Note Bearing PDF
Note Bearing PDF
Note Bearing PDF
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 1
My hopes and goal for
today
Have you understand more about:
1. The general construction of ball and roller (rolling
element) bearings.
2. How rolling element bearing lubrication works at
different relative component speeds
3. The critical importance of correct shaft and housing
dimensions
4. The contrast between the design failure mechanism
and the most common failure mechanism.
We’ll go through some of the slides very rapidly. My intent
with those is to give you a reference for future use.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 2
Some questions
1. What is the most common metallurgy for rolling
element bearings?
2. Where are ceramic components used? Why?
3. With rolling element bearings, why is a VERY SLIGHT
internal interference a good idea?
4. What is the mechanism that allows the typical
lubricating oil to separate the rolling elements from the
bearing rings?
5. How flat should a typical machine base be?
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 3
Rolling Element Bearings
We’ll discuss ball, roller, and needle bearings, not plain or
sleeve bearings.
Earliest rolling element bearing concept was in ancient times,
i.e., logs acting as rollers
Modern steel bearings became practical in the late 1880’s with
the ability to precisely control the element dimensions.
Most bearings are through hardened and, in the industrial
world, there are several standards for through hardened
bearing steels. In North America, it’s AISI 52100.
Some Timken tapered roller bearings and some special use
bearings are case hardened with numerous alloy options.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 4
A little history
Earliest Historical Evidence – around 300 BC when
Greek engineer Diades developed a roller supported
battering ram, maybe earlier on pyramids .
Oldest existing examples of bearings are in the Danish
National Museum and date from around 200 BC.
In the mid-18th century Dutch windmills had centerposts
that were supported by cast iron balls.
Later in the 18th century carriage wheel bearings were
developed. They were filling slot bearings, used cast
iron balls, and had no cages.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 5
More History
The major problem with these early applications was the
lack of control of the rolling element diameters. (In discussing
bearings, 0.0005” [12.7 microns] is a large overall dimension. With the
rolling elements, 0.00005” [1.3 microns] is a common tolerance.)
Balls
(behind cage)
Inner Ring
(race)
Cage
Balls, rollers, and rings
are usually made from Cages are made from a variety
hardened steels of materials and really depend on the
application
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 7
Some other types
Typical Deep Groove Common everyday “general duty”
bearing. Good radial and some
(Conrad) Ball Bearing thrust capacity
Applications where heavier thrust
Angular Contact Ball loads are expected such as pumps
Bearing (more thrust and fans . Must have some thrust
capacity) loading.
ACBB
Cylindrical Roller
Tapered Roller
Spherical Roller
0 2 4 6 8
BUT, with special processing these can be doubled!
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 10
Approximate Radial
Capacities Load
Axial
Load
Cylindrical Roller
Tapered Roller
Spherical Roller
0 2 4 0 2 4
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 11
Bearing Nomenclature
2 3
0 1
8, 9 4
6209 Bearing –
Last two numbers – 09 - indicates the bore diameter, i.e.,
09 x 5 = 45 mm = 1.77 in.
6 shows that it is a deep groove ball bearing
2 shows that it is a #2 diameter series with 0 as the lightest and
3 as the heaviest in common usage (Note that the bore
doesn’t change.)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 12
Bearing Nomenclature
Says 6313 C3
Look carefully for a suffix on
that bearing number!
1. Seals, shields, special
clearances, special cage,
etc.
2. With sealed and
shielded bearings it will
specify the lubricant.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 13
Internal Clearances for ball bearings
C3 and C4 suffix
Bore Range Radial Internal Clearance
Millimeters Metric (min/max in microns) Inch (min/max in 0.0001")
over up to Normal C3 C4 Normal C3 C4
10 18 3/18 11/25 18/33 1/7 4/10 7/13
18 24 5/20 13/28 20/36 2/8 5/11 8/14
24 30 5/20 13/28 23/41 2/8 5/11 9/16
30 40 6/20 15/33 28/46 2/8 6/13 11/18
40 50 6/23 18/36 30/51 2.5/9 7/14 12/20
50 65 8/28 23/43 38/61 3.5/11 9/17 15/24
65 80 10/30 25/51 46/71 4/12 10/20 18/28
80 100 12/36 30/58 53/84 4.5/14 12/23 21/33
100 120 15/41 36/66 61/97 6/16 14/26 24/38
120 140 18/48 41/81 71/114 7/19 16/32 28/45
140 160 18/53 46/91 81/130 7/21 18/36 32/51
160 180 20/61 53/102 91/147 8/24 21/40 36/58
180 200 25/71 63/117 107/163 10/28 25/46 42/64
C3 and C4 suffix
Bore Radial Internal Clearances
mm in Metric (microns) Inch (0.0001”)
Original C3 C4 Original C3 C4
12 1/2 3/18 11/25 18/33 1/7 4/10 7/13
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 15
One reason why those
dimensions are important
Life Effective Clearance - inches
-0.002 -0.001 0 +0.001 +0.002 +0.003
vs. 1.2
Internal Clearance Relative life compared to ideal
1.0
0.8
0.6
6310 Ball Bearing with
0.4 a 3,350 N (340 kgf)
radial load
0.2 (data from NSK text
Technical Report - 1992
0
-60 -40 -20 0 +20 +40 +60 +80
Effective Clearance (micrometers)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 16
Bearing bore and housing
dimensions are critical because of
the internal temperatures and
clearances
The specified shaft and housing fits depends on the application.
