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
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
ME 350
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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
ME 350
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Rolling Contact Bearings – Some characteristics
• 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
ME 350
10
Rolling Contact Bearings – Some characteristics
• 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
Types of Antifriction Bearings
Tapered Roller
Bearings保持架
•Components:Cone
= Inner ring
Cup= Outer ring
Tapered rollers
Cage = Space retainer
Types of Antifriction Bearings
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
Types of Ball Bearings
• 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
Types of Ball Bearings
• 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
Types of Ball Bearings
• Self-aligning Bearing
Types of Ball Bearings
Spherically ground outer race allows for
alignment flexibility
Reduced load bearing capabilities, with
minimal thrust loading
Types of Ball Bearings
•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
Types of Roller Bearings
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
Types of Roller Bearings
Types of Roller Bearings
• 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
Types of Roller Bearings
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'
•Delivered also with seals
. 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
•Delivered also with seals
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
•Delivered also with seals
. 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
42
Selection Criteria
Load and Life Considerations
• Static load rating
• Dynamic capacity
• Life rating
• Equivalent load under conditions of
varying loads
ME 350
43
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
44
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
ME 350
45
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
ME 350
46
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
ME 350
47
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
ME 350
48
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
ME 350
49
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
ME 350
50
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
ME 350
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
ME 350
52
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
ME 350
53
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
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
Bearing Type Selection
ME 350
58
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