How Are the Functions of Ball Bearings and Roller Bearings Different?
When you are working with machinery that depends on rotating and reciprocating shafts, you will need to employ anti-friction bearings with rolling capabilities. Rolling element bearings serve the purpose of mitigating axial or radial movement with reduced friction. Depending on your specific application, it could make more sense to either employ a roller bearing or a ball bearing, so it is important you understand which to select before making a purchase.
A ball bearing contains a row of balls in a carrier between its inner and outer raceways. Often there are also seals or shields situated within the raceways for the purpose of keeping dirt out and grease, which is used for lubrication, in. To install a ball bearing, one should lightly press the shaft into the inner race while the outer race is held in a housing element. Designs are available for handling pure radial loads, pure axial (thrust) loads, and combined radial and axial loads.
Due to their design, each of the balls within ball bearings only make contact with the shaft at one point. As the balls roll in and out of the load zone, they deform slightly and then return to their normal spherical shape; however, ball bearings do not have infinite lives and eventually fail from fatigue or other causes. Instead, they are designed with a specific lifespan in mind, with the expectation to fail after a set number of revolutions. When looking to purchase a ball bearing, you will find both single-row and double-row ball bearings available in different bore sizes for different applications. Double-row ball bearings are useful for handling two directions of thrust load. By adding additional balls between the races, one can create the so-called full-complement bearing, that of which makes use of as many balls as possible to increase radial-load capacity. Despite the strength, one drawback of this design is that their tendency to wear is much higher.
The main difference between ball bearings and roller bearings is that roller bearings make linear contact rather than point contact. There are many designs available based on the shapes and angles of their shafts and rollers, but all work with straight-edged rollers instead of rows of balls. The rollers themselves come in several shapes, namely, cylindrical, spherical, tapered, and needle. As the most basic style, cylindrical designs are limited in their load-bearing abilities, while tapered and needle designs can handle greater thrust loads. In particular, needle bearings are especially effective with reciprocating motions, but they are prone to high friction. Furthermore, spherical designs can accommodate misalignment and more thrust, and, when doubled up, thrust in either direction. When using cylindrical roller bearings on shafts with angular misalignment, it is preferred to use two short roller bearings back-to-back rather than one long roller bearing.
While there are many specifics to consider when purchasing, a general rule to follow is to employ roller bearings for circumstances with high shock and impact loading and to employ ball bearings for higher speeds and lighter loads. In general, one will procure and replace ball bearings as entire units, whereas roller bearings can be disassembled for their individual components. Additionally, ball bearings are typically standardized by manufacturers, while roller bearings are less-formally standardized, making them good for custom applications. At the same time, roller bearings are more susceptible to angular misalignment than ball bearings, so it is wise to pay attention to the allowed misalignment for your design, that of which should be provided by the manufacturer.
