This document provides information about antifriction bearings and bearing lubrication. It discusses how rolling bearings use balls or rolling elements to minimize friction between bearing rings. It also describes how grease and oil are used as lubricants, with grease being preferred for applications where continuous oil supply is not possible. Grease provides lubrication, prevents corrosion and seals out dirt. The document discusses EHD lubrication principles for rolling bearings and explains how grease properties and NLGI grades are selected based on the application and operating conditions.

1. Antifriction bearings

2. All bearings that transfer loads via rolling elements are named
rolling bearings. They use balls or other rolling elements, located
between bearing rings, to minimize friction. The rolling elements
are separated and held in position by "cages".
The fundamental purpose of a bearing is to transmit a load
between a stationary part of a machine (most likely a housing) and
a rotating part of a machine (most likely a shaft) with a minimum
of resistance.
Depending on the type of rolling element that are used, the rolling
bearing are classified into two groups:
• Ball Bearing – Load is carried over a very small surface – Point
Contact
• Roller Bearing – Load is carried over a bigger surface – Line
Contact
Rolling or antifriction bearings

3. Bearings are further classified by the specific shape of their
rolling elements:
• Ball, Cylindrical, Spherical, or Tapered.
Bearings are also classified by function depending on the
direction of the applied load.
• Radial, Thrust or Combined
Bearing classification

4. Rating life:
Rating life is defined as the life of a group of apparently
identical ball or roller bearings, in number of revolutions or
hours, rotating at a given speed, so that 90% of the bearings
will complete or exceed before any indication of failure occur.
Suppose we consider 100 apparently identical bearings. All the
100 bearings are put onto a shaft rotating at a given speed
while it is also acted upon by a load. After some time, one after
another, failure of bearings will be observed. When in this
process, the tenth bearing fails, then the number of revolutions
or hours lapsed is recorded. These figures recorded give the
rating life of the bearings or simply L10 life (10 % failure).
Similarly, L50 means, 50 % of the bearings are operational. It is
known as median life.

6. Components of Bearings
Outer Ring :
The outer ring is mounted in the housing of the machine and in
most cases it does not rotate. The raceway against which the rolling
elements run have different forms – sphered, cylindrical, tapered –
depending on the type of rolling elements.
Cage :
The cage separates the rolling elements preventing contact between
them during operation, which would cause poor lubrication
conditions. With many bearings types the cage holds the bearing
together during handling.
Inner Ring :
The inner ring is mounted on the shaft of the machine and is in most
cases the rotating part. The bore can be cylindrical or tapered. The
raceways against which the rolling elements run have different forms -
sphered, cylindrical, tapered – depending on the type of rolling
elements.

7. Rolling Elements
The rolling elements can be balls, cylindrical rollers, spherical
rollers, tapered rollers or needle rollers. They rotate against the
inner and outer ring raceways and transmit the load acting on the
bearing via small surface contacts separated by a thin lubrication
film.
Seal
Seals are essential for a long and reliable life of the bearing. They
protect the bearing from contamination and bearings with
integral seals are becoming increasingly popular.
Guide Ring
Guide rings are used in spherical roller bearings. The function of
the guide rings is to guide the rollers in the bearings so that they
rotate parallel to the shaft and to distribute the load evenly. The
quality demands for guide rings are extremely high and even the
slightest ovality is totally unacceptable
Components of Bearings

8. Cylindrical Roller Bearing

9. Deep
Groove
Ball Bearing
Angular
Contact
Ball Bearing
Double Row
Deep Groove
Ball Bearing
Self-Aligning
Ball Bearing
Types of ball bearing

10. Cylindrical
Roller
Bearing
Needle
Roller
Bearing
Taper
Roller
Bearing
Spherical
Roller
Bearing
Types of roller bearing

11. Thrust Ball
Bearing
Spherical Roller
Thrust Bearing
Types of thrust bearing

12. Bearing properties

13. Bearing type Load Comments
Bearing properties

14. Bearing Designation System
Basics
Rolling bearings can be applied universally as ready-to-mount
machine elements. This is especially due to the fact that the main
dimensions of the popular bearings are standardized according to
ISO (International Standardization Organization).
The basic designation system consists of three, four or five figures
of a combination of letters and figures.
Prefixes and suffixes provide additional information about the
bearing.
Prefixes are mainly used to identify components of a bearing. They
can also identify bearing variants.
Suffixes identify designs or variants, which differ in some way from
the original design or from the current basic design.

