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CHAPTER 6

FRICTION AND WEAR ANALYSIS FOR BUSHING

6.1 TEST RIG SETUP FOR THE FRICTION AND WEAR ANALYS IS Knowing the frictional coefficient is important for the determination of wear loss

and power loss conditions; an appropriate test rig is used to determine friction of floating

bush bearings. The frictional coefficient of bearings in lubricated conditions has been

examined in experiments. The force known as friction may be defined as the resistance

encountered by one body moving over another. This broad definition concludes two

important classes of relative motion: sliding and rolling. The ratio between this frictional

force and the normal load is known as the coefficient of friction and is usually denoted by

the symbol µ and mathematically it can be represented by,

µ = Ff /Fn

The magnitude of the frictional force is conveniently described by the value of the

coefficient of friction. The friction coefficient under relative motion and impeding motion

is defined by static and kinetic friction coefficients. These two types of friction

coefficients are conventionally defined as follows: µ = Ff /Fn and µf = Fk /Fn, where Ff

is the force just sufficient to prevent the relative motion between two bodies, Fk is the

forces needed to maintain relative motion between two bodies, and Fn is the force normal

to the interface between the sliding bodies [39].

As described elsewhere [40], six categories can be used to characterize friction testing devices: 1. Gravitation-based devices 2. Direct linear force measurement devices 3. Torque measurement devices 4. Tension-wrap devices 5. Oscillation-decrement devices 6. Indirect indications Gravitation-based devices have been proposed for at least 500 years, and some of them

are shown in the notebook sketches of Leonardo da Vinci. In some configurations, like

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flat-on-flat testing or pin-on-disk testing, the friction force can be measured directly with

mounting strain gauge on structure, a load cell or similar force sensor mounted in line

with the contact. In other systems, such as swept circular contacts (disk brakes, drum

brakes, rotation seals, etc.), friction coefficients are obtained from torque measurements

and component dimensions. Tension-wrap devices use the differences in tension resulting

between the ends of a sheet of material or a wire wrapped over a circular body.

6.2 PRINCIPLE & TEST RIG FOR BUSHING ANALYSIS

Figure 6.1 Schematic diagrams for the Experiment test rig

The basic principle used in the construction of test rig is shown in the figure 6.1

above. With the help of above principle we constructed test rig for finding out cp efficient

of friction for different bushings. The above schematic diagram helps us to understand

the frictional force offered by the system and its measurement. For measuring frictional

force transducer known as the load cell was used.

The new measuring system works on the following principle. The normal contact

of a bearing with a journal is achieved using the cable brake principle, while the friction

of the bearing is measured from the force difference at the two ends of the cable due to

friction torque during running.

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Figure: 6.2 Zoom view for test ring working principle

The measuring principle of friction in bearing can be illustrated in Figure 4.9 and

Figure 4.10. The journal of bearing is connected to the principle axis of the motor whose

rotation can be easily modulated. The two upper and lower parts of the bearing are

attached to the journal with a soft string (as the cable) wrapped around them. The upper

end of the string is fixed to a cantilever leaf spring, while the lower end is tied to a

standard weight. With the strain gauges stuck at the root of the leaf spring, the tensile

force of the upper end of the string can be precisely measured. The advantage of this

system lies in that the stretching of the string in a line provides only normal contact

between the bearing and the journal. This avoids the journal suffering from deflection,

which is of great importance for spindles.

The upper end of the string is fixed to a cantilever leaf spring and the lower is tied

and hook up to standard weight. The Load cell which mounted on the cantilever leaf

spring is calibrated with different weights at the string end before friction tests. The test

rig was operated nearly 25 to 30 minutes at no load to achieve balance conditions and

take into account the effect of friction from the bracket, shafts, motor etc.The string

forces at both the upper and lower ends are adjusted to be equal before measuring, which

determines the initial normal load. This can be achieved by adjusting the pressure gauge

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scale at 00 reading. Adjust the pointer on torque arm to match with the Stationery pointer

fitted on the frame. After this leaf spring end is fixed with load cell.

The force difference will occur while the journal rotates, which reflects the total

friction between the journal and the sleeve bearing. By analyzing the relation between

normal load and friction force, the friction coefficient of the bearing can be obtained.

Since the radius of the convex cylinder of the bearing wrapped with the string is much

larger than the radius of the concave surface of the bearing against the journal, there is no

possibility of slip taking place between the bearing and the string. For taking reading an

electronic display with mother board is used. It mainly comprising of Micro controller,

analogue to digital I.C., display , voltage regulated and operational amplifier IC Seven

segment LED were utilized for display, Whiston Bridge circuit etc. were utilized in the

construction of Mother board.

