is configured in the same manner.

Since, for this example, the maximum frictional force resultant

will be when both Fx and Fy are 10 v (120 N) the tangential force

will be Ft=(1.41) (120)=170 N. Assuming that the average moment arm

of the contact is 19 rom, an approximate maximum moment of

3.238 N-m is possible. By setting Mz to 0.4 N-m/volt, the maximum

measurable moment will be 4.0 N-m (10 v) which is consistent with

the other force measurements. To obtain an Mz setting of

0.4 N-m/v, Eq. (4.1) requires a gain of 15337. This required gain

corresponds to an output voltage of Vout=3.36 v for setting the

Gain Adjust. The final step in configuring the amplifiers is to

use the Auto-Zero button to balance the strain gages before

initiating the test. Now that the strain gage amplifiers have

been configured, both the axial load and rotational motion need to

be set.

The axial force is set according to the procedures outlined

in Section 4.2. To apply a constant axial force, the Z-axis

control panel should be set to the following positions. The

amplifier should be turned on and warmed up at least one hour. In

the Setpoint section, the toggle switch should be in the Force

position and the direction switch should be in the Up position.

Before the entire load is applied, a smaller load of 0.5 v or less

76

should be applied with the Force potentiometer. This smaller load

is to ensure the contact specimens align properly. The Z-axis

Motor switch may now be enabled, allowing the small force to be

applied.

Next, the 8-axis needs to be properly configured as described

in Section 4.3. For unidirectional rotation, the direction is set

with the CW/CCW switch, the Setpoint toggle switch should be in

the Speed position, and the Rate potentiometer is adjusted to

yield the appropriate speed. If oscillatory motion is required,

the procedure for setting the frequency and amplitude in

Section 4.3 should be followed.

Once the 8-axis is configured, the data acquisition system

must be initialized and properly configured to sample the

appropriate data. The interface between the PC and the

microprocessor is initialized by choosing the TMBMI icon from the

PC Windows environment. The interface program then initializes a

connection with the microprocessor and awaits the command to begin

sampling data. Since each of the contact types require slightly

different data to be sampled, each configuration has been given a

separate command. The commands are RLPIST, SPLATE, and RCPIST

for the counterformal, area, and conformal contacts respectively.

By entering the appropriate command at the cursor, the data

acquisition system is automatically configured to sample data for

the particular contact. To begin sampling data and saving them to

disk, the following commands, for the appropriate geometry, should

be typed but not yet entered.

For counterformal contact, type: ISAVE 15 285 "filename"

77

For area contact, type: SAVED "filename" 3600

For the conformal contact, type: ISAVE 10 285 "filename"

where filename is the name of the file to be saved to disk.

Now that the HPT controls have been properly configured, the

test may be started. First, the rotational axis motor should be

enabled. This will cause the 8-axis dc motor to begin its motion,

whether unidirectional or oscillatory. Now the full test load

should be applied. Since the Z-axis motor is still enabled, but

only a small load is applied, the Force potentiometer should be

increased until the desired load is applied. In order to ensure

repeatability of start-up conditions, the load should be increased

at a constant rate over one minute up to the test load. In order

to begin data acquisition, the appropriate save command should now

be entered from the keyboard.

The tests are run for one hour unless seizure or other

problems occur. The status of the contact should be observed

periodically. This is best accomplished by using a flashlight,

and lqoking in one of the viewports in the chamber. The presence

of wear particles, oil cloudiness, or other relevant physical

phenomena should be noted along with the time into the test. The

HPT should not be left unmonitored while a test is underway.

The procedure to stop a test is basically the reverse of

starting the test. The load should be removed and the contacts

should be separated by about 3 mm to 6 mm. The 8-axis and z-axis

motors should then be disabled, but the chamber should remain

closed to allow the oil-refrigerant mixture to be sampled, and the

remaining refrigerant to be reclaimed.

78

4.10 Sampling

Before an oil-refrigerant sample can be drawn off from the

chamber, the sample cylinder should be cleaned and purged. The

cylinder and fittings should be cleaned with mineral spirits and

then rinsed with 2-proponal to remove any residues. The Thermal

vacuum pump should then be connected to the sample cylinder, and

the cylinder purged for 5 minutes. This amount of purging has

been found to be sufficient to remove nearly all atmosphere from

the cylinder and leave a vacuum of less than 100 microns. The

valve of the sample cylinder should then be closed, it should be

disconnected from the vacuum pump, and then the sample cylinder

should be weighed on the Sartorius balance. This weight, Wl, of

the empty sample cylinder will be used to determine the weight

percent of refrigerant in the oil.