15. The first letter (not the Prefix) represents the bearing type
0 Angular contact ball bearings, double row
1 Self-aligning ball bearings
2 Spherical roller bearings and spherical roller thrust bearings
3 Taper roller bearings
4 Deep groove ball bearings, double row
5 Thrust ball bearings
6 Deep groove ball bearings, single row
7 Angular contact bearings, single row
8 Cylindrical roller thrust bearings
N Cylindrical roller bearings (N = outer ring no groove; NU =
inner ring no groove; NJ = only one shoulder on inner groove;
NUP = only one shoulder on inner groove plus one loose inner
flange)
Bearing Designation System

16. Bearing Designation System

17. • The first figure of the two-digit number for the dimension
series indicates the width series (the height series for thrust
bearings) and the second figure the diameter series.
Popular diameter series are 8, 9, 0, 1, 2, 3, 4 (increasing outside
diameters in this order).
There are several width series in each diameter series e.g. 0, 1,
2, 3, 4 (the higher the figure the greater the width).
• The last (or the last two numbers) indicate the bore diameter.
Just multiply the number by 5 and you will get the bore
diameter in mm. For bearings which have a bore diameter
smaller than 10 mm and equal to or greater than 500 mm, the
bore diameter is generally given in millimeters direct, the size
identification being separated from the rest of the bearing
designation by an oblique stroke, e.g. 618/8 (d = 8 mm) or
511/530 (d = 530 mm).
Bearing Designation System

18. Additional Designation Codes

19. Additional Designation Codes

20. Examples to Bearing designation

21. Internal Bearing Clearance
Bearings are designed with a specific internal clearance that
measures the total clearance between the rings and the rolling
elements. Internal clearance provides:
• Free rotation of rolling elements
• Compensation for thermal expansion
• Optimum load distribution
Choosing the correct internal clearance is important because
bearings hold shafts, armatures, gears and other rotating devices in
proper alignment. The amount of internal clearance influences
noise, vibration, heat build-up and fatigue life. Impact loads, severe
vibration, and ring fit also affect internal clearance. To obtain the
optimal internal clearance for specific application, those parameters
must be balanced.
Internal clearance can be separated into two categories: radial and
axial. The total internal clearance is the amount that one ring can
be displaced relative to the other ring, either radial or axial.

22. Normally, bearings are installed with interference on either the
inner or outer ring. This leads to its expansion or contraction,
which causes a change in clearance. During operation, the
bearing temperature will increase until it reaches saturation
temperature. However, the temperature of the inner ring, outer
ring and rolling elements are all different from each other, and
this temperature difference changes the clearance. In addition,
when a bearing operates under load, an elastic deformation of
the inner ring, outer ring and rolling elements also leads to a
change in clearance. Quantifying all these changes can make
calculating bearing internal clearance a complex task.
Internal Bearing Clearance

23. Internal Bearing Clearance

24. FUNCTIONS OF LUBRICATION:
Antifriction bearing lubricants serve the following primary
functions:
1. To lubricate the sliding contact between the retainer and
other parts of the bearing.
2. To lubricate any contact between the races and rolling
elements which is not true rolling.
3. To lubricate the sliding contact between the roller and guiding
elements in roller bearings.
4. To lubricate all true rolling contacts in the bearing.
Secondary functions are:
5. To protect highly finished surfaces from corrosion.
6. To help seal housings against foreign material. Grease
contributes to this objective.
7. To provide a means to transfer heat (cool).

25. Definition of EHD lubrication. The lubrication principles applied to
rolling bodies, such as ball or roller bearings, is known as
elastohydrodynamic (EHD) lubrication . An oil wedge, similar to
that which occurs in hydrodynamic lubrication, exists at the lower
leading edge of the bearing. Adhesion of oil to the sliding element
and the supporting surface increases pressure and creates a film
between the two bodies. Because the area of contact is extremely
small in a roller and ball bearing, the force per unit area, or load
pressure, is extremely high. Roller bearing load pressures may
reach 34,450 kPa (5000 lb/sq in) and ball bearing load pressures
may reach 689,000 kPa (1,000,000 lb/sq in). Under these
pressures, it would appear that the oil would be entirely squeezed
from between the wearing surfaces. However, viscosity increases
that occur under extremely high pressure prevent the oil from
being entirely squeezed out. Consequently, a thin film of oil is
maintained.
Definition of EHD lubrication

