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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.