Both fits can’t be tight because the heat generated within the
bearing will cause dimensional changes and increased stresses.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 18
Shaft and Housing Fits
Allowances have to be made for thermal expansion.
On a typical shaft, with over 18” between centers,
one bearing will be fixed and the other will be floating
to allow for thermal growth. On shorter spans the
growth allowance depends on the internal clearance.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 21
Cage Materials
The common “original” cage material was stamped and
riveted steel. (In 1903 Robert Conrad patented the machinery
that enabled the stamped steel cage to be automatically assembled
and you’ll sometimes hear people talk about “Conrad bearings” )
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 23
Bearing Load Capacities
Bearing designs are based on speed and two properties, the
static load rating and the dynamic load rating.
The dynamic load rating (C) is the radial load that 90% of a group
of identical bearings, with rotating inner rings, can withstand for
1,000,000 revolutions without suffering from Hertzian fatigue.
(The actual load for rolling applications is essentially never more
than 30% of the static load rating.)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 24
What is Hertzian Fatigue? Operating
Stress (Load)
Hertzian fatigue
crack Rolling Travel
Element
Direction
Elastic Surface Deformation
from Hertzian Stresses
Original Ring
Surface
Bearing Ring
As the ball or roller passes over an area of the ring, the ring
alternately goes into compression, then tension. Meanwhile,
the same type of cyclic loading is happening inside the rolling
element. If the stresses are high enough, this will cause
Hertzian Fatigue cracking below the surface.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 25
Dynamic Load Rating
Fatigue Life L = [C/P]3 x 106 for a ball bearing
L = [C/P]3.33 x 106 for a roller bearing
where
L = fatigue life
C = basic dynamic load rating
P = the dynamic equivalent load
These are the Lundgren-Palmgren equations and
are the foundations for all ratings of rolling element
bearings and are also used in ISO 281.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 26
Calculating the Basic
Dynamic Rating
For a ball bearing:
C = bm fc (i cos α)0.7 Z0.667 DB1.8
where:
bm = rating factor depending on material quality
fc = shape and material coefficient
i = number or rows of elements
α = contact angle (0)
Z = rolling elements per row
DB = ball diameter (mm)
But it’s a lot easier to look it up in a
manufacturer’s catalog
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 27
Almost all bearing design
is based on the bearing
failing from
Hertzian Fatigue at
1,000,000 stress cycles
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 28
Design Load Capacities
C = the basic dynamic load rating of the bearing
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 29
Calculating the L10 life of that 6209
with a 700 lb (318 kg) load
Fatigue Life L = [C/P]3 for a ball bearing
L = [C/P]3.33 for a roller bearing
where, looking at just a radial load for a 6209 ball bearing:
L = fatigue life (in millions of revolutions)
C = basic dynamic load rating (Given as 7460 lbs/3465 kg))
P = the actual load (Given as 700 lbs/318 kg)
L10
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 31
Typical L10 lives
Domestic hand tools - 1500 hrs Domestic electric
motors - 1500 hrs
Industrial Motors – Large (40 hp and up) - 50,000 hrs,
Small – 20,000 hrs
Fans - General industrial - 15,000 hrs, Mine ventilation -
50,000 hrs
Industrial compressors - 40,000 hrs
Industrial reducers - 5,000 hrs (x service factor)
Pickup trucks - Light duty - 150,000 miles,
Heavy duty -220,000 miles
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 32
Going back to those design equations where
Fatigue Life L = [C/P]3 for a ball bearing
L = [C/P]3.33 for a roller bearing
and then looking at the forces
acting on the Bearing - the
Design Loads plus the
Parasitic Loads
we see that doubling the total
load cuts the life by a factor
of 8 to 10!
