50-Most frequently asked Questions Answers on Bearings

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
Its all about HP heaters

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
  • 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
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

  • 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.
  • 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
IBR forms & Acts

36-List down the Do’s and Don’ts for bearings handling and storing.
Following are the Do’s and Don’ts for bearing:
  • 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.
  • 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.

Questions answers & calculation on Boiler draught & Chimney

1-What do you mean by the term draught?

It is the pressure difference, which causes flow of gases to take places.

2-What is the function of draught in Boiler?
  • To supply the combustion air required for proper combustion
  • To evacuate the combustion products from the furnace or combustion chamber
  • To evacuate the combustion products to the atmosphere
  • So draught is the pressure difference between the inside the boiler and outside air. Inside the boiler pressure is due to combustion products (Flue gas) & outside pressure is due to fresh atmospheric air.
3-What are the different types of draughts used in power plant boilers?
  • There are mainly two types of draughts.
  • Natural draught & artificial draught
  • Natural draught is generally obtained by Chimney
  • Artificial draughts have main two types
  • Steam jet: It is again classified into Induced & forced draught
  • Mechanical draught: This is classified into Induced draught, forced draught & Natural draught

4-What do you mean by the Natural draught?

Natural draft is obtained by the use of Chimney. It produces the draft where the air and gas are forced through the fuel bed, furnace, and boiler passes.

5-How do you produce the mechanical induce draft?

It is produced by Induced draft fan

6-How do you produce the forced mechanical draft?

It is produced by Forced draught fan & secondary air fans.

7-How do you produce the balanced mechanical draft in Boilers?

Balanced draught is produced by ID & fans.

8-What are the advantages of mechanical draught?

  • Reduced chimney height
  • Easy control of combustion & evaporation
  • Improvement in the efficiency
  • Prevents smoke
  • Can consume low grade fuel
  • The fuel consumption per HP of mechanical draught boilers is 15% less than that of artificial boiler
  • Grate required for fuel burning is less

9-What do you mean by static draught?

The difference of pressure causing the flow of gases is known as static draught. Its value is very small
10-What do you mean by Static pressure?

It is the potential energy put into the system by the fan. It is given up to the friction in the ducts and at the duct inlet as it is converted to velocity pressure.

11-What do you mean by velocity pressure?

It is the pressure measured along the line of the flow that results from the air flowing through the duct.

12-What do you mean by total pressure?

It is the sum of static pressure & velocity pressure

13-How do you calculate the air flow in the duct?

Volumetric air flow Q m3/sec = Duct area in M2 X Velocity in m/sec

14-How much draft is maintained in the boiler furnace zone?

It is -3 to + 3 MMWC for proper combustion

15-What is the FD air draught loss in Air Pre heater?

It is 50 to 60 MMWC

16-What is FD air draught loss in SCAPH (Steam coil air pre heater)?

It is 20 to 30 MMWC

17-What is the draught loss in FD air in flow element?

It is around 10 to 20 MMWC

18-What is the draught loss of flue gas in Economizers?

Around 70-90 MMWC

19-What is the draught loss of flue gas in APH?

Around 50-60 MMWC

20-What is the draught loss of flue gas in ESP?

It is 20 to 30 MMWC

21-What is the draught measured at ID fan outlet?

Draught measured at 2 to 4 meter from ID fan discharge nozzle is up to +5 MMWC & as you go nearer to Chimney the draught goes into negative

22-How do you calculate the differential pressure of a ID fan?

Differential pressure of a ID fan =Pressure (draught) at suction side – Pressure at the discharge side

23-How do you calculate the differential pressure of a FD fan?

Differential pressure of a FD fan =Pressure (draught) at discharge side + Pressure at the suction side

24-What are the reasons for more positive draught in Boiler furnace?

It is due to:
  • More FD air flow
  • Sudden load drop on Boiler
  • Improper spreading of the fuel
  • Variation of fuel moisture

25-What are the reasons for more negative draught in Boiler furnace?

It is due to:
  • Higher ID fan speed & lower FD fan speed
  • Low load on Boiler
26-Why the FD fan size and power rating is less than ID fan?

Because FD fan is used to handle cold air & ID fan for hot gases (FD air + Fuel). Hot gases have lower density than cold air, hence size & capacity of the to expel the gases is more.

27-What are the reasons for draught losses?
  • Frictional resistance offered by the flues and gas passages to the flow of the flue gases
  • Loss near the bends in the gas flow circuit
  • Loss due to friction head in equipments like grate, economizer, super heater etc
  • Loss due to imparting velocity to the flue gases
28-Why do prefer artificial draught over natural draught?
  • To reduce chimney height
  • To get more draught
  • To reduce fuel consumption
29-What do you mean by the term Chimney?

