ME3481 Mechanical Engineering Lab I
EXPERIMENT# D3
Evaluation of Friction and Wear
Characteristics
Lab Report
DEPARTMENT OD MECHANICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY MADRAS
CHENNAI-600036, INDIA.
July-Nov 2021
Contents
1 Introduction
3
2 Experiment objective
3
3 Theory
3
4 Archard wear equation
3
5 Equipment
3
6 Procedure
4
7 Results and calculations
6
8 Measurements
7
9 Report
8
10 Useful note for calculation
8
2
1
Introduction
Friction is defined as the resistance to relative motion between two bodies of contact.
The force of friction is independent of the area of surfaces in contact and is directly proportional to the applied load. Wear is defined as the phenomenon of the gradual removal,
damaging or displacement of material at solid surfaces that occurs as a result of relative
motion occurring between two contact surfaces. Wear that occurs in machinery components has the potential to cause degradation of the functional surface and ultimately loss
of functionality. Various factors, such as the type of loading, the nature of the motion
between the contact surfaces and temperature, determine the rate of wear.
2
Experiment objective
The objective of the experiment is to validate the law’s of friction and evaluate the characteristics of the given specimen. Also to determine the wear constant and understand
the effect of parameters on friction and wear characteristics.
3
Theory
Law’s of friction:
1. The friction force acts along the surface of contact and proportional to the normal
contact force, and hence is perpendicular to it.
2. Friction force is independent of the area of contact.
3. For reasonably large limits, kinetic friction is independent of velocity.
4
Archard wear equation
The Archard wear equation is a simple model used to sliding wear and is based on the
theory of asperity contact.
The wear rate is given by,
KW V
(1)
ψ=
H
where
ψ : wear rate
V : sliding velocity
K : wear constant W : total normal load
H : is the hardness of the softer of the
contacting surfaces
To verify the above laws and also find the wear constant, we need to measure the frictional
forces, wear rates at different velocities and loads using pin on disc tribometer.
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Equipment
6 millimeter dia aluminium pin, pin disc tribometer, friction force and wear monitor.
3
Figure 1: Pin on disc tribometer.
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Procedure
The experiment procedures are explained below,
1. Thoroughly clean the specimen pin and also the disc using fine grade emery paper.
2. Specimen pin is clamped to loading lever tip by hardened jaws.
3. Position the specimen pin over disc at a track diameter of 110 millimeter.
4. Set the controller with the required speed by using the rpm knob.
5. Now add weights slowly on the loading pan and obtain load values from the load
cell.
6. Repeat the above test with a few drops of lubricant between the specimen and wear
disc.
7. To obtain the wear rate, allow the pin to rotate over the disc and obtain the
displacement reading from the LVDT sensor.
Wear rate(ψ) =
(change in length of specimen) × (track area)
time
4
(2)
The track area is calculated as
track area =
π
((D + d)2 − (D − d)2 )
4
Where D is the track diameter and d is the pin specimen diameter.
Figure 2: Pin, disc and wear track area.
Friction force measurement
1. Friction force between pin and disc is measured by a load cell.
2. The load cell is a strain gauge load cell with high stability and corrosion resistance.
3. Tangential force on the specimen is transmitted to the load cell beam and the
deformation of the beam is sensed by the strain gauge.
4. Strain gauge is connected to a Wheatstone Bridge circuit which is calibrated with
strain conditioning and balancing.
Wear measurement
1. Linear wear rate is monitored using LVDT (Linear Variable Differential Transformer).
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2. LVDTs operate on the principle of a transformer. As shown in figure 3, an LVDT
consists of a coil assembly and a core. The coil assembly is typically mounted to
a stationary form, while the core is secured to the object whose position is being
measured. The coil assembly consists of three coils of wire wound on the hollow
form. A core of permeable material can slide freely through the center of the form.
The inner coil is the primary, which is excited by an AC source as shown. Magnetic
flux produced by the primary is coupled to the two secondary coils, inducing an
AC voltage in each coil.
Figure 3: General LVDT Assembly
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Results and calculations
One set of experimental data for dry friction test is provided.
Sliding speed (V) = 1 m/s
Track diameter (D) = 110 mm
Rotating speed of disc (N) = 6000 ×
V
= 173.7rpm
πD
Table 1: Normal load and frictional load in dry test.
Sl. No.
1
2
3
4
5
Normal Load (kg)
Calculations
Coefficient of friction(µ)
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Frictional force (N)
Now plot the normal reactions and frictional forces and fit a straight line through the
data points. The slope of the fitted line is the coefficient. We shall use least square
regression fit as it gives minimum error and the slope of the regression line is given by,
P
P P
n( xy) − ( x)( y)
P
P
m=
(3)
n( x2 ) − ( x)2
where, n = number of readings = 5.
Now find the value for the coefficient of friction from the fitted line.
Wear rate of the specimen
D = 110 mm (track diameter)
d = 8 mm (diameter of the specimen)
ψ = wear rate
Using Archard wear equation 1 find wear constant for the specimen,
K=
ψH
WV
(4)
where,
H = Hardness (N/m3 )
= 3Y, where Y is yield strength of softer material
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Measurements
Evaluation of friction characteristics
Material of test specimen
Diameter of the specimen (mm)
Density
Hardness value
Tract diameter (D) (mm)
Sliding speed (V) (m/s)
Rotating speed of disc (N) (rpm) :
Sl no.
1
2
3
4
5
:
:
:
:
:
:
:
Normal load (kg)
Evaluation of wear characteristics
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Dry friction force (N)
Speed of disc (N)(rpm)
Normal load (kg)
Initial weight
Final weight
:
:
:
:
Sl no.
9
Time
Wear
Report
1. Write your name, roll no, section, group no and experiment name.
2. Aim of the experiment.
3. Plot friction forces vs normal load (use table 1)
4. Find the friction coefficient.
5. Plot wear vs time.
6. Find wear constant.
7. Conclusion.
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Useful note for calculation
Step-1 Plot normal load and frictional force data, fit a straight line using equation 3, and
find the value for coefficient of friction. As we know,
F = µN
Step-2 For wear measurement, first calculate wear rate using equation 2.
Step-3 Now use equation 4 to find the value of wear constant.
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