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Introduction
Bearings provide either a sliding or a rolling contact whenever relative motion exists between parts of a machine. Sliding contact bearings are referred to as plain bearings and rolling contact bearings are often called antifriction bearings.
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Bearings that provide sliding contact fall into three general classes: radial bearings that support rotating shafts; thrust bearings that support axial loads on rotating shafts; and linear bearings that guide moving parts in a straight line. Radial bearings are also called sleeve bearings and they can either be full journal bearing or partial journal bearing. The former has 360° contact with its mating surface where the latter has less than 180° contact. The relative motions between the mating surfaces of a plain bearing may take place in the following ways:
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1. As pure sliding with any lubricating medium between the moving surfaces.
2. With hydrodynamic lubrication where a film buildup of lubricating medium is produced.
3. With hydrostatic lubrication where a lubricating medium is introduced under pressure between the moving surfaces.
4. With a combination of hydrodynamic and hydrostatic lubrication.
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Antifriction bearings minimize friction by removing any possible sliding between bearing surfaces and replacing all contacts with rolling interfaces. They substitute balls or rollers for a hydrodynamic or hydrostatic fluid film to carry loads with reduced friction. They utilize a separator to space the hardened rolling elements apart. The Anti-Friction Bearing Manufacturers Association Standards (AFBMA) provides standardized dimensions, tolerances and fits of ball and roller bearings.
Below is a summary of the advantages and disadvantages of plain bearings when compared with antifriction bearings.
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Advantage
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Disadvantage
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| Quieter in operation |
High friction between mating surfaces result in high power consumption. |
| Lower cost |
More susceptible to damage from impurities in lubrication system. |
| Require less space |
More susceptible to damage from impurities in lubrication system. |
| Bearing |
life is not limited by fatigue Have more stringent lubrication requirements. |
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Plain Bearings
Plain bearings can be classified into two types: hydrodynamic bearings and hydrostatic bearings. Hydrodynamic bearings attained lift between the mating surfaces by wedging lubricant into the contact area with a relatively high rotational speed. The disadvantage of this design is the lack of lubricant on the surfaces when the shaft begins to rotate. Thus machineries that utilize this type of bearings should not be subjected to a high load during startup.
Hydrostatics bearings utilize an external source to force lubricant into the contact. They are used in heavily loaded and slow moving machines where the rotation speed is not great enough to form full film lubrication. Below is a summary of the most commonly used plain bearings; with the first three being hydrodynamic bearings and the last one is hydrostatic bearing.
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Circumferential Groove Bearings
This type of bearings has an oil groove extending circumferentially around the bearing. The oil is maintained under pressure in the groove. The groove divides the bearing into two shorter bearings that tend to run at a slightly greater eccentricity. This design is most commonly used in reciprocating load main and connecting rod bearings because of the uniformity of oil distribution.
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Pressure Bearings
Pressure bearings employ a groove over the top half of the bearing. The groove terminates at a sharp dam about 45° beyond the vertical in the direction of shaft rotation. Oil is pumped into this groove by shear action from the rotation of the shaft and is then stopped by the dam. In high speed operating, this situation creates a high oil pressure over the upper half of the bearing. The pressure created in the oil groove and surrounding upper half of the bearing increases the load on the lower half of the bearing. This self-generated load increases the shaft eccentricity. Stability under high speed and low-load condition can be attained if the eccentricity is increased to 0.6 or greater. The primary disadvantage of this design is dirt in the oil will tend to smooth out the sharp edge of the dam and impair the effectiveness to create high pressures.
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Multiple Groove Bearings
Multiple groove bearings are sometimes used to provide increased oil flow. The interruptions in oil flow film also appear to give this bearing some merit as a stable design.
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Hydrostatic Bearings
Hydrostatic bearings are used when operating conditions require full film lubrication that cannot be developed hydrodynamically. The hydrostatically lubricated bearing is supplied with lubricant under pressure from an external source. Advantages of the hydrostatic bearing over bearings of other type are lower friction, higher load capacity, higher reliability, and longer life.
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Antifriction Bearings
Antifriction bearings can be categorized to two different configurations: axial ball and roller bearings. They are more desirable than plain bearing due to their lower friction and reduced lubrication requirement. However, the life of antifriction bearing is limited by the fatigue life of the material they are made of and the type of lubricant being used. The types of antifriction bearing are group by the shape of the rolling element and they are ball bearings, cylindrical roller bearings, tapered roller bearings, and needle roller bearings. There are variations within each type of bearings and only the most commonly used ones are discussed below. |
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Single-Row Ball Bearing, Non-Filling Slot
This type of ball bearing is also known as the Conrad or Deep-groove type. It is a symmetrical unit capable of taking combined radial and thrust loads. This type of bearing is not self-aligning therefore accurate alignment between shaft and housing bore is required.
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Cylindrical Roller
These bearings have solid or helically wound hollow cylindrical rollers. The free ring may have a restraining flange to provide some restraint to endwise movement in one direction or maybe without a flange so that the bearing rings may be displaced axially with respect to each other.
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Tapered Roller
Tapered roller bearings are a variation on the cylindrical rollers. They are held in accurate alignment by a guide flange on the inner ring. The shape of the roller is tapered rather than straight right cylindrical. This allows thrust loads to be withstood in addition to the radial loads.
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Needle Roller
Needle bearings are characterized by their relatively small size rollers. The diameter of needle roller is usually less than ¼" in diameter. The length of needle roller can range from 3 to 10 times of its diameter. The loose-roller is the most widely used needle roller and it has no integral races. The needle rollers are located directly between the shaft and the outer bearing bore. This type of bearing is capable of high radial load capacity.
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