INTERVIEW SHORT QUESTION ANSWERS-BEARINGS

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INTERVIEW SHORT QUESTION ANSWERS-

BEARINGS

Question answers are very helpful

in understanding. It increases lot

of clarity. It helps in using the concept

in real life applications.

  1. Define bearing pressure? When is bearing pressure used in design?

There is a convex surface (shaft) in contact of a concave surface (bearing).  The load is perpendicular to the convex and concave surfaces. Bearing pressure is bearing load divided by the PROJECTED AREA).
(i) We know, Bearing pressure = radial load / projected area. It is normally applied to connectors such as rivets, bolts, pins, cotters and shafts.
(ii) Projected area = hole diameter x hole length

Q. Advantages and disadvantages of rolling Contact

Bearing Over Sliding Bearing.

Advantages
(i) Less friction
(ii) Low starting torque
(iii) Compact as small in size
(iv) Can withstand shock loads
(v) Highly reliable
(vi) Less maintenance cost
(vii) Easy to install
(viii) More easy to clean
(ix) Accurateness of shaft alignment
(x) No lubrication required
Disadvantages
(i) Less resistance to shock loads
(ii) More noisy at high speeds
(iii) Initial cost is high.
(iv) Difficult design of bearing house
Mention the  its practical applications of bearing pressure.
PRACTICAL APPLICATIONS
  • Shaft and a bearing
  • bolt and bolt hole
  • Rivet and rivet hole
  • Cotter pin and the hole
  • Knuckle pin and its hole

Q. Explain the meaning of rated life

of a bearing. How is this different from the median life?

It is defined in terms of number of revolutions. That the bearing runs before the first evidence of fatigue occurs in the material of the ring or  the ball. The rated life of a bearing is defined as per ISO 281:2007.
It is represented by the symbol L10. It is given as
L10 = (C/P)p
Where L10 is basic rated life in million of revolutions at 90 % reliability
C is basic dynamic load rating(design life) in k N
P = Equivalent dynamic bearing load in k N
N= RPM
p=exponent of the life equation
For ball bearings, p=3
For roller bearings, p=10/3
NOTE: The basic dynamic load rating ‘C’
constant radial load, a bearing can support safely for I million cycles of the inner rotating ring.
(i) The static load rating is the load acting on a non rotating bearing.
(ii) Static rated load
Load which acts and causes maximum permanent deformation of 0.0001 D  without impairing the bearing operation.

LIFE OF BEARING

Life of a bearing can be defined in terms of the followings:
(i) Number of million revolutions
Or
(ii) number of working hours
OR
(iii) In number million of kilometers
Or
(iv) In number million oscillation cycles
Table given below can be used to convert life from one unit to another unit

TABLE: Conversion factors for life of a bearing

Basic Unit

                        Conversion factors

Millions of revolutions
Number of operating hours
 kilometers
traveled
Number of millions of
oscillating cycles
Million of RPM
1
106/60 n
D/103
180/2γ
Number of operating hours
60 n/106
1
60 n D/109
(180 x 60 n)/(2γ 106)
 Kilometers traveled
103/ D
60 n D/109
1
(180 x 103)/ 2γ D
Number of millions of oscillating cycles
2γ/180
(2γ106)/(180 x 60 n)
2γ D/(180 x 103)
1
n= RPM
D = rotating member diameter in ‘m’
γ = Oscillation amplitude (Angle of maximum deviation from the center position) in degrees
These conversion factors are applicable only when γ > 100

MEDIAN LIFE

Median life of a bearing is the average life of a bearing. It is also called the Mean Time Between Failure (MTBF). It is represented as L50.  L50 means Median Life. It means that life which 50% of the group of ball bearings will achieve or exceed the designed life. Median life is usually not greater than five times the rating life. The rating life is when 90 % of the group of ball bearings will achieve or exceed the designed life.