Ball (bearing)

Bearing balls are special highly spherical and smooth balls, most commonly used in ball bearings, but also used as components in things like freewheel mechanisms. The balls come in many different grades. These grades are defined by bodies such as the American Bearing Manufacturers Association (ABMA), a body which sets standards for the precision of bearing balls. They are manufactured in machines designed specially for the job.

In 2008, the United States produced 5.778 billion bearing balls.

Bearing balls are manufactured to a specific grade, which defines its geometric tolerances. The grades range from 2000 to 3, where the smaller the number the higher the precision. Grades are written "GXXXX", i.e. grade 100 would be "G100". and lack of defects, such as flats, pits, soft spots, and cuts. The surface smoothness is measured in two ways: surface roughness and waviness.

Size refers to how tight the tolerances are on the size, as measured by two parallel plates in contact with the ball surface. The starting size is the nominal ball diameter, which is the , or theoretical, ball diameter. The ball size is then determined by measuring the ball diameter variation, which is the difference between the largest and smallest diameter measurement. For a given lot there is a lot diameter variation, which is the difference between the mean diameter of the largest ball and the smallest ball of the lot.

Sphericity, or deviation from spherical form, refers to how much the ball deviates from a true spherical form (out of roundness). This is measured by rotating a ball against a linear transducer with a gauge force of less than 4 grams (0.14 oz). The resulting polar graph is then circumscribed with the smallest circle possible and the difference between this circumscribed circle and the nominal ball diameter is the variation.

The manufacture of bearing balls depends on the type of material the balls are being made from.

Metal balls start as a wire. The wire is sheared to give a pellet with a length approximately the size of the desired ball (OD). This pellet is then headed into a rough spherical shape. Next, the balls are then fed into a machine that de-flashes them. The machine does this by feeding the balls between two heavy cast iron or hardened steel plates, called rill plates. One of the plates is held stationary while the other rotates. The top plate has an opening to allow balls to enter and exit the rill plates. These plates have fine circumferential grooves that the balls track in. The balls are run through the machine long enough so that each ball passes through many of these grooves, which ensures each ball is the same size, even if a particular groove is out of specification. The controllable machine variables are the amount of pressure applied, the speed of the plates, and how long the balls are left in the machine.

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