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

Power plant Maintenance Calculations

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

Why do vibrations occur in machines???

• Bearings overloading
• Wrong or miss application of the bearings
• Misalignment
• Lack of lubrication or improper lubricant
• Contaminants in lubrication

Read Why do bearings fail???

• 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

Know about valves.

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

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:

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.

Turbine oil and transformer oil standard testing parameters

Why do the vibrations occur in machines???

What do you mean by the term VIBRATION?

A periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed.

Why do the VIBRATIONS occur in machines?

Vibrations in the machine due to:

• Misalignment of the driver & driven equipments
• Bearing failure or more clearance in bearing
• Unbalance in machine
• Loose parts in machine
• Loose foundation bolts
• Lack of lubricant
• Low viscosity or high temperature of lubricants
• Bent shaft or more run out of shaft
• Over loading of machine
• Operation of machine in critical speed band
• Wrong design of machine or its parts
• Wrong installation of machine & accessories

 What are the forces responsible for vibrations in a machine?

Three basic types of forces which cause vibrations in a machine:

• Impact – loose parts, hammering in a piping system, rolling element in a bearing hitting a spall.

• Periodic – repetitive force such as unbalance or misalignment.

• Random – varies with time, for example, turbulence in piping, pump cavitations.

Each type of force produces a different reaction in the machine.

What are the effects of VIBRATION on machine?

• Bearing failure
• Coupling failure
• Machine parts failure
• Internal rubbing & seizing related problems
• More lubricants consumption
• More power consumption
• More noise
• Machine is required to run at lower loads & hence lesser machine efficiency
• Eventually machine will fail

 Questions & Answers on Bearings

If a machine has higher vibration in only vertical/horizontal/axial direction then what does it indicate?

• If there is more vertical vibration and other direction vibrations are minimum, then indicates looseness in machine components.

• If there is more horizontal vibration and other direction vibrations are minimum, then indicates unbalance of rotating part.

• If there is more axial vibration and other direction vibrations are minimum, then indicates misalignment.

Protections & Interlocks in power plants

Methods of Vibration measurement:

In power plants, generally equipments vibrations are measured in displacement & velocity

Let us discuss on the methods of vibrations measurement

1-Displacement amplitude:

Displacement amplitude measures the distance the vibrating part travels in one direction from a reference position during oscillations. (Note that the peak-to-peak displacement value, which measures total travel in both directions, is sometimes used.) This vibration measurement is important because vibrations with high displacement amplitude can cause machine components to exceed their yield point and experience catastrophic failure. Displacement measurements are typically used when vibration frequencies are low.

2-Velocity amplitude

Velocity amplitude measures the speed of the oscillation. This measurement is typically considered the industry standard for evaluating the condition of a machine based on its vibrations; because it takes into account both vibration frequency and displacement. (Recall that velocity is the rate of change of displacement.) In fact, ISO standards refer to velocity amplitude when specifying the severity of machine vibration. Velocity amplitude can be expressed in terms of peak value or, more often, in terms of the root mean square (RMS) value, which is an indicator of the vibration energy.

3-Acceleration amplitude

Acceleration amplitude is directly related to the force imparted by the vibration and is especially useful for assessing the likelihood of fracture for equipment that rotates at high speed. The high forces associated with acceleration can also cause lubrication breakdown, which can lead to excessive wear, heat, and premature failure. Acceleration is typically measured in “g,” or multiples of earth’s gravitational acceleration.

Units of vibrations measurement:

• Displacement: mm, microns, mils
• Velocity: mm/sec., inch/sec.
• Acceleration: mm/sec2.

How do you avoid machine VIBRATIONS?

Vibrations can be avoided by

• Running the machine/equipment at or below the rated load
• Following preventive maintenance regularly
• Following Condition based maintenance (CBM) regularly
• Replacing worn-out parts timely
• Implementing proper design & installation methods
• Following lubrication schedule timely with correct quantity & quality lubricants
• Following precision alignment (Rim & Face type) method

Relation between displacement, velocity & acceleration & their conversions

Velocity = (2pi X RPM/60) X Displacement (microns peak-peak)

Acceleration = (2pi X RPM/60) X Velocity (mm/sec. pk)

Acceleration = 4 pi X (RPM/60) X Displacement (microns pk-pk)

What is the critical speed of a machine? On what factors critical speed of a machine depends?

Critical speed of a shaft is the condition, where the number of natural vibrations or natural frequency equals the shaft speed in rpm.

At this speed rotating shaft becomes dynamically unstable and vibrations occur.

Critical speed depends on,

• Shaft speed
• Distance between the supports
• Type of support

Questions & ANswers for power plant fitters & Maintenance Engineers

Guidelines for selection & installation of machine foundations:

• For centrifugal machines, Mass of the foundation = 3 X Mass of the machine

• For reciprocating machines, Mass of the foundation = 5 X Mass of the machine

• The top of the foundation block should be at least 12” above the finished floor level

• The width of the foundation should be 1.25 to 1.5  X Vertical distance from the base to the machine centre.

