design for a machine used for superfinishing the internal...

Design for a Machine used for Superfinishing the Internal Surfaces of

Gears that are Components of a Gearbox of a Truck

BADEA LEPADATESCU 1)

VALENTIN DITU 2)

ANISOR NEDELCU3)

1), 2), 3)

Transilvania University of Brasov, Faculty of Technological Engineering and Industrial

Management, B-dul Eroilor nr.29, Brasov

ROMANIA

[email protected], [email protected], [email protected]

Abstract:- In the paper is presented a machine tool that is used to accomplish the surface quality that is required for

the internal surfaces of gears that are components of a gearbox of a truck. As is known in a gearbox of a truck are many gears that are mounted on a needle bearings. For this reason, the internal surfaces of these gears need to have

a very high surface quality to work properly. The machine that is presented in the paper was designed to realize the entire requirement regarding the surface quality for all the types and sizes of gears that are in the truck gearbox.

Key-Words: - surface finish, reliability, gear, high production, load bearing capacity

1 Introduction In many instances, superior workpiece quality in regard to load bearing capacity, efficiency,

smoothness of running, operational safety, and last but not least, longevity, requires superior surface finishing of these workpieces. By logical extension,

this means continuous adaptation and optimization of

existing production processes. The superfinishing

process gives to workpieces as called “final touch”, being an economical machining operation and tipically the last and therefore the quality-

determining process, that makes it more important.

Some of the advantages of using superfinishing

process are presented below: - High flexibility;

- Low noise volume; - Low energy costs;

- Low initial investment;

- Low tool costs; - Less friction;

- Short processing time. The superfinishing process allows the targeted

influencing of the following parameters:

- Reduction of surface peak-to-valleys and increase of material contacta rea to over 90%, due to

the plateau like surface structures; - Improvment of roundness, creation of microgeometries with particular applications;

- Optimization of tribological properties through criss-crossinf finishing marks.

Superfinishing removes the amorphous structure of the material, or ‘soft skin”. This layer is usually 2 to

8 µm thick and is created by high temperature

generates by the grinding wheel. It enables the user

to achieve virtually any surface texture parameter, because only the roughness peaks are removed. The

geometry of the workpiece remains unchanged. The main advantage of this method is consistent

finish over the entire surface. Superfinish improves

the bearing ratio resulting in greatly improved wear resistance. Stock removal is typically in the order of

a few microns. The achievable surface finish quality is comparable to levels reached only by honing or

lapping. Superfinishing process is a very clean

process, because coolant or grinding emulsion is used of honing oils like Stone grinding. Recycling and

waste disposal are simplified. This process is suitable for any material that could

also be finished with geometrically undefined blades.

In addition to the most diverse metal alloys and grades, other materials can also be finished, such as

ceramics, plastics, non-ferrous metals as well as coatings, such as tungsten carbide , chromium, or

copper. After the process of superfinishing the workpiece

surface has a tightly controlled cross-hatch patter,

obtained by the interaction of three interrelated motions. These motion are the oscillation of the

stone, rotary movement of the workpiece and pressure of the abrasive tool on the workpiece.

During the superfinishing process, parts pass through

several distinc phases. When the abrasive tool makes initial contact with the part, dull grains fracture or

Mathematical Methods and Optimization Techniques in Engineering

ISBN: 978-960-474-339-1 171

pull away from the matrix to produce a new cutting surface. As the tool “self dresses”, relatively large

amounts of stock removal phase, abrasive grains begin to dull, while surface irregularities and geometry continue to improve. This results in a

cross-hatced surface free of irregularities and amorphous material.

2 Machine to superfinish the internal

surfaces of the gears In this paper is presented a machine to superfinish the internal surfaces of the gears which are components

of a gearbox of the truck. It is known that in the

gearbox of the truck are a lot of gears with different diameters that are used as the intermédiate gears,

without transmitting the torque, only to transmit the rotary motion to other shafts. For this reason, these gears have the internal surfaces smoothly and these

surfaces need to have a very high surface finish in the values of roughness between 0,4-0,2 µm Ra. The

previous process is grinding and the last machining process is superfinishing that gives the final

dimension and roughness for these internal surfaces. These gears from the gearbox have an internal diameters between Ø 35-110 mm and the width

between 20-50 mm. One of these gears is shown in Fig.1.

Fig.1 Gear with internal surface obtained after the process of superfinishing.

To can machining all these types of gears was

designed a machine that is presented in the sketch of Fig.2. In this figure the workpiece (4) is held in jaws (3) and has a rotational motion through the machine

gearbox (1). The superfinishing head has a oscillation movement with the amplitude of 1-3 mm and a

frequency of 1500cd/min. These oscillation movement is realized by the motor (8) and the mechanism (7).

It is used a plunge process when the workpiece is held and rotated, while the superfinishing multistone

(the superfinishing stones are between 3-6, according

with the internal diameter of the workpiece) makes

contact with the internal surfaces of the part. During the superfinishing process were used abrasive stones with silicone carbide, which is a combination of pure

white quartz, petroleum coke, sawdust and salt in an electric furnace.

