The advent of automobile, high speed engines and automatic production machinery provided the impetus for extensive research and development of the rolling bearing (also called the antifriction bearing).
Advantage of rolling bearing compared to journal bearing.
1.Starting friction torque is low (not much greater than the running frictional torque).
2.Ease of lubrication either with prepacked grease or with relatively simple oil system.
3.Less axial space for a comparable shaft diameter.
4.Capable to supporting both radial and thrust bearing.
5.Early warning of impeding failure signalled by increasing noisiness at the same speed of rotation.
6.Readily replacable as stock item.
7.Standarization and the employment of close tolerances make preferable their use in the satisfactory operation of cams and gear.
8.Can be preloaded causing a shaft to become stiffer.
9.Can be used for mounting a shaft placed in any position in space.
10.Wide versatility with respect to mounting because they are supplied in special housings.
Disadvantages
1. Greater diametral space required for comparable shaft diameter.
2. Initial cost is usually higher.
3. Noisier in normal operation.
4. Dirt, metal chips and so on, entering the bearing can limit their life causing early failure.
5. Finite life due to evantually failure by fatique.
6. Lesser capacity to withstand shock.
The stresses developed between the ball(s) and the races or the roller(s) and the races are ver complex. We shall describe the most common types of bearing and explain the procedure for selecting a bearing besed upon load and life requirements.
Ball bearing are categorized into three area, (1) radial ball bearing, (2) angular contact ball bearing and (3) thrust ball bearing.
Radial ball bearing primarily designed to support radial loads. This type of construction permits the bearing also to support relatively high thrust load in either direction. In the fact, the thrust load capacity is about 70% of radial load capacity.
Angular-contact ball bearing is the one directional thrust type, which has a single row of balls and is so designed that the centerline of contact between the balls and raceway is at an angle to a plane perpindicular to the axis of rotation. This angle called contact angle.
Thrust ball bearing are designed to carry pure thrust load and if any radial load is present, separate radial bearing must be used.
Roller bearing serve the same purpose as ball bearing, but they can support much higher loads than comparably sized ball bearing because they have line contact instead of point contact. Most type of radial roller bearing cannot resist thrust load of any significant magnitude and with execption of the cylinder type, operate at speeds lower than those for ball bearing.
Cylinder roller bearing comes in a variety of forms. They are available in a wide range of bore sizes and function with rollers having length to diameter ratios from 1:1 to 3:1. The outside diameter of the roller is often crowned to increase the load carrying capacity by eliminating any edge loading. The variety of types available permit the consideration of a wide range of shaft and housing design.
Needle roller bearing are like cylinder roller bearing in that they can withstand high radial loads, but are different in that their roller (called needles) have a much greater length to diameter ratio. Also needle bearings have a much smaller diametral silhoutte than cylinder roller bearing.
Tappered roller bearing is design specifically to withstand high radial load, high thrust load and combined high radial and thrust loads at moderate to high speeds. Tappered roller bearing are ideally suited to withstand repeated shock load that can be expected from service application.
Spherical roller bearing is available in single row, double row or single row thrust types. The important characteristic common to all spherical roller bearing is their self aligning property. The ability of this type of bearings to adjust itself readily to shaft misaligment is found in its fundamental construction, namely, grinding either the inner or outer raceway to a sperical contour.
Basis for Bearing failure.
Evidance of this failure appears as spalling or flaking of a raceway or rolling element. As rolling elements revolve within the raceways, they are subjected to a repeating contact stress. In addition, those areas of the raceways that support the shaft load as the rolling elements pass over them are also subjected to a repeating contact stress.
Because the stresses are greater than the endurance limit if the material, it is obvious that the bearing will have a limited life. In fact, if one consider that pure rolling does not occur in the bearing but that some sliding of the balls or roller takes place, it is readily understood why particles of metal are removed from the raceway surfaces. The removal of this metal is called spalling and, when this condition arises in a bearing, the bearing has failed.
Life, Rating and Basic Load Rating.
Thus the life of a bearing is establish on a purely statisticaly basis and the AFBMA (Anti Friction Bearing Manufacturers Association) has established the following definitions to ensure a uniform understanding and pocedure when selecting a bearing.
1.The life is the number of revolutions (or hours) that the bearing runs before yhe first evidance of fatique develops in the material or either ring or in a ball.
2.The rating life is defined as the number of revolutions (or hours) that 90% identical bearing will complete or exceed before the first evidance of fatique develops (L10).
3.The basic load rating apperantly identical ball bearing with stationary outer ring can endure for a rating life of 1 million (10^6) revolution of the inner rings.
4.The basic static load rating is defined as the radial load which correspondens to a total permanent deformation of rolling element and race (or washer) at the most heavily stressed contact of 0.0001 of the ball or roller diameter.
tobe continue...