The sample cylinder should now be connected to the chamber

using the quick connect on valve #3 of Figure 4.5. Both valves #3

and #4 should now be opened so that the oil-refrigerant mixture is

allowed to flow into the sample cylinder. The time required to

fill the sample cylinder will depend on the viscosity of the

mixture and the pressure in the chamber, but in general, 5 minutes

should be sufficient. Now both valves #3 and #4 should be closed

and the sample cylinder should be removed from the chamber at the

quick-connect. If any oil-refrigerant mixture is spilled on the

outside of the cylinder, the surface should immediately be cleaned

with mineral spirits, rinsed with 2-propanol, and then dried. Now

the full sample cylinder, with the oil-refrigerant mixture, should

be weighed on the balance. This weight, ws, is used in determining

the total sample weight.

79

3.05 m x 0.63 rom 0 Capillary Tube

Sample Cylinder Assembly

1/4" Plug Valve (Nupro: B-4P4T)

Figure 4.8 Sample Cylinder Refrigerant Evacuation Facility

The sample cylinder should now be connected, as shown in

Figure 4.8, to a 3.05 m by 0.63 rom 0 capillary tube assembly.

Initially, valve #12 should remain open to the atmosphere. Valve

#4 is then opened until outward flow has stopped. This may be

monitored by observing the gas flowing out of the capillary tube.

This process may take approximately two hours, but time will

depend on the amount of refrigerant saturated into the oil. To

remove any remaining refrigerant trapped in the oil, valve #12

should be connected to the Thermal vacuum pump, and the cylinder

purged for at least one hour. After one hour, valve #4 should be

closed and the capillary tube assembly removed from the sample

cylinder at the quick connect. The final weight, W3, of the sample

cylinder is now measured on the balance. This weight is the

80

weight of the cylinder and remaining oil. The resulting weight

percent of refrigerant in the oil is found from:

where Wt%= WI W3 W5

Wt% (4.3)

Weight percent of refrigerant in oil Empty weight of sample cylinder Final weight of sample cylinder and oil Full weight of sample cylinder with oil­refrigerant mixture

4.11 Refrigerant Reclamation

Refrigerant reclamation is necessary to avoid contributing to

the release of ozone depleting compounds, as well as to reclaim

the expensive refrigerants so that they may be recycled. The

procedure for reclaiming the refrigerant from the chamber is

efficient yet simple and inexpensive. An empty 6.8 kg DOT-39 NRC

260/325 MI073 E.!. du Pont de Nemour & Co container is used as the

drain tank. Figure 4.9 shows this container connected to the HPT

chamber at the drain valve.

To begin refrigerant reclamation, the 6.8 kg drain tank is

placed in an ice bath and valves #1 and #2 are opened. The cold

refrigerant in the drain tank is at a lower pressure than the

chamber so there is a pressure driving force to transfer

refrigerant to the drain tank. The weight of the 6.8 kg container

should be periodically monitored to ensure that it is not being

overfilled which could cause rupture. Equilibrium will be reached

after about one hour so that valves #1 and #2 can be closed. The

chamber can now be opened so that the oil and specimens can be

removed. There will still be a slight amount of refrigerant in

the chamber. Valve #5 can be opened while the chamber is opening

81

to permit any residual refrigerant to escape. The cartridge

heaters should now be turned off and the recirculator should be

set back to ambient temperature. Once the chamber has cooled

sufficiently, the specimens can be removed and wear analysis can

begin.

HPT Pressure Chamber

<D 1/4" Nupro Plug Valve (SS-4P4T-TB)

1/4" 0 Nupro Hose SS-7R4TA4TA4-24

DuPont 6.8 kg Drain Tank

Figure 4.9 Refrigerant Reclamation Facility

82

CHAPTER 5

RESULTS & DISCUSSION

5.1 Measurement of Wear

Wear results are based on measurements taken immediately

after completion of each test except for the area contact. While

all specimens are ultrasonically cleaned in mineral spirits after

testing and then immediately measured for wear, the bronze shoe of

the area contact is cleaned and then allowed to dry in a desiccant

chamber to remove any moisture. This is only done for the shoe

because its wear is measured as a weight loss which would be

affected by moisture. Wear on the other contacts is measured by

methods not sensitive to moisture so they do not need to be dried.

