wear and friction

8/7/2019 Wear and Friction

http://slidepdf.com/reader/full/wear-and-friction 1/10

CHAPTER 4 FRICTION, WEAR AND LUBRICATIONIn machinery design material and energy conservation is becoming more and more important.We

are going to learn something basic about friction, wear and lubrication in this chapter.

Basic Concept

Friction——The twisting force that resists relative motion between two bodies in contact

Friction force——The resistance because of friction

wear——to exhaust or lessen the strength gradually by friction

lubrication——appending grease in the interface of friction as a film between solid surfaces to

reduce friction, heat and wear and make smooth or slippery.

Tribology——a study that deals with the design, friction, wear, and lubrication of interacting

surfaces in relative motion

Several statistical data :

1. 1/3~1/2 of the energy in the world is used up in friction.

2. 30% of the failure in machine elements is caused by wearing.

3. Applying the knowledge and technique of tribology in the production , we will get the economic

benefit which is about 2%~7% of GDP

4.1 FRICTIONFriction is a principle cause of energy dissipation and the improved friction control can guarantee

considerable savings.

Classifications of Friction:

internal friction : inside the material

external friction: on contact surface

1.Classified by trend of relative movement

static friction: sliding friction (eg: sliding bearing ,belt)

kinetic friction: rolling friction (eg: rolling bearing, cam)

2. Classified by friction condition (or is there any lubricant)

(1)Dry friction: no anti-friction material or safety film

(2)Boundary friction : two parts separated by boundary film

(3)Liquid friction: two parts separated by fluid film

(4)Mixed fiction: mixture of the three

(1)Dry friction

friction mechanism①

a. coulomb's law in 18 century F=fFn

Trait: simple, powerful utility, in addition to liquid friction the equation suit for every condition,

but it only used in the open surface.

eg： belt drive, flank of thread friction and working in interference, etc.

b. Molecule---Machinery theory

It is believed that the friction between two dry surfaces arises from two main sources, adhesion

and deformation ,and the most important is adhesion. mathematical expression:

F=Fa+Fm=(fa+fm)Fn

among this

fa——adhesion component of friction coefficient



fm——machinery component of friction

coefficient

Molecul——Machinery theory

satisfactorily explains the friction between metal in

macroscopy.

Conclusion: The value of friction is foreign to

contact area, is only directly proportional with the

load and friction coefficient.

.the factors that influence the friction②

coefficient.

1) The characteristic of paired material:

If the friction pair is made of the same metal or have great intersolubility, it will conglutination

easily, so fa↑，f↑

eg1: In the design of gear drive, the driver and the engaged wheel always are made of different

material ,or are made of the same material in different hardness.

eg2: In the design of sliding bearing and the worm drive, we should choose the anti-attrition and

hard wearing material.2) Is there any safety film or lubricant between solid surfaces

when the contacted surfaces suffer from pollution or oxidization, the safety film will be formed. It

can decrease the attraction between the molecule of two material, caused Fa↓， this film has low

strength of shear, so Fm↓

3）temperature

General speaking, the influence of ambient temperature of f is neglectful. As the temperature arise

it will decrease ,but once reach the high temperature the oil film will fracture, the material will

temper, the component of conglutination will increase.

4）relative sliding speed

When V<30m/s，the influence of f is very little

When V>30m/s V↑， f↓ (the rising temperature affect the machinery characters of exterior

material)

5）smooth finish

when smooth finish in common value, smooth finish↑ ， f↓

when smooth finish in high value, f ↑

(2)Boundary Friction

Classify the film based on the form principle of film:

physical adsorption film

chemiadsorption filmchemical reaction film

① physical adsorption film

Long chain hydrocarbon molecules of lubricant are adsorbed by the molecule of metal and then

formed the physical adsorption film on the surface of metal.When the temperature is high, the

boundary film will break down.The boundary film is suit for low speed and light load.

