Ball bearings help reduce friction and improve efficiency by minimizing the frictional contact between machine parts through bearings and lubrication. Ball bearings allow rotary or linear movement between two surfaces. As the name indicates, a ball bearing involves a number of balls, typically steel, sandwiched between a spinning inner race (a small steel ring with a rounded grove on its outer surface) and a stationary outer race (a larger steel ring with a rounded grove on its inner surface). These balls are held in place by a cage or a retainer. The sides often have shield or seal rings to retain the grease or some other lubrication and to keep the bearing as clean as possible. The area of contact between the balls and the moving parts is small, therefore friction and heat is low. Bearings based on rolling action are called rolling-element bearings and contain cylindrical rollers instead of balls, but operate on the same principle.
A bearing can carry loads along its axis of rotation or perpendicular to its axis of rotation. Those carrying loads along the axis of rotation are referred to as thrust bearings. Rolling-element bearings carrying loads perpendicular to the rotational axis are called radially loaded bearings.
Ball bearings are generally used for small to medium size loads and are suitable for high speed operation. The type of bearing used is dependent on the application. An engineer will evaluate a bearing based on load, speed that they can carry this load, and required life expectancy under specified conditions. Friction, start-up torque, ability to withstand impact or harsh environments, rigidity, size, cost, and complexity also are important design considerations.
Some of the different types of bearing include:
- The rigid single row or radial ball bearing, probably the most widely used, is designed so that balls run in comparatively deep grooved tracks, which make the bearing suitable for both radial and axial loads. Sealed versions are lubricated for life and maintenance-free.
- A rigid single row bearing with filling slots for balls contain more balls than the standard. This allows it to withstand heavier radial loads but only limited thrust.
- Rigid double row bearings are good for heavy radial loads and provide greater rigidity.
- Double row bearings appear in a wide variety of applications, ranging from electric motors and centrifugal pumps to electromagnetic clutches.
- The self-aligning bearing has two rows of balls with a common sphered raceway in the outer ring. This feature gives the bearings their self-aligning property, permitting angular misalignment of the shaft with respect to the housing.
- The angular-contact bearing has one side of the outer-race groove cut away to allow the insertion of more balls, which enables the bearing to carry large axial loads in one direction only. Such bearings are usually used in pairs so that high axial loads can be carried in both directions.
Miniature ball bearings are particular useful for the automotive industry for anti-lock braking systems. These systems use a variety of precision small bearings in electric motors. Other applications include throttle assembly, cooling fans, and idle control motors.
A team of scientists from the the Weizmann Institute of Science in Rehovot, Israel, are developing molecular ball bearings from tungsten disulfide.
Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.
Rapoport, L., Yu. Bilik, Y. Feldman, M. Homyonfer, S. R. Cohen, and R. Tenne. "Hollow Nanoparticles of WS2 as Potential Solid-State Lubricants." Nature (June 19, 1997):791-793.
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