The test rig in this research work has the following parameters. The leaf spring

has a length of 600 mm, with a breadth of 205 mm and a thickness of 3 mm. The beam

type rectangular Load cell was mounted on the structure. Load cell having two strain

gauges on upper and two strain gauges on bottom of the load cell which are used to

constitute a Bridge circuit, with one serving as measuring sensor and the other as a

temperature-compensating sensor. The rotation of the motor can be modulated from 0 to

1500 rpm. Silk thread of a diameter of 3 mm that cannot elastically elongate is selected as

the cable (string) in the experiment.

Table No. 6.1 Technical specification for test rig for bushing

Sr. No. Description

Journal 31.85 mm diameter , 148 mm length

Bearing 38 mm outside diameter , 48.2 mm length with pressure gauge and arm

Loading

arrangement

Consisting of loading bracket 1 No. having 3.5 Kgs. Weight and dead

weight 5 kgs. And 2 kgs. And 1 kgs. 2 Nos. each Drive Motor ½ HP. 1500 rpm. D.C. shunts motor with armature voltage control to

speed variation. Range of speeds 100 to 1500 rpm.

Lubricant Recommended oil SAE 30

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6.3 ESTIMATION FOR COEFFICIENT OF FRICTION

The working principle of the testing system consists of two key aspects. One is to convert

tensile force of string into normal force acting on bearing using cable brake principle.

The other is to separate the bearing into two halves so that the normal pressing force can

be transferred to the interface of the bearing and the journal. The inner diameter of the

bearing is designed a little larger than the diameter of the journal to assure proper contact.

As shown in the zoom view in Figure 6.2 the tensile forces at two string ends are

set to the value of the weight F1 before the journal rotates. When the journal turns at a

constant speed, the tensile force of the upper string end will increase to F2 because of the

action of the friction force, while the tensile force of the lower end remains the same

value of F1 as the weight. From point A–B–C–D–A at the circumferential section of the

string, the tensile force varies from F1 to F2. The increase of tensile force at upper string

end from F1 to F2 due to rotating friction will result in further deflection of the cantilever

beam, and hence we can measure force with the help of Load cell mounted at the end of

cantilever. Little consideration will show us that due to the applied force through string,

there will be slight rotation angle of the bearing assembly bracket, but the value of the

rotation angle are quite small. For the given example test as shown below, the rotation

angle is about 2.50 to 30. Therefore, the contacts of the upper and lower bearing parts

against the journal can be regarded as remaining on the vertical line through the bearing

axis during testing.

6.4 EXPERIMENTAL SETUP FOR WEAR ANALYSIS FOR THE BU SHINGS

The tests on the adhesion wear has been done on three different Bushing material

specimens and its values are given in Table(6.6 , 6.7 & 6.8).With the help of

arrangement made in the wear testing machine it was possible to record reading on every

10 Minutes for the 60 Minutes. During the Test duration readings were recorded for the

Wear and Coefficient of friction. Wear results are recorded after every 10 minutes for 60

minutes cycle mentioned in Table 6.6 , 6.7 and 6.8.

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Figure 6.3 General view of test rig for bushing

With the knowledge of constant pressures and the surface velocities, the wear rate

on the gudgeon pin (also called wristpin) and connecting rod small end bush bearing

surfaces can be calculated using popular Archard law. Using the Archard law

instantaneously, and averaging over the cycle gives the cycle average wear rate. The wear

coefficient constant, which is an input to the problem, is not easily obtainable and it is the

function of several variables including lubricating conditions, sliding velocity and

lubricity of the used lubricant. In this experiment study, a single value for the wear

coefficient is used all three materials to make it more identical and realistic.

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Figure 6.4 Photographic images for Experimental setup for Bushing

The wear test rig photographic views are shown in the figure 4.11. These four

views give us complete set up details for the test rig which was fabricated and mofied at

institute. As shown in above figure 6.4, load cell was mounted on frame structure along

with rectangular section leaf spring. Also we can see the cantilever structure along with

off white string attached to hook for applying load. We can also see in the figure seven

segment red color display. We can also see the pressure gauge and lubricant oil supply

pipe. As stated earlier, in this experiment we used common lubricating oil i.e. SAE 30 oil

for all set. The same lubricant in all test setup would assist us to analyze the behavior of

all bushings from tribological point of view

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Figure 6.5 Photographic images for load bracket used to apply force on bushing

The above figure 6.5 described the load bracket and its mounting on the test rig. The leaf

spring one end whose cross section is rectangular in shape, utilized to fix string on it. We

can see that after the string is wound over load bracket strings other end is attached with

hook. The normal load is applied through hook by way of placing standard weights on it.

While referring schematic diagrams for the Experiment test rig (figure 6.1), we can see

that applying load on the bush bearing is the normal load and the force measured on leaf

spring with the help of load cell is frictional force. The load cell mounted on the frame

structure in cantilever beam shape would help us to measure precisely force developed in

the string end due to friction.

