1-Define the term Bearing
A bearing is a
machine element which reduces friction between two members, having relative
motion with respect to each other. It’s a device used to support and guide
rotating, oscillating or sliding shaft.
2-What are the functions of bearings?
- Supports & guides the rotating parts
- Reduces the friction & hence no or less wear & tear in moving parts
- Reduces the noise
- Reduces the lubricants consumption
- Reduces the power consumption of an equipment where it is being used
3-Where do you find the application of bearings?
Bearings found applications in almost all type of
industries from a small trimmer to big & big Machines.
4-What type of loads act on bearings?
Bearings can take in both axial, radial directions with
small misalignment
What are the main types of bearings used in industries?
Main types of bearings are Journal bearing & Rolling
contact bearings
5-Briefly explain the Journal bearings used in Industries
Journal or plain bearings
consist of a shaft or journal which rotates freely in a supporting metal sleeve
or shell. There are no rolling elements in these bearings.
It is a cylindrical
bush, add up of suitable material and containing properly machined ID and
ODs.It is a part of shaft or pin that rotates inside the bearing. They handle
high load and velocities because metal to metal contact is minimal due to oil
film. Operation is smoother. They require large supply of lubricating oil. For
high speed need forced cooling/lubrication there may be possibilities of failures
of bearings in start up and shutdown.
6-Where do you find the applications of Journal bearings?
Journal bearings used
High speed & high load machines.
In power plant
journal bearings found application in;
- Steam Turbine
- Boiler feed pumps
- Slat chain conveyors
7-What are the two types of Journal bearings?
- Oil lubricated: Used for high speed & high load carrying machines
- Grease lubricated journal bearings: Used for High load & low speed machines
8-What are the different materials used for the manufacturing of
Journal bearings?
Materials used are
Copper & its alloys,
generally Gun metal, bronze
White metal & Babbitt metal:
A tin base alloy containing 88% of tin, 8% of antimony and 4% of copper &
bismuth
9-Why do you select the copper based alloys for journal
bearings?
Because they have low
coefficient of friction
10-What is the maximum operating temperature of Journal
bearings?
Journal bearings can be
operated up to the temperature 100-105 Deg C
11-Classify the rolling contact bearings.
A. Ball Bearings:
- Deep groove ball bearing
- Angular contact ball bearings
- Self-aligning ball bearing
- Thrust ball bearing
B. Roller Bearings:
- Spherical roller bearing
- Cylindrical roller bearing
- Taper roller bearing
C.
Thrust Roller and Needle Bearings
12-What are the parts of rolling contact bearings?
Parts of Bearings:
- Outer race
- Inner race
- Cage
- Rolling elements
13-What is the material of composition of
rolling contact bearings?
It
is hardened Chromium steel
14-What is the hardness of bearing
materials/parts?
Hardness
is up to 55-60 HRC
15-How do you specify the Rolling contact bearings?
Rolling contact bearings are
specified as;
- Bearing bore size
- Bearing outer diameter
- Bearing width
- Bearing cage type & material
- Bearing clearance
16-What do you mean by Bearings Prefixes & Suffixes? & Explain some of bearing suffixes
- Prefixes are
mainly used to identify bearing rolling component.
- Suffixes identify special designs, variants & characteristics, which differ in some way from the original design or from the current basic design
Bearing suffix:
- Z: Steel shield on one side of bearing
- 2Z: Steel shields on both the sides of bearing
- ZZ: Rubber shields on both the sides of bearing
- K: Tapered bearing bore in the ratio of 1:12
- W33: Lubricating groove and 3 holes on outer race
- W33X: Lubricating groove and 6 holes on outer race
- C3: Normal bearing clearance (Clearance more than C2)
- C4: More bearing clearance (Clearance more than C3)
- J: Pressed steel cage
- F: Machined steel cage
- M/Y: Machined brass cage/Pressed brass cage
- NR: Bearing with snap ring
17-What is the maximum operating temperature of Rolling contact
bearings
It is 85 to 90 degree C for
grease lubricated bearings
18-What are the various loads considered in bearings designing?
Basic static loads, involves
mainly Dynamic (C) & Static loads (Co)
19-When the static load occurs on bearings?
Static load occurs on
following conditions
- When bearings are under load & stationary for long time
- When bearings rotate < 10 RPM
- When bearings are performing slow oscillating movements
20-How do you identify the ball bearings, taper roller bearings,
Spherical roller bearings & Angular contact bearings?