26. The general rule is to use oil for lubrication, if possible, because of
the ability to clean and cool oil versus doing so with grease. There
are, however, many applications where oil use is not possible or
practical. Greases are typically applied in areas where a continuous
supply of oil cannot be retained, such as open bearings or gears.
Grease is the most widely used lubricant for roller bearings and low
velocity applications, mainly because grease type lubricants are
relatively easy to handle and require only the simplest sealing
devices. Greases will not leak out as easily as oils. Greases are also
used when the component cannot be lubricated often or are not
accessible during operation. Greases are thick or viscous and,
therefore, unlike oil, cannot be pumped continuously through
equipment to remove heat. Greases are used for lubrication to
prevent friction and wear, to protect against corrosion, to provide a
seal from dirt and water, to provide lubrication that does not leak or
drip off the surface to which it is applied, and to lubricate for a long
time without breaking down.
Lubricating Grease

27. Grease is a semifluid to solid mixture of a fluid lubricant, a
thickener, and additives. The fluid lubricant that performs the
actual lubrication can be petroleum (mineral) oil, synthetic oil, or
vegetable oil. The thickener gives grease its characteristic
consistency and is sometimes thought of as a three-dimensional
fibrous network or sponge that holds the oil in place. Common
thickeners are soaps and organic or inorganic nonsoap
thickeners.
The majority of greases on the market are composed of mineral
oil blended with a soap thickener. Additives enhance
performance and protect the grease and lubricated surfaces.
Grease has been described as a temperature-regulated feeding
device: when the lubricant film between wearing surfaces thins,
the resulting heat softens the adjacent grease, which expands
and releases oil to restore film thickness.
Lubricating Grease

28. Lubrication: grease versus oil:
The choice between grease lubrication and oil lubrication is chiefly
determined by the following factors:
Grease should be used in applications where the following
requirements apply:
– Simplified maintenance
– Improved cleanliness (fewer leaks)
– Better protection against contaminants
Oil lubrication should be used in applications where normal
operating temperatures are high as a result of an external heat
source or excess heat generated by the machine or its bearings at
high speed.
Note: A temperature rise due to friction as in a bearing, is generally
lower with grease than with an oil bath, provided that the
appropriate type and amount of grease is used and that it is
supplied to the bearing in a suitable manner. Oil lubrication should
be used when the relubrication interval for grease is too short.

29. Applications suitable for grease.
Grease and oil are not interchangeable. Grease is used when it is
not practical or convenient to use oil. The lubricant choice for a
specific application is determined by matching the machinery
design and operating conditions with desired lubricant
characteristics. Grease is generally used for:
(1) Machinery that runs intermittently or is in storage for an
extended period of time. Because grease remains in place, a
lubricating film can instantly form.
(2) Machinery that is not easily accessible for frequent
lubrication. High-quality greases can lubricate isolated or
relatively inaccessible components for extended periods of time
without frequent replenishing. These greases are also used in
sealed-for-life applications such as some electrical motors and
gearboxes.

30. 3) Machinery operating under extreme conditions such as
high temperatures and pressures, shock loads, or slow speed
under heavy load. Under these circumstances, grease
provides thicker film cushions that are required to protect
and adequately lubricate, whereas oil films can be too thin
and can rupture.
(4) Worn components. Grease maintains thicker films in
clearances enlarged by wear and can extend the life of worn
parts that were previously oil lubricated. Thicker grease films
also provide noise insulation.
Applications suitable for grease

31. NLGI Grade Application Consistency
6 Slow-moving
journal bearings
Block
5 Low-speed
journal bearings
Very stiff
4 Very high speed
and low load
Stiff
3 High-speed
rolling element
bearings.
Typically ball
bearings.
Medium
2 Most common
grade for all
rolling element
bearing types
Medium soft
1 Centralized
lubrication
systems and
low
temperatures
Soft
0 Centralized
lubrication
systems
Very soft
00 Enclosed gears Semifluid
.

32. Even when bearings are being used under ideal conditions,
failures of bearings are caused by deterioration of the material
due to rolling fatigue. Generally, the service life of bearings is
expressed either as a period of time or as the total number of
rotations before the occurrence of failures in the inner ring, outer
ring or rolling element because of rolling fatigue, due to repeated
stress. Rolling bearings sometimes fracture earlier than expected.
The following causes should be considered;
1. Inappropriate use of bearings
2. Faulty installation or improper processing
3. Improper lubricant, lubrication method or sealing device
4. Inappropriate speed and operating temperature
5. Contamination by foreign matter during installation
6. Abnormally heavy load
Bearing failure analysis

34. Types of failure