A types of bearings is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. The bearings are used in variety of machine elements to guide and allow the relative motion between the parts with minimum friction and maximum accuracy.
Types of Bearings
Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.
Based on the directions of applied loads the bearings are classified as radial and thrust bearings. The bearing which is used to provide radial support to the rotating shaft is called radial bearing. In addition to radial loads, they carry substantial thrust loads at high speeds, in either direction. Ball bearings are the most widely used radial bearings.
The bearing which is used to support very large thrust loads. along the axis of the shaft is called thrust bearings. They cannot take much radial load and are mostly used for low-speed applications.
The most common types of bearings based on their principle of operation are
- Plain journal bearing
- Rolling element bearing
- Fluid bearing
- Magnetic bearing
Plain journal bearing
In plain journal bearing rubbing surfaces are usually provided with lubricant. This is the simplest types of bearings, widely used, relatively high friction, suffers from stiction in some applications. Some bearings use pumped lubrication and behave similarly to fluid bearings. At high speeds life can be very short.
Rolling element bearing
Ball or rollers are used to prevent or minimize rubbing. It is used for higher loads than plain bearings with lower friction.
Fluid is forced between two faces and held in by edge seal. It can fail quickly due to grit or dust or other contaminants.
Faces of bearing are kept separate by magnets. It needs considerable power. Maintenance is free.
Plain Journal Bearings
A journal bearing sometimes is also referred as a friction bearing which is a simple bearing. It consists of a shaft which is also called as journal which rotates in the bearing with a layer of lubricating oil separating the two parts through fluid dynamic effects. The bearing is generally a simple polished hollow cylinder, made of white metal, lead based babbit, tin based babbit, leaded bronze, copper lead alloy, gun metal, phosphor bronze, aluminum alloy, nylon, in which the journal rotates with lubricant filling the gap.
Rather than just reducing friction between the contact surfaces, the lubricant should be thick enough so that the surfaces do not come in contact at all while rotation. If oil is used, it is generally fed into a hole in the bearing under pressure, as it is done for the most heavily-loaded bearings in an automobile engine. Sometimes the bearings may be used dry without lubrication when the bearings are made of plastics such as nylon or PTFE.
Types of bearings – journal
Plain journal bearings are classified according to the nature of lubrication under which it operates. These bearings may be hydro-dynamically lubricated or hydrostatically lubricated. The difference between hydrostatic and hydrodynamic forces is in the way the pressure that supports the bearing is initially and subsequently maintained.
In a hydrostatic lubrication, the pressure is always present at a desirable value and is achieved by an external pump which forces lubricant into the system. The pump provides a magnitude of pressure that aims to supplement the pressure which is
created by the bearings rotation, if any. This pressure makes the separation of journal and bearing filled with oil during rotation as shown in Figure.
In hydrodynamic lubrication, the pressure in the oil film is maintained by the rotation of the shaft itself. Hydrodynamic lubrication is also called as full-film lubrication.
Hydrodynamic bearings require much greater care in design and operation than hydrostatic bearings. They are also much more prone to initial wear because lubrication does not occur until there is rotation of the shaft. At low rotational speeds the lubrication may not attain complete separation between journal and bearing.
As a result, hydrodynamic bearings are often aided by secondary bearings which support the shaft during start and stop periods, protecting the fine tolerance machined surfaces of the journal bearing. Figure shows mechanism of hydrodynamic lubrication.
Solid film lubrication – Types of Bearings
In solid film lubrication, a thin film of solid lubricants such as hexagonal or flake graphite, molybdenum disulphide, and metal sulphide are coated on the inner surface of the bearing. Solid and dry film lubricants form a dry layer or coating that
excludes moisture and reduces friction, binding, and wear. Solid lubricants maintain a low coefficient of friction up to 400° C and are available in powder, sprayable coating, and solid machinable forms. Wax, paraffin and stearate compounds are
suitable for some lubrication, anti-corrosive and anti-static applications.
Boundary layer lubrication
Sometimes the following factors may prevent the built up of a film thick enough for full-film or hydrodynamic lubrication.
- Insufficient surface area
- A drop in the velocity of the moving surface
- A lessening in the quantity of lubricant supplied to a bearing
- An increase in the bearing load
- An increase in the lubricant temperature resulting in decrease in viscosity
At this time the contact surfaces may be separated by lubricant films of thin layer which adheres to the bearing surface.
Rolling Element Bearings
Rolling element bearings minimize friction by removing any possible sliding between bearing surfaces and replacing all contacts with rolling interfaces. They substitute balls or rollers for 2 hydrodynamic or hydrostatic fluid film to carry loads
with reduced friction. They utilize a separator to space the hardened rolling elements apart. The rolling element bearing is also called as antifriction bearing since the friction is negligible as compared to the journal bearing.
A rolling element bearing has four parts (i) outer race, (ii) inner race, (iii) rolling element and (iv) retaining cage or separator. In rolling element bearing the balls or rollers are held in position between inner and outer races by the retaining
cage which is used to keep the balls or rollers separately and thereby preventing them from rubbing against each other as shown in Figure. If the balls are inserted between the outer and inner races it is called ball bearing whereas in roller bearing rollers are inserted.
Ball bearings use spherical shaped balls as rolling elements. Ball bearings can support both radial (perpendicular to the shaft) and axial loads (parallel to the shaft). For lightly-loaded bearings, balls offer lower friction than rollers. Ball bearings can operate when the bearing races are misaligned.