Bearings can be identified
based on their starting nos
- Ball bearings Nos start with 1 & 6
- Spherical roller bearings 2
- Angular contact bearings 7
- Taper roller bearings 3
- Cylindrical roller bearings start with NU
21-How do you nomenclature the bearing 6205 2Z
Nomenclature:
- 6 indicates the type of bearing
- 2 Indicates OD & Width sizes of the bearings
- 05 indicates the bore size of the bearing
- 2Z is a Suffix, bearing having metal shields on its both sides
22-What is the difference between the bearing No.6205 & 6305
6205 has lesser OD &
width than 6305 bearing, however both bearings have same bore
23-Write an example of bearing number for self aligning ball
bearings
1219, 2207 etc
24-Write an example of Spherical roller bearing?
22220 EK/C3, 23215 EK/W33
25-Write an example of Angular contact ball bearing
7205, 7305
26-Write an example for taper roller bearing
30305, 31205
Note:
Suffixes can be added as per specific requirement
27-Write an example for cylindrical roller bearing
NU 203 ECP, NU 2204
28-Calculate the bore diameter of Deep groove ball bearing 6208
C3
Bearing bore size is
calculated as 08 X 5 = 40 mm
Similarly for bearings 6315
2Z, Bore size = 15 X 5 = 75 mm
29-Calculate the shaft size for a bearing 22222 K/C3 having
tapered bore & sleeve thickness 5mm
In tapered bore spherical roller
bearings 22222 K/C3,
Shaft size = (Last two
digits) 22 X 5 -2 X Sleeve thickness =110 -2 X 5 = 100 mm
30-Why it is necessary to maintain minimum load on Bearings?
It has
been learned from experience that bearings require a minimum applied load to
insure traction for the rolling elements so they roll as the shaft starts to
rotate. If the balls or rollers do not roll, they will skid on the moving
raceway, wiping away the lubricating oil, and causing damage to the rolling element
O.D.s and raceway surfaces. This is called skidding
and the resultant damage is referred to as
smearing, which will shorten bearing life.
A good
approximation of the minimum load for each is:
Pmin = 0.02 x C
where:
Pmin = required
minimum equivalent load on the bearing, radial load for radial bearings and
thrust load for thrust bearings.
C = Bearing Dynamic
Capacity
31-Why the bearings operating at higher temperature are having
lower life?
Bearing Dynamic and Static
Capacities will reduce at high operating temperatures. The main reason is the
reduction of raceway and rolling element hardness at high temperatures.
32-What are the potential reasons for bearings failure?
Following are the main
reasons for bearing failure
- Bearings overloading
- Wrong or miss application of the bearings
- Misalignment
- Lack of lubrication or improper lubricant
- Contaminants in lubrication
- Over lubrication
- Operating bearing at higher temperature & vibration for long time
- More or less bearing clearance
- Jerk or fatigue loads
- Improper methods of bearing installation
- Bearing manufacturing defect
33-List
down the reasons for bearing seize
- Lack of lubrication
- Less clearance
- Over speed
- Contaminant in lubricant
34-List down the steps for installation/mounting of new bearing
- Ensure all the tools are in hand before installing the bearing.
- Clean the shaft and bearing.
- Measure and record the values for correct size, roundness and surface roughness.
- Unpack the bearing.
- For special purpose lubrication, clean the bearing with low pressure jet of kerosene.
- After cleaning apply the correct method of fitting Press fit, adapter sleeve, withdrawal sleeve and thermal expansion type
Note:
- For ball bearings, bearings need to apply the heat up to 90-100 Deg on induction bearing heater for easy installation.
- Do not use oil bath for bearing heater
- Do not cross the bearing or oil temperature above 100 deg C
- For fitting the bearing use gun metal or nylon rod & hammering should be done on inner race only.
35-List
down the procedure for bearings dismounting.
- Select the proper tool and person
- Decide the method of removal
- Use special wrenches, if required.
- Use puller, Induction heater and Oil injection method.
- Apply force to inner ring: If removing from shaft and to outer race if removing from housing.
Note:
- Apply load on outer race for bearing removing, do not apply load on inner race
- If required heating, plan to apply the heat on bearing only not on shaft. May wrap wetted cloth on shaft to avoid shaft expansion
36-List down the Do’s and Don’ts for
bearings handling and storing.
