CHAPTER 6:- SELECTION PF BEARING AND SPUR GERAR
1.State application of following bearing with suitable reasons:
Ans:- Application of bearings :……………1 mark for Each
i) Deep Groove Ball bearing :
Application: Electric Motor
Reason: Capacity to take heavily axial load with high rotational speed
ii) Taper roller bearing :
Application: axle housing of automobile
Reason: ability to take high radial load as well as thrust load
iii) Thrust collar bearing:
Application: Clutch of automobile
Reason: ability to combine radial & axial load with min. speed
iv) Needle roller bearing:
Application: Differential of automobile
Reason: takes less radial space. it has high radial load carrying capacity.
2. explain the selection procedure of bearing from manufacture’s catalogue.
( Correct Procedure OR Flow chart - 4 Marks)
1) Calculate radial and axial forces and determine dia. of shaft.
2) Select proper type of bearing.
3) Start with extra light series for given diagram go by trial of error method.
4) Find value of basic static capacity (co) of selected bearing from catalogue.
5) Calculate ratios Fa/VFr and Fa/Co.
6) Calculate values of radial and thrust factors.(X & Y) from catalogue.
7) For given application find value of load factor Ka from catalogue.
8) Calculate equivalent dynamic load using relation. Pe = (XVFr + YFA) Ka.
9) Decide expected life of bearing considering application. Express life in million revolutions L10.
10) Calculate required basic dynamic capacity for bearing by relation.
11) Check whether selected bearing has req. dynamic capacity, IF it not select the bearing of next series
and repeat procedure from step-4.
3. Differentiate between Sliding Contact and rolling contact bearing.
ANY 4 points: 4 Marks
4. State any four Disadvantages of rolling bearing as compared to Journal Bearing.
( Any Four , 4m)
Ans:- Disadvantages of rolling bearing as compared to Journal Bearing:( Any Four , 4m)
1) Initial cost is very high
2) Noisy in normal operation.
3) Shock capacity is less.
4) Finite life due to failure by fatigue.
5) Dirt & metal chips can enter the bearing & may lead it to failure.
6) Occupies greater diametral space compared to journal bearing.
5. State one application of each.
Application of bearings : ……1 mark for Each
i) Deep Groove Ball bearing : Electric Motor
ii) Taper roller bearing : axle housing of automobile
iii) Thrust collar bearing: Clutch of automobile
iv) Needle roller bearing: Differential of automobile
6. State any four advantages of ball bearings over plain journal bearings.
Ans:-
Advantages of ball bearings over plain journal bearing: …….Any Four 1 M Each
1) The ball bearings have a far smaller contact area and thus have a lower frictional drag coefficient.
2) Due to less frictional drag means better response and less power consumption.
3) The turbo can spool up much faster, which reduces turbo-lag and offers a major performance
advantage over journal bearing turbochargers at lower to mid turbocharger speeds.
4) The reduced contact area of the ball bearings means that it requires far less lubrication, allowing for
lower oil pressure feeds.
5) The ball bearing more reliable.
6) Less expensive.
7) Ball bearings generate less heat and require simple and inexpensive lubrication - by oil ring, oil mist, or
oil bath method.
7. List out & explain in brief any for properties of good journal bearing Material
1. Compressive strength. The maximum bearing pressure is considerably greater than the average
pressure obtained by dividing the load to the projected area. Therefore the bearing material should have
high compressive strength to withstand this maximum pressure so as to prevent extrusion or other
permanent deformation of the bearing.
2. Fatigue strength. The bearing material should have sufficient fatigue strength so that it can
withstand repeated loads without developing surface fatigue cracks. It is of major importance in
aircraft and automotive engines.
3. Conformability. It is the ability of the bearing material to accommodate shaft deflections
and bearing inaccuracies by plastic deformation (or creep) without excessive wear and heating.
4. Embed ability. It is the ability of bearing material to accommodate (or embed) small particles
of dust, grit etc., without scoring the material of the journal.
5. Bendability. Many high capacity bearings are made by bonding one or more thin layers of a
bearing material to a high strength steel shell. Thus, the strength of the bond i.e. bondability is an
important consideration in selecting bearing material.
6. Corrosion resistance. The bearing material should not corrode away under the action of lubricating
oil. This property is of particular importance in internal combustion engines where the same oil is used to
lubricate the cylinder walls and bearings. In the cylinder, the lubricating oil comes into contact with hot
cylinder walls and may oxidise and collect carbon deposits from
the walls.
7. Thermal conductivity. The bearing material should be of high thermal conductivity so as to permit the
rapid removal of the heat generated by friction.
8. Thermal expansion. The bearing material should be of low coefficient of thermal expansion, so that
when the bearing operates over a wide range of temperature, there is no undue change in the clearance.
8. Define the following term related to bearings ( 1 mark each)
(a) Bearing characteristics Number
The factor ZN / p is termed as bearing characteristic number and is a dimensionless number.
Z = Absolute viscosity of the lubricant, in kg / m-s,
N = Speed of the journal in r.p.m.,
p = Bearing pressure on the projected bearing area in N/mm,
= Load on the journal / l × d
d=diameter of the journal
l=length of the bearing
c=diametral clearance
The factor ZN/p helps to predict the performance of a bearing.
(b) Bearing modulus.
From Fig., we see that the minimum amount of friction occurs at value of ZN / p is known as bearing
modulus operated at this value of bearing modulus, because a slight decrease in speed or slight increase
in pressure will break the oil film and make the journal to operate with metal to metal contact.
9. A taper roller bearing has a dynamic load capacity of 26 KN. The desired life for 90% of bearing
is 8000 hr. And speed is 300 rpm. Calculate equivalent radial load that the bearing can carry.
...................4 marks
Ans:-