② chemiadsorption film

Under the action of chemical bond, the molecules of lubricant attach to the surface of the metal,

and then formed the chemiadsorption film.It is more steady than physical adsorption.

chemical reaction film③

Under the high temperature(150°~200°),the sulfur, chlorine and phosphorus will occur chemical

reaction with metal, formed compound , and then form the chemical reaction film on the interface of

oil and metal . Having low shear strength and high melting point, so it is steady and suit for high

speed and heavy load.The friction discipline of boundary lubrication is identical to the unlubricated



friction ,the only difference is the value of f is less.（falittle）

(3)Liquid frictionWhen friction is carry through the molecule of liquid F≈0.001~0.008,there is nearly no wearing

on the surface.

eg：the friction of dynamic or hydrostatic lubrication sliding bearing

(4)Mixed friction

Definition: when the friction condition is between the boundary friction and fluid friction, we

always use the thickness of the film to determine the friction condition.

thickness of the oil film

arithmetic average deviation of the two surface outlineλ =

Generally speaking：

λ<1 boundary friction

λ=1~3 mixed friction

λ 〉3 liquid friction

In the mechanical transmission, the majority condition of friction surface is mixed friction.

4.2 WEAR

Wear is a major cause of material wastage, so any reduction of wear can affect considerable

savings.

wear: The friction will cause the displacement and removal of surface material, so wear can causethe failure of the element, which is in 70 to 80 percent of the total failure.

The harm caused by wear:

influence the performance quality of machine① ；

eg: The wear of the gear tooth surface will destroy the involute tooth surface, and then cause the

impact and vibration.

The wear of the machinery mainshaft bearing will influence the machining precision of element.

impaired the efficiency of machine;②

eg: The wear of cylinder sleeve of the diesel engine’s will cause inadequate functioning of power.

reduced the reliability of machine:③

eg：snaggletooth，wheeltrack

The Wear Process

Three Phases of Machine Wear:

1 running-in



time: the initial stages of wear

characteristic: high rate of wear( the contact area is small, so the contact stress is large.)the

function of running-in: the material of high spots will be deformed, so the contact area becomes

sufficiently large to support the load. It is a process that gradually to load, gradually to speed.

attention to running-in：

a. load from light load to heavy load should be slow.

b. the lubricant should prevent from pollution; should replace after running-in.

② steady wear phase:

time：late stage of running-in, long-lasting; In the working stage of machine (stand for the useful

time)

characteristic: the speed of wear of slow-motion and reposeful

eg：the cylinder sleeve of locomotive and diesel engine replace every period

③ severe wear phase:

time：late stage of normal wear (late stage of life)

characteristic： rate of wear will speed up, so the failure of surface element will occur or the

clearance of kinematic pair will augment

Adhesive wear

Classification of Wear

Fatigue wear

Abrasive wear

Corrosive wear The type of

wear

Based on

mechanism

Based on exterior

appearance

Pitting

Gluing

Brush-burn



1. Adhesive wear

The asperity of friction surface after cold welding

the material will transfer to another surface when there is

relative sliding, that is called adhesive wear.

2. Abrasive wear

The wear is because of the dissociated rigid particles

or the peak of asperity

Influenced factor ：

(1)The harder the material is ,the higher the staying

quality is.

(2)The wearing capacity is increasing along with the

augment of the particles’ size.

(3)The wearing capacity is increasing along with the augment of the hardness of the paticles.

3.Fatigue wear(pitting)

It occurs when there is a slight vibratory movement among loaded surfaces in contact and which

manifests itself by the pitting of the surfaces and accumulation of oxidized debris. The debris

occupies a greater volume than that of the metaldestroyed and in a limited space.

Influenced factors：

(1)The harder of the element surface is, the safer of the fatalness of the endurance crack is;

(2)When the value of the roughness concentration is comparatively large, the probability of element

fatigue is increased;

(3)When the viscosity of oil is quite low, the probability of element fatigue is increased.

4. Fluid and cavitation erosion

It is caused by small drops of liquid including rigid particles can occur in steam turbines add fast



flying aircraft though the impact of water droplets causing plastic depressions on the surface.

eg: processing the exiguous curvilinear holes

5. Corrosive Wear

The acid and fuel from the air combusting to little inorganic acid (like sulphuric acid), and the

chemistry or electrochemical action of water together cause the lessening of the surface of material.