Following are the Do’s and Don’ts for
bearing:
DO’s
- Keep the bearings in protective packing till they are mounted.
- Cover the assembled bearings appropriately to avoid dust ingress.
- Store the bearings in dry area.
- Use tubes, Puller, Hydraulic nuts, and other tools for removal/fixing of the bearings.
- Use correct amount of specified lubrication at right time with correct procedure.
Don’ts
- Over lubrication is a silent killer for bearings. Do not over lubricate.
- Never allow welding at bearings that will damage the contact surfaces.
- Don’t let the bearing stand upright, store them flat on their sides.
- Do not use water to clean the bearings.
37-What do you mean by the term bearing
clearance?
Bearing clearance is the clearance or gap between outer race
or inner race and rolling elements. And in journal bearings it is the clearance
between shaft and bearing liners.
38-How do you calculate the bearing
clearance of bearing having bore diameter D?
As a thumb rule Bearing clearance
minimum = 0.00185 X D
Bearing
clearance maximum = 0.00254 X D
39- What do you mean by basic rating life
of a Bearing?
Basic rating life of a bearing is the life
that corresponds to 90% reliability using standard materials &
manufacturing quality at normal operating conditions.
It
is given by
L10
= (C/P)e
Where
C is bearing dynamic load in KN and P is dynamic equivalent load (KN) in axial
or radial directions.
e = Exponent (e = 3 for ball and 10/3
for roller bearings).
40-What is meant by bearing life and how do
you calculate it?
Bearing life is the number of
revolutions that 90% group of bearings can run without causing flaking due to
rolling fatigue.
Bearing life L10 = (C/P)e X
106/60 N
41-What factors do affect the bearings
life?
Following
factors affect on bearings life
- Operating speed
- Operating environment
- Load on bearing
- Type of lubricant
42-What is the misalignment tolerance for
ball bearings?
Deep groove ball bearing : 2 Minutes of arc
(1 Degree=60 minutes of arc)
Self aligning ball bearing
- 2.5 Deg-SR1200
- 3 Deg-SR1300
- 2.5 Deg-SR2200
- 1.5 Deg-SR 2200 2RS1
- 3 deg-SR 2300
- 1.5 Deg-SR 2300 2RS1
Angular contact ball bearing: No much misalignment
tolerance is allowed
43-What is the misalignment tolerance for
spherical roller bearings?
- Spherical roller bearing
- series 21300 - 1°
- series 22300 - 2°
- series 23100 - 1.5°
- series 24100 - 2.5°
44-What is the misalignment tolerance for
taper roller bearings?
Taper
roller bearing: 3 Minutes of arc
45-What is the misalignment tolerance for
thrust bearings?
Thrust
bearings: Thrust Roller bearing-2 to 2.5 degree & Cylindrical thrust
bearing-No misalignment
46-What are the operating, alarm & trip
level vibrations for bearings?
- Operating level vibrations:1 to 4 mm/sec
- Alarm level vibrations: 4 to 6 mm /sec
- Trip level Vibrations: > 6 mm/sec
47-How do
you calculate the grease required for re-lubrication of a Bearing?
Grease required for re-lubrication in grams is = Bearing OD X
Bearing width X 0.005
48-On what
factors bearing re-lubrication & frequency depends?
- Bearing operating speed
- Bearing operating environment
- Operating temperature
- Bearing operating vibrations
- Bearing operating hours
49-What causes bearing currents?
When
voltage is present on the motor shaft it can overcome the insulating effect of
the bearing lubrication film. This cause a current flow that results
effectively in electric discharge machining (EDM) of the bearing, causing
premature wear and ultimately, early failure.
50-where
& why do you use the insulated bearings?
Insulated bearings are used in Alternators & VFD motors. Insulated
bearings are fitted at Non drive end side of the machine.
The purpose of insulated
bearing for Motors with VFD drives or the Alternators
is to prevent the flow of circulating current in a closed circuit
through bearing and ground and consequently prevent
the bearing from damage.
An important new solution for motors in IEC frame
sizes 132 to 250 is to install a motor shaft grounding brush that
directs the current to the ground via the brush, rather than through the
bearing. This protects the motor itself and the complete installation.
The brush can be pre-installed on new motors by specifying a variant code. Or
it can be retro-fitted on site.
