Rolling Element Bearings
Bearings
Bearing is a mechanical element that permits relative
motion between two parts, such as the shaft and the
housing, with minimum friction. The functions of the
bearing are as follows:
(i) The bearing ensures free rotation of the shaft or the
axle with minimum friction.
(ii) The bearing supports the shaft or the axle and holds
it in the correct position.
(iii) The bearing takes up the forces that act on the shaft
or the axle and transmits them to the frame or the
foundation.
Rolling Element Bearing
Bearings can be classified depending on various criteria such as the
design, loading conditions, arrangements, type of lubrication and so on.
Proper selection of the bearing is important, but not as important as
proper installation.
Classification based on the rolling elements
Rolling Bearing
Roller Bearing
Ball Bearing
(High Load
(High Speed)
Carrying Capacity)
Ball Bearing Classification
Ball Bearing
Deep Grove Ball Bearing Angular Contact
Self Aligning Ball Bearing
Ball Bearing
(DGBB)
Roller Bearing Classification
Roller Bearing
Cylindrical
Bearing
Taper Roller
Bearing
Spherical Roller
Bearing
Needle Roller Bearing
Rolling Element bearing
Classification based on load carried
Rolling Element Bearing
Radial bearing Thrust bearing
Components of Bearing
Cage
Outer raceway
Inner
raceway
Rolling Elements
Types of Rolling Element
Needle Roller Spherical Roller
Ball Roller
Cylindrical Roller
Taper Roller
Bearing Cages
Bearing Cages
Bearing component Assembly
Types of bearings
Deep Groove Ball Bearing
Single Row DGBB Double Row DGBB
Deep Groove Ball Bearing
Deep groove ball bearing has the following advantages:
(a) Due to relatively large size of the balls, deep groove ball
bearing has high load carrying capacity.
(b) Deep groove ball bearing takes loads in the radial as well as
axial direction.
(c) Due to point contact between the balls and races, frictional loss
and the resultant temperature rise is less in this bearing. The
maximum permissible speed of the shaft depends upon the
temperature rise of the bearing. Therefore, deep groove ball
bearing gives excellent performance, especially in high speed
applications.
(d) Deep groove ball bearing generates less noise due to point
contact.
(e) Deep groove ball bearings are available with bore diameters
from a few millimetres to 400 millimetres.
Deep Groove Ball Bearing
The disadvantages of deep groove ball bearings
are as follows:
(a) Deep groove ball bearing is not self-aligning. Accurate alignment
between axes of the shaft and the housing bore is required.
(b) Deep groove ball bearing has poor rigidity compared with
roller bearing. This is due to the point contact compared with the
line contact in case of roller bearing. It is unsuitable for machine
tool spindles where rigidity is important consideration.
Typical arrangements for housing of bearing
Bearings may be considered analogous to ground connection in the
case of electrical circuit.
Angular Contact Ball Bearing
Angular Contact Ball Bearing
Angular contact bearings offer the following advantages:
(a) Angular contact bearing can take both radial and
thrust loads.
(b) In angular contact bearing, one side of the groove in the
outer race is cut away to permit the insertion of larger
number of balls than that of deep groove ball bearing.
This permits the bearing to carry relatively large axial and
radial loads. Therefore, the load carrying capacity of
angular contact bearing is more than that of deep groove
ball bearing.
Angular Contact Ball Bearing
The disadvantages of angular contact bearings are as
follows:
(a) Two bearings are required to take thrust load in both
directions.
(b) The angular contact bearing must be mounted without
axial play.
(c) The angular contact bearing requires initial pre-loading.
Angular Contact Ball Bearing
Arrangements
Stiffness of Ball Bearing
• The stiffness of a rolling bearing is characterized by the
magnitude of the elastic deformation (resilience) in the
bearing under load. Generally, this deformation is very
small and can be neglected. In some cases, however, e.g.
spindle bearing arrangements for machine tools or pinion
bearing arrangements, stiffness is important.
• Depending on the application it may be necessary
to have either a positive or a negative operation-
al clearance in the bearing arrangement. In the
majority of applications, the operational clearance
should be positive, i.e. when in operation, the bear-
ing should have a residual clearance.