Summary：

Take note of design:

(1)Improve the rigidity, surface finish quality and the strength of material

(2)Choose the appropriate lubricant, and add refinements such as gland bush and filtering apparatus.

main index

1.Penetration Number: signing the

value of the essential resistance and

flow properties. The cone of G=1.5N, the

stuck depth in1.5S.

2. Drop Point: signing the fireresistant

capability of grease lubricant.In theregulated condition, the temperature at

which the first drop of the grease from the

hole of the standard cup.

lubrication method

1.Periodical manual application of

lubricant —low velocity, light load or

discontinuous running .

little Quantity of oil

nozzle tip, oil cup and oil gun

2.Oil bath, oil control ring and splash

lubrication— the important machine of

midvelocity, moderate duty.

Supply the oil without interruption, and

cool off circumferential velocity of

rotating part is1m/s ≤ v ≤ 10～

15m/s

The lubricant of the positive speed

gearbox

3. Mist lubrication —gear and bearing of high velocity and low load

gear ： circumferential velocity is v

＞ 5～15m/s

bearing ： dn ＞ 600000mm·r/min(d bearing bore

diameter, n working speed )

Form of oil mist lubrication

device： Thrust jet, straw and oil

regulator mist lubrication device

4. Forced feed lubrication —the

important element of high speed, heavy

load and high quantity of fuel oil

supply.



Headstock of the machine tool, internal-combustion engine forging and pressing equipment .

Form of forced feed system: lubricating press, gasoline tank, filter, cooler ,pressure control valve

(PCV) and fuel-flow control valve ,etc.

A－lubricating press, B－filter, C－cooler, D－holding valve , E—manometer, F－fuel-flow control

valve, G－ pressure regulating valve, T－oil sink

4.4 AIRPROOF

—prevent the leak of the

liquid, gas or lubricant, and

the enter in the lubricated

part of the dust and

moisture

selection of airproof

Select the type and the

structure of the economic

reasonable airproof is based

on the working conditionlike pressure, velocity and

working temperature(14-4).

Standard and specification of airproof is generally normative, so in the design we generally choose

the stan-

dard element. Forced feed system of positive speed gearbox

generally normative, so in the design we generally choose the standard element.

The airproof can be classified to silent seal and moving seal, basing on whether there is relative

movement on the airproofed surface.

1. Silent seal

Under clamping pressure of the bolt on the neat and glabrous junction surface, the surfaces are

press against each other to airproof.

In common, the clearance of the clamping pressure should be less than 5µm, and need lapped

finish.

Silent seal

2. Moving Seal

The stuffing produce the radial force to enclasp the shaft

Simple structure, it only suit for low speed (v ＜ 4～5m/s), and its main function in grease



lubrication is dustproof.

felt seal

O-ring seal J-ring seal U-ring seal

Obturating ring seal—obturating ring is made of elastic material. The function of seal is supported

by it own elasticity or pressing force of the spring.

Mechanical Seal (mechanical (end face) seal)

slit seal

The seal is supported by the contact between the graphite block and the rotatingdisc.Make use of

the throttling effect of the throttling groove to dustproof and caulk.

Labyrinth seal



axial type radial type

Grease lubrication or oil lubrication: the rotate speed of the shaft is high, but its working

temperature must under the drop point of the lubricative grease.

Making the clearance between the revolving part and still part as labyrinth, fill the clearance of

lubricant to strength the result of the seal.

Reliable seal, sealing element not easy to damage, but the structure is complex.

Axial type: considering the elongated of the shaft, the axial clearance choose the large value, 1．5

～2mm。Radial type: diametral clearance <0．1—0．2mm。

Centrifugal seal

Overcome the leakiness by the centrifugal force of liquid.

Thackeray washer seal——prevent the lubricating oil from marching off by the lubricating grease.

Slinger oil seal — oil lubrication

Lathing groove or installing oil slinger on the shaft to sling the oil in the radial direction, and

then reflow from the oil hole and the oil gathering reservoir.

The shaft must whirl in the orient

Combination seal



Clearance + labyrinth seal ——one of the combination seal, bringing each merit into fullplay may

improve the result of the seal.

wear and friction

Documents