Each test was repeated at least once to compute the average

friction and wear.

The reported wear for the counterformal contact is only that

measured on the tool steel pin. The surface of the mating piece,

the gray cast iron plate, showed only minor polishing wear. This

wear probably resulted in slight changes in the surface finish,

but no measurements were taken. A typical wear scar on the tool

steel pin is shown in Figure 5.1. The wear scar width is measured

with a Nikon SMZ-2T stereoscopic microscope. This method permits

the width of the scar on the pin to be measured to an accuracy of

0.005 mm. The average wear scar width was determined by taking

three measurements of the wear scar along the length of the pin.

The volumetric wear may be calculated by:

v ~ s r - w( r - 0 )]L 2

83

(5.1 )

where V volumetric wear (mm3 )

wear scar width (mm) r = radius of pin (mm) w

L length of pin (mm) and

s=2r</> where </>= asin(2:) and

Figure 5.1 Typical Wear Scar on Pin of Counterformal Contact. This particular contact, 31RL, was run in HFC-134a, by itself, at 1.55 MFa. (25x)

The reported area contact wear is computed as a weight loss

on the bronze shoe. Figure 5.2 shows a typical surface of the

bronze shoe after testing. The weight of the shoe, Wi, is recorded

prior to testing. After the test, the shoe is cleaned and dried

before re-weighing it. This final Height, W"f, is the Height after

testing. The difference betHeen these tHO Heights is the total

84

weight loss due to wear. All of these weights are measured on a

Mettler model AE 163 precision balance, with a readability of

0.01 mg. Because the mating piece, the ductile cast iron plate,

is very much harder than the bronze, no wear was observed on the

plate. Only small amounts of bronze are transferred to the

surface of the plate during testing.

Figure 5.2 Typical Worn Surface of Bronze Shoe of Area Contact. This contact, 30SP, was run with PAGI-F plus HFC-~34a. (25x)

The wear reported for the conformal contact is that measured

only on the aluminum pad. Figure 5.3 shows a magnified view of a

typical wear scar on the pad. The entire wear scar on the surface

of the aluminum pad is similar to that in Figure 5.4. Using a

Talysurf 10 surface profiler, the depth of the wear scar was

accurately measured to within 0.0025 mm. The scar depth was

measured at three separate locations on the pad and then an

85

average wear scar depth was computed. The mating piece, the casc-

hardened steel pin, showed no wear.

Figure 5.3 Typical Wear Scar on Aluminum Pad of Conformal Contact. This contact, 25RC, was run with Est3-F plus HFC-134a. (25x)

Talysurf Traces

Wear Scar

Figure 5.4 Wear Scar on Aluminum Pad of Conformal Contact

5.2 Surface Analysis

In order to determine critical surface "properties, both

before and after testing, two surface analysis tools were used.

The Talysurf 10 was used to determine surface profiles as well as

86

roughness average (Ra) values for all samples tested. As

previously mentioned, the Talysurf 10 was also used to measure

wear scar depths on the aluminum pad for the conformal contact.

The second surface analysis tool was X-ray Photoelectron

Spectroscopy (XPS); which was used to determine the existence, if

any, of surface films formed during testing.

A Perkin-Elmer/PHI 5400 X-ray Photoelectron Spectrometer was

used in the small spot XPS studies. A regular MG Ka x-ray source

is used to bombard the surface of the specimen with x-rays which

dislodge photoelectrons from the surface. These photoelectrons

are then analyzed by a spherical capacitor analyzer with a

computer data acquisition system. This analysis can be used to

show the formation of surface films, if any, such as metallic

chlorides or metallic oxides, that formed on the specimen surfaces

during testing.

5.3 Counterformal Contact Results

Wear data for the counterformal contact are shown in

'Figure 5.5. The formulated versions of the three lubricants

(Alkbenz-F, Est1-F, and PAG2-F), by themselves, provide better

wear resistance than their base counterparts. This can be

attributed to the additive packages that are included in the

formulated oils to improve wear. Once refrigerant is added to the

lubricants, however, the wear results are less predictable.