Preload in Ball Bearing
• However, there are many cases, e.g. machine tool
spindle bearings, pinion bearings in automotive
axle drives, bearing arrangements of small electric
motors, or bearing arrangements for oscillating
movement, where a negative operational clearance,
i.e. a preload, is needed to enhance the stiffness of
the bearing arrangement or to increase running ac-
curacy.
• Effect of preload
The main effects of bearing preload are to:
• Enhance system stiffness
• Reduce running noise
• Enhance the accuracy of shaft guidance
• Provide long service life
Back-to-back arrangement (load lines diverge)
• Also known as an O arrangement, this is known to give
maximum stability and rigidity. In this arrangement, inner
ring faces are designed to touch each other to achieve
preload, and radial and axial loads in both directions can be
taken.
Back-to-back arrangement (load lines diverge)
• this constant reaction force from an outside stationary body
(ground) acting on the shaft and with L being more than
the width of the bearings, the shaft will be tightly held in
the position covering length L at the bearing location. This
tightness is realized in terms of higher rigidity and the
stability of the shaft. That is why back-to-back
arrangements have better resistance to moments in the
shaft.
Face-to-face arrangement(load lines converge)
• The preload is achieved by closing the gap between the outer
races. One can imagine that while pushing the outer races
closer, it will move the rolling elements with the cage along.
the reaction will be given by the inner races against the applied
force.
Face-to-face arrangement(load lines converge)
• Also known as an X arrangement, this is known to tolerate
misalignments and cannot support moment loads as effectively
as back-to-back arrangements. If misalignment cannot be
avoided between the bearing positions, face-to-face bearing
arrangements are recommended.
Self aligning ball Bearing
Roller Bearing
Taper Roller Bearing
Taper roller bearing subjected to pure radial load induces a thrust
component and vice versa.
In many applications tapered roller bearings are used in back-to-back
pairs so that axial forces can be supported equally in either
direction. Pairs of tapered roller bearings are used in car and vehicle
wheel bearings where they must cope simultaneously with large
vertical (radial) and horizontal (axial) forces.
Taper Roller Bearing
Taper roller bearings offer the following advantages:
(a) Taper roller bearing can take heavy radial and thrust loads.
(b) Taper roller bearing has more rigidity.
(c) Taper roller bearing can be easily assembled and disassembled
due to separable parts.
The disadvantages of taper roller bearing are as follows:
(a) They can support an axial load very well in one direction, less well
in the opposite direction. If you don’t know what direction the
axial load will take, then you’d better put the tapered bearings in
pairs with one bearing opposed to the other.
(b) (b) It is necessary to adjust the axial position of the bearing with
pre-load.
(c) Taper roller bearing cannot tolerate misalignment between the
axes of the shaft and the housing bore.
(d) Taper roller bearings are costly.
Thrust bearings
Roller Thrust Bearing
Ball Thrust Bearing
Needle Thrust bearing
Arrangement in the machine components
Single Row Bearing arrangement in the machine component
Typical arrangement of taper roller
bearing
Load carrying capacity
• Static load carrying capacity
• Static load is defined as the load acting on the
bearing when the shaft is stationary. It produces
permanent deformation in balls and races.
• The static load carrying capacity of a bearing is
defined as the static load which corresponds to a
total permanent deformation of balls and
races, at the most heavily stresses point of
contact, equal to 0.0001 of the ball diameter.
Load carrying capacity
• Dynamic load carrying capacity
• The dynamic load carrying capacity of a
bearing is defined as the radial load in
radial bearings or thrust load in thrust
bearings that can be carried for minimum
life of one millions revolutions.
Life of Bearing
• The life of an individual ball bearing is
defined as the number of revolutions that the
bearing runs before the first evidence of
fatigue cracks in balls or races.
• The rating life of a group of apparently
identical ball bearing is defined as the
number of revolutions that 90% of the
bearings will complete or exceed before the
first evidence of fatigue crack.
Life of Bearing
• 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).
Life of Bearing
• 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).
