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How are balls of ball bearings made?

A ball bearing consists of one or more rows of balls that are held, by means of a cage, between an inner and outer ring known as the raceway, in addition to a number of other features including seals to protect the lubricant, and screws to hold the bearing in place.This article will only review the main components of a bearing, namely the raceway, the balls, and the cage.

Most bearings and bearing components are made of steel, the most commonly used is SAE 52100 bearing steel, a chromium steel alloyed with 1% carbon and 1.5% chromium, this material is stable at temperatures in excess of 120°C. Commonly, cages are made of polyamide plastic, which is less expensive to produce but is not suitable for use in harsh environments, especially high temperature applications.

The inner and outer raceways are manufactured by a very similar process, using automated machines to cut the steel tube into the basic shape of the raceway, leaving a small amount of additional material to address warpage during the heating process, and the outer ring is stamped with the bearing number and manufacturer’s information.

The rings are then hardened in a heat-treating oven and heated to about 1,550 degrees Fahrenheit (840 degrees Celsius) in a time range of 20 minutes to several hours, depending on the size of the part, then cooled in oil and tempered in a second oven at about 300 degrees Fahrenheit (148 degrees Celsius), a process that makes the raceways both hard and durable.

Every part of the raceway must be ground to ensure that the bearing width, radius, seating position, and geometry are correct, and for some bearings (such as angular contact bearings), additional grinding is required later in the process to ensure that the rings are dimensionally accurate.

Bearing balls are manufactured through a very precise process. A perfectly round and smooth ball minimizes friction within the bearing, which begins as a wire or rod containing the necessary material to form the finished ball, and is then subjected to a process called “cold heading”, in which the ends of the wire collide with each other to form a ball. During this process, the ends of the wire collide with each other to form a ball.

The ball is then repeatedly fed into a groove between two cast iron discs, one of which rotates while the other remains stationary. The rough groove effectively tears away the fretting edge, making the ball fairly round but slightly oversized for subsequent grinding, and the balls are then subjected to a raceway-like heat treatment process to improve durability before being ground to the proper size and roundness.

Finally, the balls are moved to a grinder which polishes them using a soft cast iron disk, similar to the tumbling process, with less pressure, and a polishing paste is used to make the surface completely smooth without the need for further removal of material, and the balls are left in the grinder for 8-10 hours to produce completely smooth balls.

The cage is part of the bearing and can be made from a number of different materials, for steel cages or metal cages the cage is stamped from a thin piece of metal and placed in a mold-like structure that bends the cage into the proper shape, the cage is then removed and ready for assembly, for plastic cages a process called “injection molding” is used where molten metal is pressed into the cage to create a smooth surface without further removal of material. For plastic cages, a process called “injection molding” is used, where molten plastic is injected into a mold and then hardened into shape.

After a process such as the one described above, all the bearing parts are complete, after which the bearings can be assembled. First, the inner race is placed inside the outer race, then the balls are inserted and evenly spaced between the races, finally, cages are installed to hold the balls in place, plastic cages snap into place easily while steel cages usually need to be riveted together, and finally, the bearing is coated with a rust inhibitor or other special surface treatment for the particular application and packaged for shipment.

Ball bearings are inexpensive to produce and have been favored for centuries, but there are more efficient constructions of ball bearings, such as magnetic levitation bearings that use magnets or compressed air to prevent two objects from coming into complete contact, which have zero friction because there is no direct contact between the structures and can therefore withstand ultra-high rotational speeds, even if rotation at the speed of light is possible, but they are too costly to produce and operate, and are therefore largely unused.

Balls also have a very sci-fi production process, such as the manufacture of “space ball”, bearing ball in the space shuttle manufacturing, melted steel free floating in zero gravity, the formation of complete and perfect sphere, however, this manufacturing method is also more expensive than grinding molding.