When CFC-12 is added to the alkylbenzenes, the wear results

depend on the formulation of the oil. When CFC-12 is added to the

base alkylbenzene, wear decreased. XPS analysis of the tool steel

pin showed formation of iron chloride films (FeC12) on the surface.

87

The hypothesis is that these FeC12 films essentially act as EP

agents in the oil thus reducing wear. A comparable increase in

wear resistance for the formulated alkylbenzene was not observed

when it was tested with CFC-12. This is most probably due to the

additives which already exist in the oil.

0.004

o.ooo~~-

Ref Base Form Base + Ref

Form + Ref

Figure 5.5 Wear Data For Counterformal Co.ntact

While CFC-12, by itself, provides very good wear resistance,

testing in a HFC-134a environment shows extremely high wear rates,

equivalent to testing in air. XPS analysis of the tool steel pins

tested in HFC-134a showed that no surface films were produced.

The addition of HFC-134a to the Estl and PAG2 lubricants also show

no improvements to wear resistance. In fact, testing of both the

esters and PAGs with HFC-134a t~nds to increase wear over the

respective lubricant alone. This is due to the fact that the

88

addition of HFC-134a to the lubricant decreases the effective

viscosity. This lower viscosity decreases the probability of

generating protective oil films between the surfaces, thus

producing higher wear. The raw data, given in Appendix B, shows a

maximum scatter between repeated tests of 8.7%. This suggests

that the repeatability of experimental data is quite good.

s:: 0

• .,f .jJ u

• .,f 1-1 r.. Ii-! 0

.jJ s:: CI)

• .,f u

• .,f Ii-! Ii-! CI) 0 u CI) 01 cu 1-1 CI)

~

0.15

0.10

0.05

Ref Base Form Base + Ref

Form + Ref

Figure 5.6 Friction Data For Counterformal Contact

While wear data are quite dependent on the type of lubricant

and type of refrigerant tested, the friction results, shown in

Figure 5.6, do not generally follow these tendencies. Overall,

only slight variations in coefficient of friction are observed for

all tests conducted. The coefficient of friction for HFC-134a, by

itself, does show the highest value of any of the tests. This

correlates well with the wear results but other general trends are

89

difficult to discern. The only other tendency of the coefficient

of friction results which parallels the wear results, with the

exception of Est1-B, is that the tests involving HFC-134a with a

lubricant, show slightly higher coefficients of friction than the

corresponding tests without HFC-134a. The repeatability of the

coefficient of friction for all tests is within 16%.

5.4 Area Contact Results

The wear results from area contact tests are shown

graphically in Figure 5.7. When the base oils are used alone, the

mineral oil seems to provide the best wear resistance. Although

the two formulated oils by themselves give better wear

characteristics than the base mineral oil, once refrigerants are

added to the lubricants, the mineral oil-CFC-12 mixture provides

the best wear resistance. The presence of CFC-12 promotes

chemical reaction on the bronze shoe, producing copper chlorides

(CUCI2) and traces of zinc fluoride (ZnF2) surface films. As with

the counterformal contact, the surface films help to protect the

surface and therefore lower wear. Although not verified, the R12

most likely formed FeCl2 surface films on the mating ductile cast

iron disk as well. The wear resistance of base PAG and ester oils

in a HFC-134a environment is much lower than that for mineral oil

in CFC-12. From the limited number of lubricants tested, it is

seen that the formulated ester with refrigerant (Est2-F+R134a)

provides wear characteristics similar to the mineral oil-CFC-12

mixture.

As with the counterformal wear results, HFC-134a alone lacks

lubricative properties. Though not shown, very high wear rates

90

and temperature rises, equivalent to testing in air, are observed

for tests run with HFC-134a by itself. When HFC-12 was tested

alone, it also showed higher wear rates than those of the base

oils, but not nearly as high as the HFC-134a. Similar to the

counterformal contact results, the addition of HFC-134a to both

the PAGs and esters tends to increase wear. The raw data, given

in Appendix B, show a maximum scatter between repeated tests of

13.5%.

