Any time a machine contains moving components, there must also be a solution to dampen the effect of the wear-causing friction that is created. For this purpose, engineers rely upon bearings. This class of devices is integral to increasing the operational lifespan of equipment from countless industries. Depending on the type and magnitude of stress that is applied to the machine, an appropriate bearing must be chosen in order to obtain maximum efficiency. In this blog, we will discuss the operating principle and design of ball bearings and how they may be implemented to effectively provide motion limitation and friction reduction.
To understand ball bearings' mechanism of action, it is first necessary to review two physical forces that contribute to friction. For example, we will examine the various loads acting upon a cylindrical shaft. Beginning with axial loads, it is the force applied in the same direction as the shaft. This force may either be tensile or compressive in nature, and depending on the source, the location of maximal force load may be centered or askew. Conversely, radial loads are forces which act upon the shaft at a right angle to the axis. Many linear drive mechanisms may have some natural resistance to axial loads, but very few can tolerate even the slightest of radial forces without the aid of a bearing.
Ball bearings are most commonly placed between two machine members which, if not supported, would create significant frictional resistance to motion. Although varying between applications, the most common configuration includes a moving and fixed element. Unique to ball bearings is the combination of three components: balls, races, and cages.
The balls are considered the functional aspect of the bearing in that they are capable of transmitting a load. When a load is applied, either axial or radial, the balls respond by rotating about a circular axis, thereby equally distributing the load across the entire assembly. Depending on the load and thermal demands of an application, balls may be manufactured using thermoplastics or more common materials such as glass, ceramic, nylon, and polypropylene.
Races are a pair of concentric rings that provide a fixed pathway for the balls to travel on. Since the races are the elements that are in contact with the associated components, they are commonly made from a rigid metal or ceramic.
Regardless of the material, bearing balls tend to roll together in unison, thereby lowering their efficacy. As a result, small components called cages are installed between each ball. While they do not support any meaningful load, cages are indispensable in ball separation.
When purchasing any bearing component, it is necessary to anticipate its operational lifespan. The general rule for ball bearings calculates the lifespan to be equal to 1/ (load)^3. As a result, even a three-fold increase in bearing load translates to a 27-fold reduction in operational lifespan. However, it is important to note that these calculations were formulated by researchers studying metal fatigue in the 1940s, and there have been numerous leaps in bearing technology since then.
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