To achieve the desired surface finish was used abrasive materials with fine grit sizes between 400 and 1200 on the FEPA scale.

The time of machining was between 30-40 seconds, depending on the size of part diameter.


ISBN: 978-960-474-339-1 172

Fig.2 Sketch of the superfinishing machine.

One of the important problem that must be solved was to design a device that can be hold and center the

workpiece in those three jaws. For this reason were realized for every size of workpiece a special device

as can be seen in Fig.3.

This device has as a body a ring (6) where are fixed by screws three small parts (4) with two balls (3) on

each. This device allows to center the part and to transmit the rotational movement that is needed

during the machining process.

Fig.3 Device to hold the workpiece in the superfinishing machine.


ISBN: 978-960-474-339-1 173

3 Factors that influence the workpiece

surface finish To obtain the surface finish for the workpiece is

needed to take into consideration the next main factors:

- the pressure of the abrasive stone over the worpiece

surface; - the grain size of the abrasive stone;

- the cutting speed during machining process. During the tests was used the superfinishing machine that is shown in Fig.4. This machine was build to

superfinish all the range of the gears from the gearbox of a truck and is used because we need to

know the optim process parameters for each of the workpiece type.

Fig.4 The superfinishing machine used in machining internal surfaces of the gears.

3.1 The influence of the abrasive stone

pressure on the part surface finish

In Fig.5 is shown the graph of the influence of the abrasive stone on the part surface finish when was

used a cutting speed of 15 m/min and an abrasive stone with grain size of 600. It can be seen that if is

used a pressure of the abrasive stone on the part

surface finish more than 25.10-4 Pa, is possible to obtain the better surface quality of the part in the

same processing time as with smaller presuure.

The dependence between part the surface finish and pressure of the abrasive stone on the part surface is

shown by equation(1): Ra = 0, 00404∗ t2 – 0,036634∗ t + 0,937818

(1)

with an error compared with the tests results of: Er = 1, 24441∗ 10-2 .


ISBN: 978-960-474-339-1 174

Fig.5 The influence of the abrasive stone pressure on the part surface finish.

3.2 The influence of the grain size on the part

surface finish

The dependence between the surface finish and the

abrasive grain size is shown in Fig.6. It was used two type of abrasive stones, with grain size of 600 and

1200, maintaining the cutting speed at a value of 15 m/min and a pressure of stone on the part surface of

25.10-4 Pa. It can be seen that in case of using abrsive

stone with grain size of 1200 the surface finish is with better quality for the same time of processing.

Usually, two types of grain size is often used in the superfinishing machines that can have

simultaneously semifinishing and finishing stations

during machining. Based on mathematical programs was obtained the

value of surface roughness Ra according with the garin size of abrasive stone:

Ra = 7,622178∗ (grain size)-0,558

(2)

with an error compared with the experimental values

of: Er = 7,8632∗ 10-3 .

Fig.6 The dependence between abrasive grain size and part surface finish.

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

10 200 30 40 50 60 70 80 90

gain size =400

v=20m/min

I-p=0,5x10 MPa3

Ra[ m]

t(s)

II-p=1,3x10 MPa3

III-p=2,5x10 MPa3

III

II

I

0 15 30 45 60 75

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

t(s)

RI-gain size 600II-gain size 600-1000

v=25m/min

p=2,5x10 MPa3

a[ m]

B

A

I

II

I: p=15.10-4 Pa

II: p=25.10-4 Pa

III: p=35.10-4

Pa

V=15 m/min

I: grain size 600

II: grain size 1200

V=15 m/min

P=25.10-4Pa


ISBN: 978-960-474-339-1 175

3.3 The influence of the cutting sped on the

part surface finish

Cutting speed has an important influence on the part surface finish specially on the machine tools with

two stations of machining: semifinishing and finishing. For the finishing station the cutting speed

is bigger which ensure obtaining a high surface quality of the part after processing. It was used a

pressure of abrasive stone on the part surface during

machining of 25.10-4 Pa. Fig.7 shows this dependence between the cutting speed and part

surface finish.

Fig.7 The dependence between the part surface finish and cutting speed.

The value of surface roughness depending on the cutting speed is giving by the equation:

Ra = 0,00026∗2

sF - 0,006408∗Fs + 0,474286

(3)

with an error compared with test results of Er = 2,5463∗ 10-3 .

4 Conclusions

The superfinishing machine that was presented is

usefully to machining process of all types of gears that are component of a gearbox of a truck.

The device of holding worpieces ensures a good centering and its simple construction gives flexibility

when is changing the oart to be machined. This operation is the last in the operations plan and ensure

a good roughness for internal surfaces of the gears.

By this process is reduced wear, energy consumption and leads to savings in service and maintenance

costs. Using application of artificial neuron network for modeling and analysis, we can predict and obtain the

best process parameters of superfinishing technology for each type of workpiece material and size

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0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

gain size =600

I-F=2daN/cm2

a[ m]

R

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III-F=1daN/cm2s

III

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I

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P=25.10-4 Pa

5 10 15 20 25 30 35 40 45 50 t[sec]

Ra

Fs

V


ISBN: 978-960-474-339-1 176

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ISBN: 978-960-474-339-1 177

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