Base Form Base + Ref

Form + Ref

Figure 5.7 Wear Data for Area Contact

The friction data for the area contact are shown in

Figure 5.8. In general, these data correlate reasonably well with

the wear data. As with the wear, the friction obtained with the

formulated ester and HFC-134a compares favorably with the

presently used mineral oil-CFC-12 mixture. Also the formulated

91

versions of the PAG and ester provide lower coefficients of

friction than their base counterparts. The maximum scatter for

the coefficient of friction between repeated tests was 22%.

s:: 0

• .-1 .j.J 0

• .-1 1-1 r.. "-I 0

.j.J s:: Q)

• .-1 0

• .-1 "-I "-I Q) 0 u Q) 0'1 lIS 1-1 Q)

~

0.08

0.06

0.04

0.02

Base Form Base + Ref

Form + Ref

Figure 5.8 Friction Data for Area Contact

5.5 Conformal Contact Results

Wear data for the conformal contact are shown in Figure 5.9.

As with the area contact results, the mineral oil provides the

best wear resistance for this contact. The addition of CFC-12 to

the base mineral oil improves wear resistance. XPS analysis shows

the formation of FeCl2 films on the steel pin. This film acts like

those formed in the counterformal contact by protecting the

surface against wear. The addition of HFC-134a to the PAG2-B

tends to increase the amount of wear. Although not graphically

shown, when HFC-134a was tested by itself, seizure occurred after

92

15 minutes of testing. The difference in the wear results for the

two esters is probably due to the fact that the viscosity of Est3

is much less than that of Est2. Thinner oil films are generated

for low viscosity oils than for high viscosity oils. This allows

closer proximity of surfaces and therefore more asperity

interaction and wear. The scatter between repeated tests, from

raw wear data given in Appendix B, show a maximum of 11.5%.

i'O ~1\1

p., .c +J S o..::s Q) t: Q .g 1-1 ::s 1\1~ u ..:e U)

t: 1-1 0 1\1 Q)'O :l: Q)

1-1 Q) ::s OIUl 1\1 1\1 1-1 Q) Q) ~

~

0.25

0.20

0.15

0.10

0.05

Base Form Base + Ref

Form + Ref

Figure 5.9 Wear Data for Conformal Contact

Figure 5.10 shows the coefficient of friction data for the

conformal contact. Even though the mineral oil-CFC-12 mixture

shows very good wear characteristics compared to the esters and

PAG with HFC-134a, its friction characteristics tend to be worse

than those of the latter mixtures. Overall, the PAG2-F gives the

93

() I-' 0 0 CD ~ HI CD HI CJ) .... rt () .... ()

Average Coefficient of Friction CD 0 ~ (1)

hj rt HI .... 0 0 0 a a HI \Q a a ...... ...... '" 0 .... s:: 0 lJ1 a lJ1 a HI () Ii .... CD HI CD

Ii ~ U1 .... rt

tIl () I-' III Est2-B rt 0 0 CII .... HI

Ct> Est3-B 0 hj

PAG2-B ~ HI Ii Ii .... 0' .... () CD () rt rt rt .... ~ .... 0 CD 0 ~ t'rJ CD ~

0 ~ t:l

~ Est3-Pl Ii '" rt CD 1-3 "'" Pl PAG2- "0 ::T

CD CD HI Pl 0 rt a Ii CD Pl

tIl 0- X (") Est2-B+R134a .... 0 :x:J III rt e ~ Ct> CII

Est3-B+R134a CD HI

HlCt> CJ) a

0 + PAG2-B+R134a rt Ii CJ) CJ)

a () Pl .... Pl I-' CJ) rt

rt (")

t'rJ I-' CD 0 :x:J 0 Est2-F+R134a w Ii ~ Ct> 11 d(J

rt HlS Est3-F+R134a HI Pl 0 () + PAG2-F+R134a Ii rt

rt ::T CD

6.1 Research Summary

CHAPTER 6

CONCLUSIONS

An experimental facility has been developed to test the

,friction and wear characteristics of oil-refrigerant mixtures. A

high pressure tribometer has been designed to permit specimen

testing in pressurized refrigerant environment at temperatures

that can vary from -300 C to 1500 C and pressures of up to 1.724

MFa. The HPT has been outfitted with various apparatuses for

charging, purging, and sampling as well as permitting data

acquisition by a personal computer. In addition, procedures and

standards were developed to ensure repeatability of the test

results.

The critical contacts were found to be the counterformal,

area, and conformal contacts in the rolling piston, swash plate,

and reciprocating piston compressors, respectively. These

contacts along with their approximate operating conditions were

tested in the HPT. Two refrigerants and various mineral and

synthetic lubricants were evaluated in the HPT to determine

lubricative properties of the mixtures.

