BITS, PILANI-DUBAI, ACADEMIC CITY, DUBAI

SECOND SEMESTER 2020-2021

11-03-21
DE G513 Tribology
TEST 1
DURATION: 20 MINUTES MAXIMUM MARKS: 40 WEIGHTAGE: 20%
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Answer all the questions

1. If the equation of a velocity profile of an oil flowing over a plate is u= 4y3/5, in which u is
the velocity in m/sec at a distance of y meters above the plate, determine the kinematic viscosity of
the oil in centistokes if the shear stress at a distance of 0.075 m is 0.6 Pascals. Given the density of
the oil is 850kg/m3. (8 marks)

2. A cubical block of 30cm edge and mass 40kg is allowed to slide down a plane inclined at
25ᵒ to the horizontal on which there is a thin film of oil of viscosity 15 cS and density 800 kg/m3.
What will be the sliding velocity of the block if the oil film thickness is estimated to be 0.025mm.
(8 marks)
3. Explain the different types & regions of lubrication mechanisms with the Stribeck curve and
also discuss the stable and unstable regions of the curve with proper explanation. (5 marks)

4. Explain the psedoplastic and thixotropic behavior of the lubricants with suitable graphs
(5 marks)

5. List out the components of an IC Engine which requires lubrication and explain how those
components are lubricated in pressure feed lubrication system. (6 marks)

6. The pressure distribution of a linear pad bearing using Ocvirk’s approximation is given by
the following expression, P = 106×(9.5×e10x – 3.5×e20x- 7.5) N/m2. The dimension of the bearing in
X-axis is 600mm and the dimension of the bearing in the Y-axis is 150mm. Find the load capacity
of the bearing in kN. (8 marks)

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 BITS PILANI DUBAI CAMPUS, ACADEMIC CITY, DUBAI
SECOND SEMESTER 2020-21

DE G513 TRIBOLOGY
TEST 2
DATE: 22-04-21

DURATION: 50 MINUTES MAXIMUM MARKS: 40 WEIGHTAGE: 20%

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1 An infinitely long linear pad bearing of length 40mm and width 200mm is operating under an
inlet film thickness of 20µm. Assuming a constant moving pad velocity of 12m/sec and the
viscosity of the oil used to be 7cP find out the following a) the maximum load capacity of the
bearing and its corresponding film thickness at the outlet of the bearing c) the intensity of
maximum pressure in MPa that can be attained with the maximum load capacity and its
location and d) the oil flow rate required in liters per hour to maintain the minimum coefficient

of friction for the bearing. 16

2. An infinitely long secant wedge bearing of 30mm×150mm is operating with an inlet film
thickness of h0= 25µm. Assuming a constant velocity of 10m/sec and viscosity of the oil used
to be 5cP find out the load capacity of the bearing and the intensity of the maximum pressure

in MPa and its location assuming the Reynolds boundary conditions. 12

3. A journal bearing of 160mm diameter and 40mm length is operating under a speed of 3200
rpm. The viscosity of the oil used is 9cS with an oil density of 850kg/m3 and the bearing
operating temperature is 80ᵒC. Assuming a clearance ratio of c/R= 0.0015 and assuming
optimum eccentricity ratio estimate the load capacity of the bearing taking the Sommerfeld
number from the graph, the Petroff Multiplier, the attitude angle and the oil flow rate required

in liters per hour to maintain the stability of the bearing. 12

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 BITS PILANI DUBAI CAMPUS, ACADEMIC CITY, DUBAI
SECOND SEMESTER 2020-21

DE G513 TRIBOLOGY
COMPREHENSIVE EXAMINATION
DATE: 27-05-21
DURATION: 75 MINUTES MAXIMUM MARKS: 80 WEIGHTAGE: 40%

PART A
1 a. What is ZDDP? Why it is added with lubricants.
b. What are monograde and multigrade lubricants. Explain their characteristics and how they are
rated.
c. What type of engines the mist type of lubrication is employed. What are the advantages of dry
sump lubrication systems. What is the use of pressure relief valve in this system.
d. What is pour point of a lubricant? What is wax pour point and viscosity pour point? What is the
requirement for the engine with respect to the pourpoint of the lubricants.
e. What do you mean by the oxidation stability of the lubricant and explain how it is determined.

