Bearing Introduction

Consider how many things in your life turn or revolve. skate wheels, electric

motors, car wheels, microwave turntables, even your PC has bearings in

it.The humble bearing makes many of today's machines a reality. Without

them we would not be able to make precision items on a massive scale and

things would wear out quickly due to excessive friction. This page is designed

to give you an idea of what bearings are, what they do and the formats they

come in.

Simply put...

All things roll and rotate better than they slide. If the wheel did not exist we

would be stuck with sliding things everywhere. Consequently little progress

in the world would be achieved. Sliding causes friction. Friction is caused by

two surfaces resisting movement between them. If however two surfaces can

contact each other by rolling then friction problems are significantly reduced.

Bearings reduce friction either by using hard smooth balls or rollers, and a

smooth inner and outer surface for the balls to roll against or by introducing a

low friction surface between the surfaces. These balls or rollers "bear" any

loads which they may be subjected to thus allowing the bearing to rotate

smoothly.

How bearings 'bear' load

Ball bearings are typically capable of dealing with two kinds of loading

condition; radial load and thrust load. Depending on the type of application

the bearing is used in, it may experience radial load only, thrust load only or a

combination of both. A classic example being the car wheel as shown below.

BEARINGS

Roller bearings like the one illustrated above are used in applications

like conveyer belt rollers, where they must hold heavy radial loads. In

these bearings, the roller is a cylinder, so the contact between the inner

and outer race is not a point but a line. This spreads the load out over a

larger area, allowing the bearing to handle much greater loads than a ball

bearing. However, this type of bearing is not designed to handle much

thrust loading.

Tapered roller bearings are used in car

hubs, where they are usually mounted in

pairs facing opposite directions so that they

can handle thrust in both directions.

Roller thrust bearings like

the one illustrated below

can support large thrust

loads. They are often

found in gear sets like car

transmissions between

gears, and between the

housing and the rotating

shafts. The helical gears

used in most

transmissions have

angled teeth -- this

causes a thrust load that

must be supported by a

bearing.

Ball thrust bearings like

the one shown below

are mostly used for

low-speed applications

and cannot handle

much radial load.

Barstools and Lazy

Susan turntables use

this type of bearing.

• Rotating elements (usually shafts) need to be supported

and friction must be minimized at the supports

• Mating parts in sliding contact - Introduction of

lubricant in between – Hydrodynamic or Hydrostatic

journal bearings

• Introduction of a rolling element between the shaft and

the support and lubricated – Rolling contact bearings

Need for bearings

Sliding or Rolling Bearings

Sliding bearings:

sliding friction

µ

Rolling bearings:

rolling friction µ

http://upload.wikimedia.org/wikipedia/commons/3/30/BallBearing.gif

Principles of Operation

Rolling Friction (Rolling Bearing)Roller/ball

Lubrication

Outer Ring

Inner Ring

Sliding Friction (Sleeve Bearing)

Sleeve

Lubrication

Circumferential

pressure profile

Hydrodynamic lift is

generated by fluid being

dragged into gap by

viscous shear

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Rolling Contact Bearings – Some characteristics

• Rolling contact bearings also called as Anti-friction

bearings

• But coefficient of friction is comparable with thick film

hydrodynamic journal bearings Average coefficient of

friction for rolling contact bearings ranges between

0.0010 to 0.0018 (catalogued values)

• Due to small contact area and large stresses, the

components of rolling contact bearings are made from

hard and superior materials compared to the shaft and

housing

• So the rolling contact bearing is made available as an

assembly – Outer ring, Inner ring, Rolling element,

Separator (Retainer or Cage), and in some cases – shield

or seal

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• Load transfer is through rolling elements – like balls and

rollers – Small value of coefficient of friction

• Starting friction is very less compared to sliding contact

bearings, suitable for intermittent operation (On/Off

cycles)

• Rolling bearings can take high overloads for short

duration

• Lubrication is simple – Bearings pre-packed with grease

are common in use

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• The bearings can take combination of radial and thrust

loads

• Advantageous, if limited space is available in axial

direction (Needs more space in radial direction)