EQUIVALENT BEARING LOAD
• Then the hypothetical load can be compared with the dynamic
load capacity. The equivalent dynamic load is defined as the
constant radial load in radial bearings (or thrust load in thrust
bearings), which if applied to the bearing would give same life
as that which the bearing will attain under actual condition of
forces. The expression for the equivalent dynamic load is
written as,
P = XVFr + YFa
where,
P = equivalent dynamic load (N)
Fr = radial load (N)
Fa = axial or thrust load (N)
V = race-rotation factor = 1 for inner race rotating and
1.2 for outer race is rotating
• X and Y are radial and thrust factors respectively and their
Load –life Relationship
• The relationship between the dynamic load carrying capacity,
the equivalent dynamic load, and the bearing life is given by,
Selection of Bearing life
Load Factor
Load factors are used in applications involving gear, chain and belt
drives. In gear drives, there is an additional dynamic load due to
inaccuracies of the tooth profile and the elastic deformation of teeth.
In chain and belt drives, the dynamic load is due to vibrations.
SELECTION OF BEARING FROM MANUFACTURER’S CATALOGUE
(i) Calculate the radial and axial forces acting on the bearing and
determine the diameter of the shaft where the bearing is to be
fitted.
(ii) Select the type of bearing for the given application.
(iii) Determine the values of X and Y, the radial and thrust factors,
from the catalogue. The values of X and Y factors for single-row deep
groove ball bearings are given in Table
SELECTION OF BEARING FROM MANUFACTURER’S CATALOGUE
The static and dynamic load capacities of single-row deep groove
ball bearings of different series are given in Table. To begin with, a
bearing of light series, such as 60, is selected for the given diameter
of the shaft and the value of C0 is found from Table
SELECTION OF BEARING FROM MANUFACTURER’S CATALOGUE
(iv) Calculate the equivalent dynamic load from the equation.
P = XFr + Yfa
(v) Make a decision about the expected bearing life and express
the life L10 in million revolutions.
(vi) Calculate the dynamic load capacity from the equation
C = P ( L10)1/3
(vii) Check whether the selected bearing of series 60 has the
required dynamic capacity. If not, select the bearing of the next
series and go back to Step (iii) and continue.
Ball bearings are thus selected by the trial and error procedure. The
above procedure is also applicable to other types of bearings.
Example
• A transmission shaft rotating at 720 rpm and transmitting power
from the pulley P to the spur gear G is shown in Fig. The belt
tensions and the gear tooth forces are as follows:
P1 = 498 N P2 = 166 N Pt = 497 N Pr = 181 N
The weight of the pulley is 100 N. The diameter of the shaft at bearings
B1 and B2 is 10 mm and 20 mm respectively. The load factor is 2.5 and
the expected life for 90% of the bearings is 8000 h. Select single row
deep groove ball bearings at B1 and B2.
Example
• A shaft transmitting 50 kW at 125 rpm from the gear G1 to the gear
G2 and mounted on
two single-row deep groove ball bearings B1 and B2 is shown in Fig.
The gear tooth forces are
• Pt1 = 15915 N Pr1 = 5793 N
• Pt2 = 9549 N Pr2 = 3476 N
The diameter of the shaft at bearings B1 and B2 is 75 mm. The load
factor is 1.4 and the expected life for 90% of the bearings is 10000 h.
Select suitable ball bearing.
Example
Design for cyclic loads
In certain applications, ball bearings are subjected to cyclic loads and
speeds. As an example, consider a ball bearing operating under the
following conditions:
(i) radial load 2500 N at 700 rpm for 25% of the time,
(ii) radial load 5000 N at 900 rpm for 50% of the time, and
(iii) radial load 1000 N at 750 rpm for the remaining 25% of the time.
Suppose that the work cycle is divided into x elements. Let P1, P2, …
Px be the loads and n1, n2,…, nx be the speeds during these elements.
During the first element, the life L1 corresponding to load P1, is given
by
Design for cyclic loads
Design for cyclic loads
In case of bearings, where there is a combined radial and axial load,
it should be first converted into equivalent dynamic load before the
above computations are carried out.
