Bearings
Bearing
types and styles are classified into: plain and anti-friction
bearings. Plain bearing operates in sliding motion
principle and the anti-friction bearing operates in rolling
motion principle.
Plain
Bearing
There is
a relative sliding movement between shaft and bearing
surfaces. A lubricant is used to keep the surfaces separated and
minimized friction. A film of lubricant should be maintained to
prevent from making contact and to have long service life. The common
practice is to use a steel shaft and to make a plain bearings
made of bronze, babbitt or some other materials soften than steel. A
bearing material that is softer than steel will wear before
the shaft and usually can be replaced more easily, quickly and
cheaply.
Another
factor that has a major influence on plain bearing operation
is the surface finish of both journal and the bearing. Simply
stated, the rougher the surface, the thicker the film required to
separate them. Oil film thickness, even in very large bearings,
does not exceed a couple of thousands and in small bearings
may be a few ten-thousands. This gives some indication of the
importance of fine surface finishes to satisfactory bearing
performance.
Anti-friction
Bearing
It has
series of rollers or balls interposed between the moving and
stationary members. These rollers or balls are usually mounted in a
cage or separator and enclosed between rings or races. In theory, the
rolling elements in anti-friction bearings perform the
function that the lubricant film performs in the plain bearing.
In practice however, anti-friction bearings require
lubrication as some sliding action occurs between rolling elements
and races.
Bearing
Types
Single-Row
Deep Groove Ball Bearings
Single-row
deep groove ball bearings are the most widely used of all
bearings and probably of all anti-friction bearings.
They can sustain combined radial and thrust loads, or thrust loads
alone, in either direction even at extremely high speeds.
Double-Row
Deep Groove Ball Bearings
Double-row
deep groove ball bearings embodies the same principle of
design as the single-row deep groove ball bearing. However,
the grooves for the two rows of balls are positioned so that the load
through the balls tends to push outward on the outer ring races. This
bearing has substantial thrust capacity in either direction
and high radial capacity due to two rows of balls.
Angular-Contact
Radial-Thrust Ball Bearings
Angular-contact
radial-thrust ball bearings can support radial loads when
combined with thrust loads in one direction. The inner and outer
rings are made with extra high shoulder on one side only ( thrust
side ). It is designed for combination loads where the thrust
component is greater than the capacity of single-row deep groove ball
bearings. They may be mounted either face to face or back to
back and in tandem for constant thrust in one direction. When
mounting in pairs, care must be taken that the bearings have
been ground for the style of assembly used.
Four-Point
Contact Ball Bearings
This
bearings are radial single row angular contact ball bearings
with raceways designed to support axial loads in both directions.
Radial loads can be supported up to certain fraction of axial loads.
This have less axial space than double rows. This bearings
have a prefix of QJ and have a contact angle of 35° to 45° and with
split inner ring. This enables for a large number of balls to be
incorporated in the bearing making it to withstand heavy
loads.
Self-Aligning
Ball Bearings
This
type of bearing is very suitable for angular misalignment. The
angular movement is possible because the two rows of balls are
rolling on the spherical surface of the outer ring than in usual fix
races. This bearing can support moderate radial loads, since
the balls have only point-contact support on the spherical outer ring
surface.
Ball
Thrust Bearings
The ball
thrust bearing can support thrust loads in only one direction,
which precludes radial loading. The thrust load is transmitted
through the balls parallel to the axes of the shaft, resulting in
very high thrust capacity. The rings of these bearings are commonly
known as washers. For a successful operation, it must be thrust
loaded moderately at all times. This should not be operated at high
speeds, as centrifugal force will cause excessive loading of the
outer edges of the races.
Cylindrical
Roller Bearing
The
cylindrical roller bearing has straight cylindrical shape
rolling elements. This rolling elements are approximately equal in
diameter and length. These equal dimensioned rollers distinguish
cylindrical roller bearings from other roller bearings
that have rollers with a much greater length to diameter ratio. They
are designed for heavy radial loads because of it huge point of
contact but of lower speed application because greater point of
contact generates heat.
Spherical
Roller Bearings
The
double row spherical roller bearing is a self aligning bearing
utilizing rolling elements shaped like barrels. The outer ring has a
single spherical raceway. The double shoulder inner ring has two
spherical races separated by a center flange. The rollers are
retained and separated by an accurately constructed cage.
Journal
Roller Bearings
Journal
roller bearings are straight roller bearings; however,
the rollers have greater length to diameter ratio than rollers in
straight cylindrical roller bearings. They are used in variety
of low speed applications where loads are light to moderate and
limited spaced. Because of their length and long line of contact, the
journal bearings develop a considerable amount of internal
friction. Therefore, they are made with large internal clearances to
compensate for normal heat expansion. Its name takes place from the
fact that a common practice is to use the surface of the shaft as the
inner race for the rollers. The journal bearings are also made
with an inner race, outer race or both.
Installation
of the Individual Bearing
An
anti-friction bearing requires an interference fit on one of
the bearing races, usually the inner race. Care should be
considered in mounting a bearing into shaft; it is critical
that the internal clearance is not totally removed when the inner
race stretched to fit over the shaft. Checking of dimensions are the
key of proper mounting. The bearing manufacturer provides
checking measurements for the shaft. When replacing a bearing,
make sure that proper fitting of bearing and shaft is
obtained.
There
are two common ways of bearing mounting to a shaft: hydraulic
press mounting and thermal
mounting. In press mounting, a force capable of moving the
bearing into the shaft is applied to the face of the ring that
has the interference fit, usually the face of the inner ring. Always
exert force on the ring to be mounted to prevent damaging the
bearing.
Thermal
mounting uses a technique of heating a bearing for expansion.
In most applications, the inner ring should be uniformly heated to a
temperature not to exceed 250°F. Never use direct flame or hot
plate. Heating the bearing in excess of 250°F for extended
time will anneal the metal and reduce the hardness. A bearing
can be heated using a cone heater, an electric oven, an induction
heater, a hot oil bath, or even an electric light bulb centered in
the inner race.
Bearing
Handling
There
are two most important rules in handling anti-friction bearings
as follows:
- Keep the bearing and parts clean. Most anti-friction bearing failures are the contamination of dirt or grit.
- Apply force to the tight ring only. Transfer of force from one ring through the rolling elements to the other ring can cause indentation of the races, which will cause bearing failure.
The
following practices and procedures for mounting and dismounting
anti-friction bearings are recommended by the Anti-friction
Bearing Manufacturers Association:
- Clean shafts and bearing housing thoroughly.
- Clean dirt out of keyways, splines and grooves.
- Remove burrs and slivers.
- Clean and oil bearing seats.
- Press bearing on straight and square.
- Press only on the ring that takes the tight fit.
- Press bearings until they are seated against the shaft or housing shoulder.