• Noisy at high RPM

Types of Antifriction Bearings

Ball Bearings

Bearings Components

Seal Rolling elements Inner ring

Outer ring Cage Seal


Tapered Roller

Bearings保持架

•Components:Cone

= Inner ring

Cup= Outer ring

Tapered rollers

Cage = Space retainer


Roller and Ball Contact Area/Form

Rolling Elements

Cylindrical

Needle

Taper

Ball Spherical Asymmetrical

Types of Ball Bearings

Bearing Bearing Bearing Bearing Bearing

Bearing Bearing Bearing Bearing Bearing


• Single Row Deep Groove (Conrad) Bearing

Spherical balls roll in deep groove in both races

Space maintained by separators (retainers/cages)

Ball radius smaller than groove radius

Mostly take radial loads, some thrust load

Theoretical point contact (actually a small circular area), so high local

contact stress

Some permissible misalignment


• Double Row/Deep Groove Bearing

Add another row to increase load capabilities

Greater load capabilities than SRDG

Smaller space requirement than 2 SRDG

More misalignment problems

• Angular Contact Bearing

One side of race is higher

Can accommodate a larger thrust

Force resultant preferred between 15º and

40º


• Self-aligning Bearing


Spherically ground outer race allows for

alignment flexibility

Reduced load bearing capabilities, with

minimal thrust loading


•Thrust BearingLarge axial loading capabilities

Shaft speeds must be kept low because of

centrifugal forces

Types of Roller Bearings

A)Cylindrical Bearing

B) Spherical Bearing

C) Tapered Roller, Thrust

Bearing

D) Needle Bearing

E) Tapered Roller

(Bearing)

F) Steep-angle Tapered

Roller Bearing


Cylindrical (Straight)Bearing

Greater radial load capacity

Theoretical line contact (actually a rectangle), so lower contact stresses

Do not use for thrust - causes rubbing not rolling

外圈无挡边内圈无挡边内圈单挡边内圈单挡边

并带斜挡圈内圈单挡边

并带平挡圈

Needle Bearing

Roller with small diameter

Small d, makes them radially compact, good for large radial loads at high speeds

Thrust capabilities and misalignment poor



• Spherical Roller Bearing

One type of self-aligning

If misaligned - relative rotation of outer race to rollers and inner race

Load capability increased

• Thrust Roller Bearing

Only resist thrust

Several types: rollers, tapered rollers

• Tapered Roller Bearing

Combine advantages of straight roller and ball

type bearings

Can accommodate radial and axial loading

High load bearing capabilities


Selection Criteria

•Magnitude and proportion of axial and radial loads

•Misalignment between shaft axis and bearing

housing

•Deflection of shaft during operation

•Space availability in radial and axial direction

•Facility of lubrication

Basic types of rolling bearings are internationally

standardised. Within the scope of each type the bearings are

produced in various designs whose properties may differ

from the basic design. The following text gives brief

characteristics of individual types of rolling bearings; a

comparison of their utility properties can be found in the

table at the end of this document:

A. Deep groove ball bearings

•The cheapest and most commonly used bearing type; produced

in many designs and sizes

•The bearings are characterised by simple designs and cannot be

disassembled

•The bearings can handle operational conditions and their

maintenance is simple

•The bearings show relatively good load rating both in radial and

axial directions

•The bearings are suitable for high and very high speeds

•The bearings require good alignment of the journal and bearing

body; the permissible tilting angle is approx. 10'

•The bearings are delivered with shields or seals

B. Angular contact ball bearings

•Orbital paths are offset mutually in the direction of the bearing axis

•Designed to retain combined load with relatively large axial forces (axial load

of bearings increases with increasing contact angles)

•Single-row bearings enable the retention of axial forces only in one direction;

therefore these bearings are installed in pairs in opposite positions and as close

to each other as possible

•The bearings can be paired or double-row bearings can be used to retain

axial forces (pairs of bearings are delivered in one packing; as the bearings are

paired, bearings from different pairs must not be used together)