Roller bearings use cylinders of slightly greater length than diameter. Roller bearings typically have higher radial load capacity than ball bearings, but a low axial capacity and higher friction under axial loads. If the inner and outer races are
misaligned, the bearing capacity often drops quickly when compared to either a ball bearing or a spherical roller bearing.
Types of bearings – Ball
- Deep groove ball bearing [Figure (a)]
This is the most common type of bearing and widely used. A deep-groove radial bearing is one in which the race dimensions are close to the dimensions of the balls that run in it. Deep-groove bearings have good radial load carrying capacity and moderate axial load capacity.
- Filling-notch or Filling-slot ball bearing [Figure (b)]
A filling-notch radial bearing is one in which the inner and outer races are notched so that when they are aligned, balls can be slipped in the slot in order to fill the bearing. It has higher radial load carrying capacity than deep groove bearings and has poor axial load capacity.
- Angular contact ball bearing [Figure (c)]
An angular contact ball bearing uses axially asymmetric races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings better supports both the radial and axial loads.
- Double row ball bearing [Figure (d)]
Double row ball bearings have two rows of balls and are made with either radial or angular contact between the balls and races. These bearings are able to withstand higher radial loads and thrust loads in either directions compared to single row bearings.
- Self-aligning ball bearing [Figure (e)]
Self-aligning ball bearings are constructed with the inner ring and ball assembly contained within an outer ring that has a spherical raceway. This construction allows the bearing to tolerate a small angular misalignment resulting from deflection or improper mounting. These bearings are able to withstand moderate radial loads and fairly poor axial loads.
- Thrust, grooved race ball bearing [Figure (f)]
These ball bearing uses side-by-side races. An axial load is transmitted directly through the bearing, while a radial load is poorly-supported, tends to separate the races, and anything other than a small radial load is likely to damage the bearing.
Types of bearings – roller
- Straight roller bearings [Figure (a)]
Straight roller bearings use cylinders of slightly greater length than diameter. These bearings have higher radial load capacity than ball bearings of same size, but a low axial capacity and higher friction under axial loads. If the inner and outer races are misaligned, the bearing capacity often drops quickly when compared to a ball bearing.
- Taper roller bearings [Figure (b)]
Tapered roller bearings use conical rollers that run on conical races. Most roller bearings only take radial loads, but tapered roller bearings support both radial and axial loads, and generally can carry higher loads than ball bearings due to greater contact area. For example, the wheel bearings of most cars, trucks, buses, and so on.
- Needle roller bearings [Figure (c)]
Needle roller types of bearings use very long and thin cylinders. Often the ends of the rollers taper to points and these are used to keep the rollers captive, or they may be hemispherical and not captive but held by the shaft itself or a similar arrangement. Since the rollers are thin, the outside diameter of the bearing is slightly larger than the hole in the middle.
Selection of Bearings
The selection of ball and roller bearings for a given installation depends upon the following factors.
- The load-carrying capacity and the nature of the load.
- The speed of shaft in RPM,
- The type of service under given conditions such as temperature, humidity, dust iness, acidity, etc.
- The anticipated life of the bearing.
- Magnitude and direction of loads.
- The proportion of thrust to radial load.
After considering the above factors, generally ball bearings are suitable for small machines running at high speeds while roller Bearings are suitable for machines to be placed under heavy loads.
- Deep groove or Angular Contact Ball Bearings are suitable where comparatively heavy thrust loads are to be carried at high speeds.
- Self Aligning Ball Bearings and Spherical Roller Bearings are suitable for such parts as are apt to be subjected to permanent bending or warping and fluctuating misalignment occurs between shaft and bearing housing.
- Cylindrical roller Bearings are suitable for those shafts which have been allowed to move freely longitudinally within certain limits, and for far larger and heavily loaded applications.
- Taper Roller Bearings are suitable where radial and thrust loads or any combination of both are required to be handled and in dealing with heavy composite loads as in case of automobile parts.
- Thrust Ball Bearings are suitable for arrangements put under light axial loads.
- Ball Bearings- with pressed cages which are light in weight and have high elasticity are suitable at normal speed.
- In very high speed spindles and machines only, bearings with precision tolerances are used.
- Ball Bearings with mechanical brass cages are more suitable at high speeds.
The capacity of the bearing decreases as the speed increases. If a ball bearing operates continuously, its life expectancy measured in hours will obviously be shorter than if operated intermittently.
Some types of bearings can carry only radial loads or thrust loads while some types of bearings can carry both radial as well as thrust loads. So if a bearing which can carry only thrust loads is put under radial loads, it will either break or damage the machine into which it is fitted. So, a proper selection is necessary one major cause for bearing failure lies the improper selection of bearings.
The proper selection of bearings is most essential and will ensure the longer life of the bearings as well as the life of the machine to which it is used.
If the bearing loads with its directions of the ratios of radial and thrust capacity and speed is accurately determined, its life span can also be established. The necessary bearing size can also be determined. The Anti-Friction Bearing Manufacturers Association Standards (AFBMA) provides standardized dimensions, tolerances and fits of ball and roller bearings.
The rolling element bearing failure occurs due to fatigue. AFBMA standard states that the failure criterion is the first evidence of fatigue. The useful life is often used as the definition of fatigue life. The life of the bearing is defined as number of million revolutions that 90% of the bearings will reach or exceed before fatigue failure. AFBMA denotes this life as rating life or L10 life and is used by most of the manufacturer.