Chapter 5

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GweeCh

iouChin

20

14

Chapter5:Be

aring

This topic explains the types of bearing

such as friction bearing, fluid coated

bearing and rolling bearing. It also

explains the method of selectingsuitable bearing based on calculation

of dynamic and life load ratio

Mechanical engineering

department

Politeknik Merlimau

[email protected]

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JJ513 Engineering Design Bearing

THE BIG PICTURE

OBJECTIVES OF THIS CHAPTER

After completing this chapter, you will be able to:

1. State the types of bearing.2. Explain the relationship between bearing load and life.3. Recommend appropriate value for the design life bearings.4. Apply the manufacturers data to specify suitable bearings for a given application.

5 - 1 ROLLING CONTACT BEARINGS

5 - 2 DESIGN LIFE BEARING

5 - 3 BEARING SELECTION

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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:

1. The bearing ensures free rotation of the shaft or the axle with minimum

friction.

2. The bearing supports the shaft or the axle and holds it in the correct

position.

3. The bearing takes up the forces that act on the shaft or the axle and

transmits them to the frame or the foundation.

Bearing are classified in different ways. Depending upon the direction of force

that acts on them, bearings are classified into two categories radial and thrust

bearings, as shown in Figure 5.1. A radial bearing supports the load, which is

perpendicular to the axis of the shaft. A thrust bearing support the load, which acts

along the axis of the shaft.

Figure 5.1 Radial bearing and thrust bearing.

The most important criterion to classify the bearings is the type of friction

between the shaft and the bearing surface. Depending upon the type of friction,

bearings are classified into two main groups sliding contact bearing and rolling

contact bearings as shown in Figure 5.2.

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Figure 5.2: Sliding and Rolling Contact Bearing

5.1 ROLLING CANTACT BEARING

Ball and roller bearing are used in almost every kind of machine and

mechanical device with rotating parts. However, such bearings cannot be used

indiscriminately without taking into account types of loads and operating conditions.

Substitution of rolling friction in place of sliding friction results in a reduction of

frictional resistance in a significant way. The surface of two elements of the turning

pairs are separated by a number of rolling elements. The introduction of rolling

friction cuts down frictional resistance significantly. To reduce the coefficient of

rolling friction further, the balls or the rollers are made of chromium stated or

chrome-nickel steel, which are heat treated to increase their hardness. These rolling

elements are precisely ground and polished. The starting friction is thus cut down

significantly.

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Figure 5.3: Rolling Contact Bearing

Classification of Rolling Contact Bearings

The rolling contact bearings are classified into two major groups with respect

to their structure as:

1. Ball Bearings

2. Roller Bearings

There are many types of Ball Bearing such as deep groove ball bearings,

angular ball bearings, and thrust ball bearings. Roller bearings such as taper roller

bearings, cylindrical roller bearings, thrust roller bearings. Both types of bearings can

carry radial loads and axial loads act individually or in combined form. Generally ball

bearings are used for light loads and the roller bearings are usually used for heavierloads.

Figure 5.4: Types of Rolling Contact Bearing

Deep Groove Ball Bearing

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The most frequently used bearing is the deep groove ball bearing. It is found

in almost all kinds of products in general mechanical engineering. In this type of

bearing, the radius of the ball is slightly less than the radii of curvature of the groove

in the races. Kinematically, this gives a point contact between the balls and the races.

Therefore, the balls and the races may roll freely without any sliding. Deep grooveball bearing has the following advantages:

a) Due to relative large size of the balls, deep groove ball bearing has high loadcarrying 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 inhigh 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

millimeters to 400 millimeters.

The disadvantages of deep groove ball bearings are as follows:

a) Deep groove ball bearing is not self-aligning. Accurate alignment betweenaxes of the shaft and the housing bore is required.

b)Deep groove ball bearing has poor rigidity compared with roller bearing. Thisis due to the point contact compared with the line contact in case of roller

bearings. It is unsuitable for machine tool spindles where rigidity is important

consideration.

Cylindrical Roller Bearing

When maximum load carrying capacity is required in a given space, the point

contact in ball bearing is replaced by the line contact of roller bearing. A cylindrical

roller bearing consists of relatively short rollers that are positioned and guided by thecage. Cylindrical roller bearing offers the following advantages:

a) Due to line contact between rollers and races, the radial load carryingcapacity of the cylindrical roller bearing is very high.

b) Cylindrical roller is more rigid than ball bearing.c) The coefficient of friction is low and frictional loss is less in high-speed

applications.

The disadvantages of cylindrical roller bearing are as follows:a) In general, cylindrical roller bearing cannot take thrust load.

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b) Cylindrical roller bearing is not self-aligning. It cannot tolerate misalignment.It needs precise alignment between axes of the shaft and the bore of the

housing.

c) Cylindrical roller bearing generates more noise.Angular Contact Bearing

In angular contact bearing, the grooves in inner and outer races are so

shaped that the line reaction at the contact between balls and races makes an angle

with the axis of the bearing. This reaction has two components radial and axial.

Therefore, angular contact bearing can take radial and thrust loads. Angular contact

bearings are often used in pairs, either side by side or at the opposite end of the

shaft, in order to take the thrust load in both directions. These bearings are

assembled with a specific magnitude of preload. Angular contact bearings offer thefollowing advantages:

a)Angular contact can take both radial and thrust loads.b) In angular contact bearing, one side of the groove in the outer races is cut

away to permit the insertion of larger number of balls than that of deep

groove ball bearing.

The disadvantages of angular contact bearings are as follow:

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 required initial pre-loading.