•Lower load rating than tapered roller bearings, however, may be used at

higher speeds

•Double-row bearings can retain tilting moments in the axial plane. However,

these types need perfect alignment and rigidity of seating and do not allow any

swinging of shafts

•Double-row bearings are also delivered with shields or seals

C. Self-aligning ball bearings

•Provided with two rows of ball bearings with spherical orbital

path on the outer ring

•Their design allows mutual tilting of rings (approx. 2-3°,

depending on the design)

•Suitable, above all, for seating, where the shaft shows deflections

or misalignment may occur

•Load rating of these bearings is lower than with single-row ball

bearings of the same size; not suitable for retaining larger axial

forces

•Produced usually with cylindrical or tapered holes

•Delivered also with seals

D. Cylindrical roller bearings

•Dismountable bearings, designed for transmissions of large radial loadings

(up to 60% higher load rating compared with ball bearings of the same size)

•High rigidity, therefore suitable for fluctuating and surge loads

•Bearings without cages (full complement) show higher loading capacities;

however, bearings with cages can be used for higher speeds

•Bearings with guiding collars on the outer and inner rings allow the retention

of higher axial forces. Other designs cannot retain any axial forces, however,

allow axial misalignment of the rings

•Cylindrical roller bearings require perfect alignment of the journal and

bearing body; the permissible maximum tilting angle is 3-4'

•Double-row cylindrical roller bearings are usually produced with both

cylindrical and tapered holes

E. Needle roller bearings

•In fact, needle bearings are cylindrical roller bearings with long slim rollers

(acc. to ISO, the roller length is min. 2.5 diameter)

•Show small installation height, high accuracy and rigidity

•Despite their low cross section, the bearings have a high load rating and are

therefore very suitable for seating where radial dimensions are limited

•Used, above all, for low speeds or swinging movements; also suitable for

fluctuating and surge loads

•Cannot retain any axial forces, however, allow axial misalignment of the rings

•One or both rings may be omitted to reduce the installation height; however,

seating surfaces on the shaft must then be hardened and machined carefully

•Show high requirements for alignment of the journal and bearing body, the

maximum permissible tilting angle is 3-4'


. Taper roller bearings

•Designed usually as dismountable, provided with tapered orbital paths on the

outer and inner rings with tapered rollers arranged in the paths

•High load rating; suitable especially for retaining simultaneously acting large

radial and axial forces

•Allow the retention of axial forces in one direction only; therefore installed in

pairs in opposite positions and as close to each other as possible

•In case of too high loading on the bearing or if axial forces must be retained

in both directions, the bearings can be paired (pairs of bearings are delivered

in one packing; as the bearings are paired, bearings from different pairs must

not be used together)

•Higher load rating than angular contact ball bearings, however, these types

are designed for lower speeds

•Seating surfaces for tapered roller bearings must be aligned; the permissible

tilting angle is 2-4'

G. Spherical roller bearings

•Two rows of spherical rollers with a common spherical path on the

outer ring

•Their design allows mutual tilting of rings (approx. 1.5-2.5°,

depending on the design)

•High load rating, retention of radial and simultaneously also axial

forces in both directions

•Suitable for large loads with misaligned seating and deflections of

shafts

•Produced usually with cylindrical and tapered holes; non-

dismountable


H. Toroidal roller bearings

•Single-row bearings with long, slightly spherical rolling elements; orbital

paths of the outer and inner rings are concave and symmetrical along the axis

running through the bearing centre

•The design combines the tilting ability of a spherical roller bearing

(permissible tilting angle approx. 0.5°) with the ability of axial balancing,

typical for rolling elements; they also show relatively small installation height