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 33
Sources of Bearing Loads
in a Typical Motor
Design Loads
1. Rotor Weight Parasitic Loads
2. Windage 1. Soft foot, i.e., base and
3. Allowable vibration and bore distortion
misalignment forces 2. Excessive forces from
misalignment and
Shaft Stator vibration
3. Axial thrust
4. Thermal changes
Rotor
Bearing
Bearing
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 34
Calculating the Actual L10 Life
Where
L10 = the load rating at 90%
reliability
L10 a1 = reliability adjustment factor
a2 = special properties adj. factor
a3 = operating conditions factor
Number of Stress Cycles (revolutions, mile, etc.)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 37
Lact = a1 x a2 x a3 x L10
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 40
As the bearing rotates…
Outer Rolling Elements - separated
Ring by Cage Bars
As the ball moves, the lubricant gets trapped between the ball and ring
and is subjected to pressures of more than 300,000 psi (2 GPa). When
This happens, the oil viscosity increases tremendously during “viscosity
transformation”, and separates the two elastically deformed steel pieces.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 42
Pressure and Viscosity
Pressure (MPa)
200 400 600 800 1000
100,000
ASME Research Committee on
Lubrication - Volume 11 (1963)
Absolute Viscosity (cP)
10,000
Honey
1000
100
10
Antifreeze
20 40 60 80 100 120 140
Pressure (1000 psi)
This chart was developed over 50 years ago. Some modern
research shows the viscosity goes up by another 10 5.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 43
Pressure and Viscosity
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 44
Film Thickness effect on
Bearing Life
λ (lambda) is the
1 2 4 10
Relative Film Thickness ( )
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 45
We know the lubricant
functions include separating
and cushioning between the
moving pieces,
and it is also important for
heat transfer.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 46
Temperature Effects on
Lubricants
•Increased temperature reduces the lubricant viscosity
and the actual film thickness.
•Increased temperature increases the rate of additive
deterioration (Arrhenius’ Rule).
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 47
Why do roller bearings need higher
viscosity lubricants than ball bearings?
The contact points of balls and rollers are very different and the line contact
of rollers requires a thicker lubricant film to ensure sufficient clearance.
Then, with roller thrust bearings, there is a huge difference in contact point
velocities as the radius becomes smaller.
Table 8.6 – Suggested minimum viscosities at operating
temperatures for typical industrial machinery
Minimum Viscosity (mm/sec) Equipment
13 Ball bearings, hydraulic
systems
23 Spherical, tapered, cylindrical
roller bearings, lightly loaded
gears
35 Roller thrust bearings
40 General spur and helical gears
70 Worm reducers
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 48
Low speed changes the lubricant
viscosity requirements
The general rule for ball bearings is that a minimum viscosity
of 13 cSt should be adequate, but at low speeds there isn’t
enough velocity to create the viscosity transformation – so a
heavier oil and/or EP additives are needed.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 50
… UNFORTUNATELY,
what often happens is that we
force them into position, feed
them contaminants, neglect good
fits, put water in their lubricants,
and Hertzian Fatigue never gets
involved.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 51
Both water (because it destroys
the lubricant film) and dirt
(because it increases the Hertzian
stresses) will have huge effects on
bearing life.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 52
In the next few slides, we’ll look at
a series of failed bearings, talk
some more about how and why
precise fits are critical, and see
how these bearings didn’t last
long enough to see Hertzian
fatigue
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 53
Conducting the Failure
Analysis
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 55
Step #1
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 56
Step #1
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 57
Step #1
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 59
Fretting
Fretting is a wear and corrosion mechanism that
results from tiny movements between the bearing and
the housing (or shaft) causing wear, followed by
oxidation of the wear particles.
How does this affect heat transfer?
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 60
Inner Ring Fit Clues
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 61
Step #2 Typical Normal Load Zones
at the
Ball Could be either outer ring rotating,
i.e., a trunnion bearing, or the load
Paths could be rotating with the inner
ring (unbalance).
(Traces)
Typical of axial loading
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 62
Typical Abnormal Ball Paths
Indicating Parasitic Loads
Element Force
Element
Travel
_
Radial Cross-Section of +
Side Cross-Section of
Ball Bearing Rings Inner Bearing Ring
Hertzian Spall
(Hertzian Fatigue Crack)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 68
Comparing Hertzian (Design)
Fatigue with Surface Fatigue
Rolling
Element
Hertzian Fatigue Ball
Damage
Travel
Surface Fatigue Original Ring
Damage Surface
Bearing Ring
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 70
Water Marks (Corrosion)
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 72
Electrical Fluting
This example came from a variable speed electric
motor. This is becoming a common failure mode.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 73
Arcing
Ball from
Damage a stopped
(from welding) machine
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 74
How to determine why it
failed prematurely:
•Look at the surrounding conditions
•Get the operating history
•Talk to the operators and maintenance people
•Look at the bearing components and chart every
symptom.
•From that, then determine the parasitic loads and
lubricant condition.
•Then look back at the human and latent roots to
that allowed the failure to happen, and change the
way you do things.
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 75
Some questions
1. What is the most common metallurgy for rolling
element bearings?
2. Where are ceramic components used? Why?
3. With rolling element bearings, why is a VERY SLIGHT
internal interference a good idea?
4. What is the mechanism that allows the typical
lubricating oil to separate the rolling elements from the
bearing rings?
5. How flat should a typical machine base be?
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 76
Thank you for listening!
Any questions about bearings you want to
talk about?
© 2019 by Neville W. Sachs, P.E. Mechanical and Materials Engineering and Failure Analysis 77