Baseline data for mineral oils and alkylbenzenes with CFC-12

have been completed. The results of these tests have shown that,

for all three contact types, CFC-12 has excellent lubricative

properties. This includes testing CFC-12 by itself as well as

with various lubricants. XPS analysis shows that metallic

chlorides are formed on the surfaces of the contact specimens.

95

These chloride films aid the lubrication process by protecting the

surfaces and, therefore, tend to decrease wear.

Comparative tests for HFC-134a and various esters and PAGs

have also been completed for the same contacts and conditions as

the baseline tests. In general, these tests show worse wear

results than the baseline tests,- especially when the refrigerant

is tested alone. XPS analysis shows no surface films produced on

the surfaces of the contact specimens, so there is no equivalent

reduction of wear. When HFC-134a is saturated into the

lubricants, the effective viscosity of the mixture is decreased

and, therefore, thinner oil films are generated and wear

increases. Although the addition of CFC-12 to the lubricant also

lowers viscosity, the formation of surface films counteracts this

effect.

The data for the coefficient of friction do not generally

correspond to their wear results. The coefficient of friction for

the counterformal contact shows only small variations from test to

test. The area contact results show somewhat greater variations

and do seem to correlate with some wear results. The friction and

wear results for the conformal contact are generally not similar.

Although the mineral oil-CFC-12 shows the best wear resistance,

its friction is among the worst. These observations suggest that

there is no clear relationship between friction and wear, and each

property must be evaluated separately.

6.2 Recommendations for Future Research

Although this research has provided excellent baseline data

for friction and wear in oil-refrigerant environments, some

96

improvements can be made. The installation of a high speed data

acquisition board would greatly enhance the quality of sampled

data during oscillatory testing. This board, along with

appropriate control software, could be used to control the dc

servo motors so that considerably more complicated loads and

motions could be tested. In addition, an in-line viscometer

should be added to the system to permit the viscosity of the oil­

refrigerant mixture to be measured. This would allow for a more

quantitative analysis of the thinning effect of refrigerant on oil

lubricative properties.

Future testing should focus on three possible avenues:

material development, surface roughness effects, and lubricant

screening. Material development includes both determining new

material pairs to be tested as well as investigating surface

coatings. The surface roughness of the contacts should be varied

to determine possible effects on friction and wear. Finally,

additional lubricants should be used to determine if they provide

better lubrication. In order for the lubricant screening to be

effective, however, lubricant manufacturers will have to divulge

additives used as well as provide more information about the

lubricant structure.

97

REFERENCES

[1] Little, J.L., "Viscosity of Lubricating Oil-Freon-22 Mixture," Refrigerating Engineering, Nov. 1952. p. 1191.

[2] Parmelle, H.M., "Viscosity of Refrigerant-Oil Mixture at Evaporator Conditions," ASHRAE Trans., Vol. 70, 1964, p. 173.

[3] Spauschus, H.O., "Vapor Pressure, Volume, and Miscibility Limits of R-22-0il Solution," ASHRAE Trans., Vol 70, 1964, p. 306.

[4] Spauschus, H.O., and L.M. Speaker, "A Review of Viscosity Data for Oil-Refrigerant Solutions," ASHRAE Trans., Vol. 93, No.2, 1987, p. 667.

[5] Pate, M.B., Van Gaalen, N.A. and Zoz, S.C., "The Measurement of Solubility and Viscosity of Oil/Refrigerant Mixtures at High Pressures and Temperatures: Test Facility and Initial Results for R-22/Naphthenic Oil Mixtures," ASHRAE Trans., Vol. 96, No.2, 1990, P 183.

[6] Pate, M.B., Van Gaalen, N.A. and Zoz, S.C., "The Solubility and Viscosity of HCFC-22 in Naphthenic Oil and in Alkylbenzene at High Pressures and Temperatures," ASHRAE Trans., Vol. 97, No.1, 1991, P 100.

[7] Pate, M.B., Van Gaalen, N.A. and Zoz, S.C., "The Solubility and Viscosity of Solutions of R-502 in Naphthenic Oil and in Alkylbenzene at High Pressures and Temperatures," ASHRAE Trans., Vol. 97, No.2, 1991, p 179.