15
2. a. Find the viscosity index of an oil that has a kinematic viscosity of 625 CS at 40◦C and 35.5 CS
at 100◦C.
b. An oil has a viscosity index of 95 and density 725kg/m3. What is the dynamic viscosity of the
oil at 15◦C. The kinematic viscosity of the oil at 40◦C is 388.8 CS for the reference oil having 100

viscosity index and the same kinematic viscosity at 100◦C as the oil investigated. 8
3. Find out the viscosity of the following oils in stokes and poise which were measured using

Redwoods Viscometer. 8
Oil Time for collecting Wt of the measuring jar Wt of the jar + 50ml oil
50ml. Redwood seconds. (W1) (W2)
A 55 secs 0.13 kg 0.21 kg
B 1350 secs 120 gms 450 gms

4. An infinitely long non-linear pad bearing of dimensions 40 mm × 160 mm is operating under the
film thickness as h= h0×e2x with an inlet film thickness of 20µm. Assume a constant moving pad
velocity of 10m/sec and the viscosity of the oil used to be 6cP. Calculate the maximum pressure in
MPa which occurs at a length of 27 mm. The constants for the integration of Reynolds equation are
C1 = -7.5*10-5 and C2 = 6.865*108.

9
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 BITS PILANI DUBAI CAMPUS, ACADEMIC CITY, DUBAI
SECOND SEMESTER 2020-21

DE G513 TRIBOLOGY
COMPREHENSIVE EXAMINATION
DATE: 27-05-21

DURATION: 75 MINUTES MAXIMUM MARKS: 40 WEIGHTAGE: 20%

PART B
1 A hydrostatic circular pad bearing operates under a minimum oil thickness of 60µm and
supports a vertical load of 80kN. The speed of rotation is 1800 rpm. The lubricant’s viscosity
at the operating temperature is 10cP, the recess oil pressure is 3mPa and the external oil
pressure outside the bearing is zero. Assuming D/D0=3.75 calculate the bearing dimensions
and the pumping power. Calculate the maximum pressure of the bearing and the pressure

acting at one third radius of the bearing. 8

2. A journal bearing of 140mm diameter and 35mm length is operating under a speed of 3500
rpm. The viscosity of the oil used is 11cS with an oil density of 850kg/m3 and the bearing
operating temperature is 80ᵒC. Assuming isothermal boundary conditions and a clearance ratio
of c/R= 0.0015 and the eccentricity between the centers of shaft and bush to be 0.011025mm
estimate the load capacity of the bearing using Sommerfeld number, the oil flow required to

maintain the bearing in liters per hour, the attitude angle and the frictional power loss. 10

3. A shaft transmitting 50kW at 125 rpm from the Gear G1 to the gear G2 and mounted on two
single row deep groove ball bearings B1 and B2 is shown in the figure. The gear tooth forces
are Pt1=15915 N, Pt2=9549 N Pr1 = 5793 N Pr2 = 3476 N. The diameter of the shaft at
the bearings B1 and B2 is 75mm. The load factor is 1.4 and the expected life of the bearings is
10000hours. Select a suitable ball bearings. If in addition to the loads an axial load of 2000N

is acting on the bearings select a suitable ball bearing in this case. 12

4. Find out the following contact parameters between a steel ball and an alloy plate. The radius of the
steel ball is 2.5 cm. Young’s modulus for the steel ball is 2.1*1011 N/m2. Young’s modulus for the
alloy plate may be taken as 3.1*1011 N/m2. The poisson’s ratio for the steel is 0.3. Calculate the
following a. contact area b. maximum and average contact pressures c. max deflection and the
maximum shear stress and the location. The density of the steel is 7800kg/m3. Calculate the minimum
thickness of the electrohydrodynamic film using Hamrock and Dowson’s equation if the ball is rolling

at a speed of 1000 rpm on the plate. 10

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