•High radial load rating even if the bearing must compensate a misalignment

or axial shifts

•Reduce vibrations in seating; axial vibrations of the shaft are not transferred

to the body

•Load rating of full complement bearings is significantly higher than bearings

with cages

•Produced with both cylindrical and tapered holes


. I Thrust ball bearings

•Designed only for retaining axial forces; radial forces cannot be retained

•Produced as single direction bearings or double direction bearings for

retaining axial forces in one or both directions respectively

•Not suitable for higher speeds; limit speeds are limited by adverse effects of

centrifugal forces

•These bearings must not work unloaded to prevent slipping of the balls

•Dismountable bearings; simple shapes and designs

•Correct functioning needs perpendicularity of the rings’ face surfaces to the

shaft axis

•The bearings designed with spherical seating surfaces can be used to

compensate for misalignment between the support surface in the housing and

the shaft

J Cylindrical roller thrust bearings

•Designed to retain large axial forces in one direction; radial forces

cannot be retained

•Form rigid seating; low sensitivity to surge loads

•Usable only at lower speeds; must not be operated unloaded to

prevent slipping of rolling elements

•Show simple shapes and designs; dismountable, require small

spaces in axial directions

•Correct functioning needs perpendicularity of the rings’ face

surfaces to the shaft axis

•Used especially where axial ball bearings cannot provide sufficient

load rating

K. Needle roller thrust bearings

•Designed to retain large axial loads in one direction; radial forces

cannot be retained

•Form rigid seating with minimum space requirements; low

sensitivity to surge loads

•Usable only at lower speeds; must not be operated unloaded to

prevent slipping of rolling elements

•Cages can be used in seating with needles independently or in

combinations with rings of various designs (all parts must be ordered

separately due to possibility to create various combinations)

•Correct functioning needs perpendicularity of the rings’ face

surfaces to the shaft axis

•Used especially where space is limited in axial directions

L. Spherical roller thrust bearings

•High axial load rating; can retain radial forces

•Suitable for retaining large axial forces even with relatively high

speeds

•Their design allows balancing of misalignment of the shaft and body

(permissible tilting of approx. 2-3° depends on the bearing design)

•The shaft ring and cage with spherical rollers form a non-

dismountable unit

•These types always need oil lubrication due to their inner

arrangement

ME 350

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Selection Criteria

Load and Life Considerations

• Static load rating

• Dynamic capacity

• Life rating

• Equivalent load under conditions of

varying loads

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Load Considerations

1. Static Load Capacity: Stribeck’s Equation

• Stribeck’s equation is based on contact stress

analysis at the point of contact between the

balls as rolling elements and the inner race

• Based on static condition

• Derivation – Book (e.g. Schaum’s series)

ME 350

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For Single Row Deep Groove Ball Bearing

2

05

KZDC

C0 – Static Capacity, Newton

K – Proportionality Constant = 61 x 106 N/m2

D – Dia. of each ball, metre,

Z – Number of balls

Stribeck’s Equation

Load Considerations

1. Static Load Capacity: Stribeck’s Equation

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2. Static Load Rating - AFBMA Standard

Co = Basic Static Load Rating – Defined as the static load on non-rotating bearing corresponding to a total permanent deformation of ball and race at the most heavily stressed contact of 0.0001 times the ball diameter

i = No. of balls in any one row; Z = No. of balls per row

D = Ball diameter, m; α = Nominal angle of contact = The nominal angle between the line of action of the ball load and a plane perpendicular the bearing axis

fo = A factor

= 3.34 x 106 N/m2 for self-aligning ball bearings

= 12.26 x 106 N/m2 for Radial and angular contact ball bearings

2 coso o

C f iZD

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3. Static Equivalent Load: AFBMA Standard

or

such that always

o o r o a o r

o r

P X F Y F P F

P F

Po = Equivalent Static Load, defined as that load which

will cause same total permanent deformation at

the most heavily stressed contact as that which

occurs at actual condition of loading

Fr = Radial load, Fa = Axial load

Values of Xo and Yo are available from table

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Values of Xo and Yo - calculation of for

Static Equivalent Load

Bearing Type

Single Row Bearings Double Row Bearings

Xo Yo Xo Yo

Radial Contact Groove

Ball Bearings0.6 0.5 0.6 0.5

Angular

Contact

Groove Ball

Bearings

α = 20o 0.5 0.42 1 0.84

α = 25o 0.5 0.38 1 0.76

α = 30o 0.5 0.33 1 0.66

α = 35o 0.5 0.29 1 0.58

α = 40o 0.5 0.26 1 0.52

Self Aligning Ball Bearing 0.5 0.22Cot α 1 0.44Cot α

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2. Static Equivalent Load

AFBMA Standard Contd. . .