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Before the test and after the test the weight of the specimen was measured by a precise

electronic weighing machine A&D Japan makes shown in the figure 6.7 with an accuracy

of 0.0001g (Model MC-1000). Using the mass loss technique wear rate can be calculated.

The lubricant used in wet condition is SAE30.The readings are mentioned in the Table

6.2, 6.3 & 6.4.Generally wear rate is calculated per Kilometer distance traveled by the

bearing. Here we have taken in all cases speed as the 900 rpm, therefore we have not

opted for such calculation.

Figure 6.6 Three different investigated Bushings

Figure 6.7 Digital weighing machine MC- 1000 (1100 grams x 0.0001 g.)

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6.5 EXPERIMENTALINVESTIGATION FOR POWER LOSS AND C. O.F.

Table 6.2 Readings for C.O.F. and power loss – first test set

Load Applied: 20 N Lubricant: SAE30 OIL Speed: 900 rpm.

FIGURE 6.8 Relationship b/w Power Loss and Time at 20 N &900 rpm

Sr.No.

Time Duration (Minutes)

C.O.F. Brass

Power Loss Brass (Watt)

C.O.F. Gunmetal

Power Loss Gun

Metal (Watt)

C.O.F. Cast

Nylon

Power Loss Cast

Nylon (Watt)

Remarks

1 10 0.472 14.226 0.256 7.709 0.083 2.502

2 20 0.392 11.818 0.242 7.300 0.079 2.381

3 30 0.379 11.426 0.233 7.015 0.076 2.291

4 40 0.357 10.753 0.2205 6.648 0.072 2.170

5 50 0.355 10.697 0.218 6.567 0.071 2.140

6 60 0.353 10.641 0.217 6.566 0.071 2.140

Average 0.385 11.594 0.231 6.967 0.075 2.271

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TABLE 6.3 Readings for C.O.F. and power loss – second test set

Load Applied: 30 N Lubricant: SAE30 OIL Speed: 900 rpm

Sr.No.

Time Duration (Minutes)

C.O.F. Brass

Power Loss Brass (Watt)

C.O.F. Gunmetal

Power Loss Gun

Metal (Watt)

C.O.F. Cast

Nylon

Power Loss Cast

Nylon (Watt)

Remarks

1 10 0.426 19.279 0.295 13.337 0.112 5.064

2 20 0.372 16.842 0.269 12.174 0.095 4.295

3 30 0.350 15.839 0.254 11.501 0.087 3.934

4 40 0.339 15.330 0.238 10.767 0.079 3.572

5 50 0.325 14.692 0.230 10.401 0.075 3.391

6 60 0.311 14.046 0.227 10.278 0.073 3.301

Average 0.353 16.004 0.252 11.409 0.087 3.927

FIGURE 6.9 Relationship b/w Power Loss and Time at 30 N &900 rpm

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TABLE 6.4 Readings for C.O.F. and power loss – third test set

Third Test Set : Load Applied : 40 N Lubricant : SAE30 OIL Speed: 900 rpm

Sr.No.

Time Duration (Minutes)

C.O.F. Brass

Power Loss Brass (Watt)

C.O.F. Gunmetal

Power Loss Gun

Metal (Watt)

C.O.F. Cast

Nylon

Power Loss Cast

Nylon

Remarks

1 10 0.535 32.260 0.399 19.169 0.131 7.898

2 20 0.468 28.228 0.364 16.966 0.102 6.149

3 30 0.463 27.892 0.344 15.906 0.092 5.546

4 40 0.437 26.324 0.322 14.846 0.084 5.064

5 50 0.418 25.203 0.311 14.356 0.079 4.763

6 60 0.394 23.747 0.307 14.112 0.079 4.763

Average 0.452 27.276 0.3414 15.892 0.095 5.697

FIGURE 6.10 Relationship b/w Power Loss and Time at 40 N & 900 rpm

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6.6 GRAPHICAL REPRESENTATION FOR C.O.F. FOR BUSHING

Figure: 6.11 Representation of the C.O.F. Vs. Time (A, B & C)

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Table 6.5: Summary of the experimental investigation for the bushings

Phase Time Duration (Minutes)

Average

C.O.F. Brass

Power Loss Brass (Watt)

Average

C.O.F. Gunmetal

Power Loss Gun Metal (Watt)

Average C.O.F. Cast Nylon

Power Loss Cast Nylon (Watt)

Remarks

I 60 0.385 11.594 0.231 6.967 0.075 2.271

II 60. 0.354 16.005 0.252 11.409 0.087 3.926

III 60 0.452 27.276 0.341 15.892 0.095 5.697

6.7 RESULT DISCUSSION FOR BUSHING’S C.O.F AND POWER LOSS

The tests for the Co-efficient of frictions were done on three different material

specimens and its average values are given in Table (6.5).With the help of arrangement

made in the wear equipment it was possible to record reading on every 10 Minutes up to

the 60 Minutes. Readings were recorded for the Wear and Coefficient of friction.