Self-aligning Bearings

These are two types of self-aligning rolling contact bearings, self- aligning

rolling contact bearing and spherical roller bearing. The self-aligning ball bearing

consists of two rows of balls, which roll on a common spherical surface in the outer

race. In this case, the assembly of the shaft, the inner race and the balls with cage

can freely roll and adjust itself to the angular misalignment of the shaft. There issimilar arrangement in the spherical roller bearing, which runs on a common

spherical surface in the outer race. Compared with the self-aligning ball bearing, the

spherical roller bearing can carry relatively high radial and thrust loads. Both types of

self-aligning bearing permit minor angular misalignment of the shaft relative to the

housing. They are therefore particularly suitable for applications where misalignment

can arise due to errors in mounting or due to deflection of the shaft. They are used

in agricultural machinery, ventilators, and railway axle-boxes.

Taper Roller Bearing

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The tapper roller bearing consists of rolling elements in the form of a frustum

of cone. They are arranged in such a way that the axes of individual rolling elements

intersect in a common apex point on the axis of the bearing. In kinematical analysis,

this is the essential requirement for pure rolling motion between conical surfaces. In

taper roller bearing, the line of resultant reaction through the rolling elements makesan angle with the axis of the bearing. Therefore, taper roller bearing can carry both

radial and axial loads. In fact, the presence of either component results in the other,

acting on the bearing. In other words, a taper roller bearing subjected to pure radial

load induces a thrust component and vice versa. Therefore, taper roller bearing is

always used in pair to balance the thrust component. Taper roller bearing has

separable construction. The outer ring is call cup and the inner ring is called cone.

The cup is separable from the remainder assembly of the bearing elements including

the roller, cage and cone. 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 bearings are as follows:

a) It is necessary to use two taper roller bearings on the shaft to balance theaxial force.

b) It is necessary to adjust the axial position of the bearing with pre-load. This isessential to coincide the apex of the rolling elements.

c) Taper roller bearing cannot tolerate misalignment between the axis of theshaft and the housing bore.

d) Taper roller bearings are costly.Taper roller bearings are used for cars and trucks, propeller shaft and

differentials, railroad axle-boxes and as large size bearings in rolling mills.

Thrust Ball Bearing

A thrust ball bearing consists of a row of balls running between two rings

the shaft ring and the housing ring. Thrust ball bearing carries thrust load in only

one direction and cannot carry any radial load. The use of a large number of balls

results in high thrust load carrying capacity in smaller space. This is the major

advantage of thrust bearing. The disadvantages of thrust bearing are as follows:

a) Thrust ball bearing cannot take radial load.b) It is self-aligning and cannot tolerate misalignment.

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c) their performance is satisfactory at low and medium speeds. At high speeds,such bearings give poor service because the balls are subjected to centrifugal

forces and gyroscopic couple.

d) Thrust ball bearing do not operate as well on horizontal shaft as they do onvertical shaft.

e) Thrust ball bearing requires continuous pressure applied by spring to hold therings together.

Thrust ball bearings are used where heavy thrust loads are to be carried, for

example, worm gear boxes and crane hooks.

5.2 LOAD-LIFE RELATIONSHIP

The relationship between the dynamic load carrying capacity, the equivalent

dynamic load, and the bearing life id given by,

L10= (5.1)

Where,

L10= rated bearing life (in million revolutions)

C = dynamic load capacity (N),and

P = 3 (for ball bearings)

P = 10/3 (for roller bearings)

Rearranging Eq. (5.1),

C = P(L10)1/P

For all types of ball bearings,

C = P(L10)1/3 (5.2)

For all types of roller bearings,

C = P(L10)0.3 (5.3)

The relationship between life in million revolutions and life in working hours is

given by

L10=

(5.4)

Where,

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L10h= rated bearing life (hours)

n = speed of rotation (rpm)

Example 5.1

In a particular application, the radial load acting on a ball bearing is 5kN and the

expected life for 90% of the bearings is 8000 h. calculate the dynamic load carrying

capacity of the bearing, when the shaft rotates at 1450 rpm.

Solution:

Given Fr= 5 kN L10h= 8000 h n = 1450 rpm

Step I Bearing Life

L10=

=

= 696 million rev.

Step II Dynamic load capacity

Since the bearing is subjected to purely radial load,

P = Fr= 5000 N

From eq. (5.2),

C = P(L10)1/3= (5000)(696)1/3= 44,310.48 N

Example 5.2

A taper roller bearing has a dynamic load capacity of 26 kN. The desired lofe for 90%

of the bearings is 8000 h and the speed is 300 rpm. Calculate the equivalent radial

load that the bearing can carry.

Solution

Given C = 26 kN L10h= 8000 h n = 300 rpm

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Step I Bearing Life

L10=

=

= 144 million rev.

Step II Equivalent radial load

From eq. (5.3), C = P(L10)0.3

P =

[]=

= 5854.16 N

Since the bearing is subjected to purely radial load,

Fr= P = 5854.16 N

5.3 BEARING SELECTION

Each bearing type displays characteristic properties, based on its design,

which makes it more, or less, appropriate for a given application. For example, deep

groove ball bearings can accommodate moderate radial loads as well as axial loads.

They have low friction and can be produced with high precision and in quiet running

variants. Therefore they are preferred for small and medium-sized electric motors.

Spherical and toroidal roller bearings can carry very heavy loads and are self-

aligning. These properties make them popular for example for heavy engineering

applications, where there are heavy loads, shaft deflections and misalignments.

In many cases, however, several factors have to be considered and weighted

against each other when selecting a bearing type, so that no general rules can be

given.

The information provided here should serve to indicate what are the most

important factors to be considered when selecting a standard bearing type and thusfacilitate an appropriate choice.

available space loads misalignment precision speed quiet running stiffness axial displacement

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Chapter 5