See Notes for additional information regarding different

configurations. e.g. double row ball bearings and if two

angular contact ball bearings are mounted together

- Machine Design by Hall et.al., Ch. 22, Pg. 258

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3. Dynamic Load Rating: AFBMA

Standard

0.7 2 / 3 1.8cosC

C f i Z D

0.7 2 / 3 1.4cos 0.23C

C f i Z D

If ball diameter is less than 25.4 mm

If ball diameter is greater than 25.4 mm

C in Newton for D in metre

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C = Basic Dynamic Load Rating – Defined as the constant stationary

radial with stationary outer ring can endure for rating life of one

million revolution of inner ring,

i = No. of balls in any one row; Z = No. of balls per row, D = Ball

diameter in meter,

α = Nominal angle of contact = The nominal angle between the line of action

of the ball load and a plane perpendicular the bearing axis

fC = A factor calculated from the table given on next slide

Dynamic Load Rating: AFBMA Standard

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51

Multiply these values by f = 24.64 x 106 to get fC

fC /f

Single row radial contact , single

and double row angular contact,

groove ball bearings

Double row radial

contact groove ball

bearings

Self-aligning

ball bearings

0.05 0.476 0.451 0.176

0.07 0.521 0.494 0.203

0.10 0.566 0.537 0.238

0.14 0.600 0.568 0.282

0.20 0.611 0.579 0.323

0.40 0.492 0.467 0.412

m

DCos

d

dm = Pitch diameter of the ball set

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Equivalent Dynamic Load

r aP XVF YF

P = Equivalent Radial Load

X = A radial factor, Y = A thrust factor

V = A rotation factor, Fr = Radial load

Fa = Radial load

Factors X, Y and V are provided by the

manufacturers

V depends on whether inner ring is rotating or outer

one, Bearing type and its design, Ratio Fa/VFr and

Fa/C0

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Life Rating

L = No. of revolutions in millions that 90% of the

bearings of a group of apparently

similar bearings will complete or exceed

before first evidence of fatigue develops

C = Basic dynamic load rating

P = Equivalent radial load

3

CL

P

ME 350

54

Bearing Numbering system

1/5th of nominal bore,

for bores greater than 20 mm

X X X XX

Type of bearing

N – Cylindrical Roller

1 –

2 – Self Aligning Sph. Roller

3 – Taper Roller Bearing

4 –

5 – Thrust ball Bearing

6 – Deep Groove Ball Bearing

7 – Ang. Contact Ball Bearing

Width Code Dia. Code

Some more prefixing/post-fixing

Letters to indicate additional features

Bearing Type Selection


Criteria:

• Type of load: radial, thrust, combination of both, steady or shock

• Magnitude of load

• Rotation speed

• Shaft misalignment

• Diameter of both shaft and housing

• Packaging constraints

• Desired life

• Maintenance requirements


ME 350

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Bearing Mountings

ME 350

59

Bearing Mountings

ME 350

60

Installation of Rolling Bearings

Plummer Blocks / Bearing Housings

Seals

Use of Circlips and Bearing Caps

Duplexing – Angular contact ball bearings used in pair,

specially manufactured, increase in resistance to

misalignment and increased shaft stiffness

ME 350

61

Installation of Rolling Bearings

Preloading – For minimizing clearances, for increased

accuracy, use of adapter sleeve

Alignment – Care during assembly, Only two supports as

far as possible

Enclosure - Protection from dust, avoid loss of lubricant by

leakage

ME 350

62

Lubrication – Points to note

Methods of Lubrication

Higher rpm – Low viscosity oil

Smaller bearings – Suitable for larger rpm – increase in

linear speed and centrifugal force with radius

Documents

Bearing Introduction