Chimney is a vertical tubular structure built either of masonry, concrete or steel. Chimneys found application in power plants, textile plants, steel industries, Diesel plant etc. For smaller boilers up to 50 TPH steel chimneys are used and the boilers of capacity more than 50 TPH concrete chimneys are used

30-What are the functions of Chimney?
  • To vent the flue gases produced in the Boiler
  • To produce draught
  • To reduce the load on Boiler ID fans
  • To protect environment by venting the flue gas at suitable height
31-Explain the construction of concrete Chimney?

Chimney is normally cylindrical in construction. It is exposed to hot gases inside & cold air at outside. Inside of the chimney is lined with refractory bricks to protect concrete part from high temperature & erosion by flue gas. Chimney is specified based on its inside, outside diameters & height.

32-How does the draught produce in Chimney?

Draught in the chimney is due to density difference between the columns of the hot flue gases in chimney & cold air outside.

33-How do you specify the Chimney?

Chimney is specified based on its Outer diameter, inside diameter and height.

34-How do you calculate the draught produced in Chimney (Hw)?

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
H = Chimney height in meters
Ta = Atmospheric temperature in K
Tg = Flue gas temperature in K
Ma = Mass of air & Mass of flue gas = Ma+1

35-How do you calculate the velocity of the flue gas in the Chimney?

Velocity of flue gas V = √(2XgXH1)
H1 = Head in terms of gas column
Calculated as H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)

36-How do you calculate the mass of flue gas flowing through the Chimney?

Mg (kg/sec)= Density of gas (kg/m3) X Area of Chimney (m2) X Velocity of flue gas in Chimney (m/sec)

37-In which type of draught flue gas temperature leaving the Boiler is higher?

Flue gas temperature in Natural draught chimney is more as compared to artificial draught

38-Calculate the height of the Chimney required to produce a draught equivalent to 20 mmwc if the temperature of the FG  is 180 deg c and ambient temperature is 25 Deg C.And air required for complete combustion per kg of fuel is 7 kg

We have formula,
Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
Ta = 273 + 25 = 298 K
Tg = 273+180 = 453 K
Ma = 7 kg
Ma+1 = 7 + 1 =8 Kg
Hw = 20 mmwc
20 = 353 X H (1/298 – 1/453 (8/7))
H = 43.17 meter

39-Calculate the mass of flue gas flowing through the Chimney when the draught produced is equal to 18 mmwc. Temperature of flue gas is 250 Deg C and ambient temperature is equal to 30 deg c. The flue gas formed per kg of fuel burnt are 15 kg.Take diameter of chimney 2 meter

Hw = 18mmwc
Tg = 250 + 273 = 523 K
Ta = 30 + 273 = 303 K
Mass of flue gas Ma + 1 =15 kg
Chimney diameter D= 2 meters
Let us calculate Chimney height H
Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
18 = 353 X H ((1/303 – 1/523 (15/14))
H = 34.28 m
We have H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)
H1 = 353 X 34.28 (((14/15) X (523/303))-1)
H1 = 20.94 meter
Velocity of flue gas inside the chimney V = = √(2XgXH1) = √(2x9.81x20.94) = 20.2 m/sec
Mass of dry flue gas Mg = A X V X Density of gas
Density of gas at temperature 250 deg c = 273 X1.293 / 523  = 0.67 kg/m3
Area of chimney A = 3.142 X 22/4 = 3.142 M2
Mg = 3.142 X 20.2 X 0.67 = 42.52 kg/sec

39-Calculate the draught produced in Chimney of height 50 m, if density of the flue gas is 0.8 kg/m3 and atmospheric temperature is 25 deg c.

Density of atmospheric air = 273 X 1.293 / (273+25) = 1.18 kg/m3
Draught produced in Chimney is given by
Hw = H (Density of cold air-Density of flue gas) = 50 X (1.18-0.8) = 19 mmwc

40-How do you increase the draught produced in Chimney?
  • By increasing the height of Chimney
  • By increasing the flue gas temperature
41-How do you calculate the minimum height required for Chimney?

Minimum height of the chimney is calculated based on Sulphur dioxide emission.
H = 14 X Q0.3
Q = Sulphur emission rate in kg/hr

42-How do you calculate the draught pressure for maximum discharge?

It is given by P = 176.5 X H / Ta
Hw = Chimney height in meters
Ta = Absolute atmospheric temperature in Kelvin
Hw = Draught in mmwc

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