[8] Grebner, Jeffery J., The Effects of Oil On the Thermodynamic Properties of Dichlorodifluoromethane (R-12) and Tetrafluoroethane (R-134a), M.S. Thesis University of Illinois, Jan. 1992.

[9] Thomas, R.H.P., Wu W-T, and Pham, H.T., "Solubility & Viscosity of R-134a Refrigerant/Lubricant Mixtures," ASHRAE Journal, Feb. 1991, p. 37.

[10] ASHRAE. 1990. ASHRAE Handbook -- Refrigeration, ASHRAE, 1990, p. 8.1-8.21.

[11] Huttenlocher, D.F., "Bench Scale Test Procedure for Hermetic Compressor Lubricants," ASHRAE Journal, June 1969, p. 85.

[12] Sundaresan, Sonny G., "Status Report on Polyalkylene Glycol Lubricants For Use With HFC-134a in Refrigeration Compressors," Procedings of the 1990 USNC/IIR-Purdue Refrigeration Conference, July 17-20 1990, p. 138.

98

[13] Sundaresan, S.G., and W.R. Finkenstadt, "Polyalkylene Glycol and Polyolester Lubricant Candidates for Use With HFC-134a in Refrigeration Compressors," ASHRAE Trans., 1992, Vol. 98. No 1, In Print.

[14] Sanvordenkerr, K.S., "Lubrication by Oil-Refrigerant Mixtures: Behavior in the Falex Tester," ASHRAE Trans., Vol. 90, No. 2B, 1984.

[15] Komatsuzaki, S., T. Tomobe, and Y. Homma, "Additive Effects on Lubricity and Thermal Stability of Refrigerant Oils," Lubrication Engineering, Journal of ASLE, Vol 43, No.1, Jan. 1987, p. 31.

[16] Komatsuzaki, S. and Y. Homma, "Antiseizure and Antiwear Properties of Lubricating Oils under Refrigerant Gas Environments," Lubrication Engineering, Journal of STLE, Vol. 47, No.3, 1991, p. 193.

[17] Komatsuzaki, S, Y. Homma, K. Kawashima, and Y. Itoh, "Polyalkylene Glycol as Lubricant for HFC-134a Compressors," Lubrication Engineering, Journal of STLE, Vol. 47, No. 12, 1991, p. 1018.

[18] Kosberg, Robert C., Jeffrey M. Stokes, and Margaret A. Schneller, Mathematical Modeling and Dynamic Analysis of The Rolling Piston Rotary Vane Compressor, Engineering Design Program, Dept. of General Engineering, University of Illinois, 1988.

[19] Young, Warren C., Roark's Formulas for Stress and Strain, New York: McGraw-Hill Book Company, 1989.

[20] ASHRAE, 1984. ANSI/ASHRAE 41.4-1984, Standard Procedure for experimentally determining the weight concentration of oil in single-phase solutions of oil in refrigerant. Atlanta: American Society of Heating, Refrigerating, and Air­~onditioning Engineers Inc.

99

APPENDIX A - Equivalent Cylinders

The distance between two rigid cylinders can be shown to be

equivalent to the distance between an equivalent rigid cylinder

and a flat plate. Figure A.l shows two cylinders, separated by a

gap height of ho at their closest point. An equivalent cylinder

separated from the flat plate is shown in Figure A.2. The

expression for the gap height, h, between two cylinders may be

written as:

(A. 1)

Using the Maclauren series expansion, cos ~ can be expressed by:

cos (A. 2)

If ~ is small, then substituting with x/R and eliminating all

higher order terms, the following can be obtained.

cos ~ = 1 - ~ 2R2

Substituting Eq. (A.3) into Eq. (A.l) yields:

(A.3)

If the final two terms are combined with a common denominator, the

expression for h reduces to:

h (A. 4)

100

The radius of the equivalent cylinder on a flat plate, for

counterformal contacts, is found to be:

R =

and for conformal contacts, the radius of the equivalent cylinder

on a flat plate is:

R =

-A

1 -I C

I 1 .x -- D

B

~ ____ ~ ____ ~~ x

(a) (b)