Thumb rules for steam Turbine displacement vibrations measurement:

• Normal vibrations in microns = 2400 / √Turbine speed in RPM

• Alarm vibration in microns = 4500 / √Turbine speed in RPM

•  Trip vibrations in microns = 6600 / √Turbine speed in RPM

Why do the Boilers Explode???

Turbine oil and transformer oil standard testing parameters

18-reasons for turbine bearings vibrations

Tools tackles used in power plant maintenance

Tools & Tackles are the main resources for power plant maintenance..Thsese are classified into Hand tools,machine tools, lifting tools & measuring tools.

Power plant maintenance tools & tackles

Powerplants of capacity 10 MW to 100 MW need following tools for routine preventive, predictive & breakdown maintenance works.

A. Hand Tools:

• Spanners (Ring, flat, box, tubular and Allen types)

• Files (Rectangle, square, triangular, round, half round)

• Chisels 6", 12"

• Screw spanner 6", 12"

• Pipe wrench 12", 24"

• Plumb

• Hammer and mallets

• Pliers (Nose and circlip pliers)

• Shim cutter

• Torque wrench

• Pipe bender

• Hole and letter punches

• Screw drivers

• Line testers

• Bench vice

B. Machine Tools:

• Single phase Hand drill machine

• Hand angle grinding machine (AG-4, AG-5 and AG-7)

• Pipe cutting machine (Chop saw machine)

• Air blower

• Vacuum cleaner

• Hot gun

• Bearing heaters

Power plant Maintenance Calculations

C. Lifting/Pulling Tools:

• Chain blocks (1T, 2T, 3T, 5T and 10T)

• Dee shackles (1T to 10T)

• Eye bolts (0.5T to 10T)

• Polyester/Nylon Lifting belts (1T to 10T)

• Chain puller (1T)

• Pulley block (0.5T to 5T)

• Hydraulic jack (25MT)

• Screw jack (2T to 10T)

• Coupling puller

• Wire ropes (1/2" and 1")

• Slings (1/2", 1", 1.5")

D. Measuring Tool (Instruments):

• Vernier caliper ( 0–150 and 0–300 mm LC: 0.02 mm)

• Measuring tape (3 meter and 5 meter)

• Inside and outside micrometer screw gauges (0–25 mm and 50–200 mm LC: 0.01 mm)

• Dial gauge (LC: 0.01 mm)

• Inside and outside calipers

• Ultrasonic thickness checking machine

• Infra-red temperature gun

• Tachometer

• Multimeter

• Clamp meter

• Earth leakage tester

• Earth resistance checker

• Insulation resistance checker

• Pressure gauge calibrator

• Balance weigher

• Sound level meters

Boiler safety valves overhauling steps

Constructional materials & welding electrodes used in power plant

What are the various materials used in power plants construction???
1-Low Carbon Steel IS 2062, IS 1239: For structural steels like plates, angles, channels, beams, platform, walkways & LP steam lines etc.

2-High Carbon Steel SA 106 Gr. B/C, SA210 Gr, B/C, SA 516 Gr. 70: For Boiler pressure parts like water wall panels, headers, economiser coils, down comers, feed water lines, steam drum etc.

3-Alloy Steel SA 213 Gr. T11, T22, T91 and SA 335 P11/P22: Super heater coils and main steam pipe lines.

4-Cast Iron: Travelling grate materials, pulley, coupling etc.

5-Copper: Air conditioning cooling lines.

6-Brass: Surface condenser and oil cooler tubes.

7-Stain Less Steel SS 304, 316, 410 etc: Ejector tubes, surface condenser/oil cooler tubes, control valve stems & valve trim materials

8-Plastics: PVC pipe lines, valves, tanks etc.

9-Steam Turbine

• ESV: Cast alloy steel
• HP Casing: Cast alloy steel
• LP Casing: Carbon steel
• Rotor: Forged chromium, molybdenum and vanadium alloy steel
• Blades: Alloy steel
• Bearings: Liner Babbitt or white metals
• Labyrinth: Phosphorous, lead and aluminum

Power plant Maintenance Calculations

What welding electrodes are generally used in power plants various welding works???

Sl No.	Particular Material and Grade	Type of Welding	Welding Electrodes Used	Application
1	Low carbon steel IS 2062/1239	Arc	E 6013	General welding works like structures, non IBR pipe lines plates etc.
2	Carbon steels SA 106 Gr.B/C, SA 210 Gr.A/C, SA 516 Gr.70, SA 234, etc.	TIG Arc	ER 70 S2 E 7018	Boiler water wall panels, economizer coils, evaporator headers, process steam lines etc.
3	Low alloy steel SA 213/SA-335 Gr.T11/P11	TIG Arc	ER 80S B2 E 8018 B2	Primary super heater coils, headers, soot blower lines etc.
4	High alloy steel SA 213/SA-335 Gr.T22/P22 and T91/P91	TIG Arc	ER 90S B3/90S B9 E 9018 B3	Secondary and radiant super heater coils, main steam line etc.