From the figure 6.11 and above Table 6.2, 6.3 and 6.3 it is quite evident that the

co-efficient of friction is very low for Cast Nylon compared to Brass and Gunmetal and

therefore we have less power loss due to friction of the Cast Nylon. In the first phase

keeping 900 rpm and load 20 N, the C.O.F. of Brass is approximately ten times more

compared to Cast Nylon. Also the C.O.F. of Gun metal is nearly eight times more than

Cast Nylon. Remaining two Tables reading 6.3 and 6.4 revealed the same facts i.e. the

COF. of Cast Nylon is quite low with respect to Brass and Gunmetal.

The graphical representation between Power loss and Time are shown in the

figure 6.8, 6.9 and 6.10. It is quite clear that in all three figures, the power loss due to

friction for the Cast Nylon is lowest and highest for the brass. The summary for all three

setup readings are mentioned above in the Table 6.5. In the first setup, Brass C.O.F. is

five times and Gunmetal C.O.F. is three times higher compared to Cast Nylon. In the

second setup Brass C.O.F. is four times and Gunmetal C.O.F. is 2.9 times higher

compared to Cast Nylon. In the last set up Brass C.O.F. is 4.75 times higher and

Gunmetal C.O.F. is 3.5 times higher compared to Cast Nylon.

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TABLE 6.6 Wear loss readings for all three bushings after every 10 minutes up to 60 minutes – first set

(Wear results were recorded after every 10 minutes for 60 minutes cycle)

Figure 6.12 Relationship b/w Wear Loss and Time at 20 N & 900 rpm

Sr.No Time (Minutes)

Reduction in weight after test (mg)x10-1

Reduction in weight after test (mg)x10-1

Reduction in weight after test (mg)x10-1

Material

Time Min.

BRASS GUN METAL

CAST NYLON

1 10 45 26 16

2 20 25 17 11

3 30 22 9 8

4 40 21 7 6

5 50 15 4 4

6 60 5 1 0

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TABLE 6.7 Wear loss readings for all three bushings after every 10 minutes

up to 60 minutes second set

Figure 6.13 Relationship b/w Wear Loss and Time at 30 N & 900 rpm

Sr.No Time Min.

BRASS mg x 10-1

GUN METAL mg x 10-1

CAST NYLON mg x 10-1

1 10 53 29 19

2 20 35 19 12

3 30 29 10 10

4 40 18 10 7

5 50 10 6 4

6 60 7 2 1

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TABLE 6.8 Wear loss readings for all three bushings after every 10 minutes

up to 60 minutes third set

Figure 6.14 Relationship b/w Wear Loss and Time at 40 N & 900 rpm

Sr.No Time Min.

BRASS mg x 10-1

GUN METAL mg x 10-1

CAST NYLON mg x 10-1

1 10 56 33 21

2 20 39 21 15

3 30 28 17 11

4 40 22 12 9

5 50 18 9 5

6 60 8 5 2

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6.8 RESULT ANALYSIS FOR WEAR WITH TIME

The entire wear test for the three bushings was divided in three setups. The

readings for wear were recorded in the Table 6.6, 6.7 and 6.8. The graphical

representation between wear and time are represented in the figure 6.12, 6.13 and 6.14.

Referring to the readings recorded in the Table 6.6. 6.7 and 6.8, its graphical

representations, it is quite clear that in the beginning of the test wear was pretty high but

as the time goes on and establishing the lubricant film between the journal and bearing,

the wear rate is reduced. After nearly 60 minutes in the all cases the wear rate reaches to

nearly negligible. It is visible that the lowest wear rate was found in the case of Cast

Nylon bushing while highest was found in the case of Brass. It is known fact that the

brass is bit hard material compared to the Gunmetal and therefore wear rate of Brass is

higher even compared to the Gunmetal.

6.9 CHAPTER SUMMARY

With reference to readings recorded in Table 6.2, 6.3 and 6.4 and it’s graphical

representation between power loss versus time shown in the figures 6.8, 6.9 and 6.10, it is

quite clear that out of three bushings the Cast Nylon bushing has got less coefficient of

friction in all three different loading conditions. Also referring to the wear test readings

recorded in the Table 6.6, 6.7 and 6.8 and its graphical representation between wear

versus time shown in the figure 6.12, 6.13 and 6.14, we found least wear for the Cast

Nylon in all three loading conditions.

Therefore, if we opt for the Cast Nylon bearing in place of conventional bearing

made either from Brass or Gunmetal, Cast Nylon would have less power loss and less

temperature induced during the operation. In this investigation for all types of set up,

speed and lubricant oil used were the same to have more realistic result with respect to

I.C. engine.