Figure A.l (a) Counterformal Contact Geometry (b) Conformal Contact Geometry

101

.-x

Figure A.2 Equivalent Cylinder on Flat Plate

102

APPENDIX B - Raw Data

Table B.1 Counterformal Contact Wear Data

Wear Scar Coefficient lest i Qils Befl:igel:aDt Ile~th (mm} Qf El:i!::tiQD

09RL Alkylbenz-F 0.165 0.090 10RL Alkylbenz-B 0.220 0.094 11RL Alkylbenz-B R12 0.140 0.094 12RL Alkylbenz-F R12 0.195 0.100 14RL Alkylbenz-F R12 0.lS5 0.101 15RL Alkylbenz-B R12 0.140 0.104 lSRL R12 0.195 0.105 19RL Alkylbenz-F 0.170 0.102 20RL Alkylbenz-B 0.230 0.104 22RL R12 0.200 0.107 23RL Est1-B 0.265 0.OS5 24RL Est1-F 0.165 0.102 27RL Est1-B 0.295 0.101 2SRL Est1-F 0.165 0.10S 30RL R134a 0.470 0.132 31RL R134a 0.505 0.12S 32RL Est1-B R134a 0.260 0.105 33RL Est1-B R134a 0.270 0.104 34RL Est1-F R134a 0.255 0.120 35RL Est1-F R134a 0.270 0.120 3SRL PAG2-B 0.235 0.076 39RL PAG2-B 0.235 O.OSO 40RL PAG2-B R134a 0.255 0.OS7 41RL PAG2-B R134a 0.250 0.OS7 42RL PAG2-F 0.140 0.102 43RL PAG2-F 0.150 0.101 44RL PAG2-F R134a 0.220 0.097 45RL PAG2-F R134a 0.240 0.105

103

Table B.2 Area Contact Wear Data

Wear Coefficient lest i Qils Bef;r:ige;r:aDt (mg} Qf E;r:i!:tiQD

06SP R12 132.21 0.182 07SP R12 149.44 0.203 16SP Min1 8.06 0.080 18SP Min1 8.99 0.081 19SP Min1 R12 1. 95 0.055 20SP Min1 R12 1.83 0.044 21SP PAG1-B 14.31 0.063 22SP PAG1-B 12.49 0.072 23SP PAG1-B 1.88 0.018 24SP PAG1-B 2.11 0.022 26SP R134a Seized 27SP PAG1-B R134a 37.65 0.042 28SP PAG1-B R134a 38.11 0.048 29SP R134a Seized 30SP PAG1-F R134a 22.08 0.065 31SP PAG1-F R134a 19.04 0.061 32SP Est2-B 1.36 0.060 33SP Est2-B 1. 52 0.067 34SP Est2-F 0.22 0.037 35SP Est2-F 0.17 0.035 36SP Est2-B R134a Seized 37SP Est2-B R134a Seized 38SP Est2-F R134a 0.33 0.057 39SP Est2-F R134a 0.26 0.065

104

Table B.3 Counterformal Contact Wear Data

Wear Scar Coefficient lest i Qils Befz:igez::ant OeJ;2tb. 0.1. in} Qf Ez::ic:tiQn

02RC Min2 1250 0.130 03RC Min2 1000 0.150 04RC Min2 R12 560 0.150 05RC Min2 R12 516 0.143 06RC R12 550A

07RC R12 460A 10RC Est2-B 4530 0.114 11RC Est2-B 4360 0.128 12RC Est2-F 5240 0.147 13RC Est2-F 5130 0.173 15RC Est2-B R134a 4410 0.081 16RC Est2-B R134a 4830 0.081 17RC Est2-F R134a 5000 0.110 18RC Est2-F R134a 5240 0.126 19RC Est3-B 8510 0.117 20RC Est3-B 8330 0.154 21RC Est3-F 8210 0.130 22RC Est3-F 8750 0.136 23RC Est3-B R134a 9000 0.106 24RC Est3-B R134a 9150 0.126 25RC Est3-F R134a 8540 0.146 26RC Est3-F R134a 8930 0.157 27RC PAG2-B 2760 0.124 28RC PAG2-B 2660 0.098 29RC PAG2-B R134a 3160 0.092 30RC PAG2-B R134a 3350 0.099 31RC PAG2-F 1400 0.110 32RC PAG2-F 1250 0.091 33RC PAG2-F R134a 2660 0.111 34RC PAG2-F R134a 2860 0.114

A These two tests show small wear scar depths, but the surface finish of the aluminum pad was very rough. The other tests had wear scars that were smooth.

105