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Ball Bearings: Races, Balls and Cage
A ball bearing has four parts: an outer race, an inner race, the balls that roll between them, and a cage that keeps the balls evenly spaced. Almost all of it is steel, usually 52100 (1% chromium, 1% carbon), which can be made very hard and tough by heat treating; 440C stainless is used where rust is a problem. The races are turned from tubing, the balls are cold headed from thick wire, both are hardened, tempered, ground and lapped to an almost mirror finish, and the cage is stamped from thin sheet. Even an inexpensive bearing's balls may be out of round by no more than 25 millionths of an inch; precision bearings allow five millionths.
Advanced
Factory process
Instructions
1
1
Turn the races
Turn the races
Cut the basic shape of both races from steel tubing on automatic lathe-like machines, leaving every dimension slightly oversize: the steel usually warps in heat treatment.
Materials for this step:
Bearing Steel Tube (52100)1 as requiredTools needed:
Automatic Turning Machine2
2
Harden and temper the races
Harden and temper the races
Heat to about 843 °C (1,550 °F) for up to several hours depending on size, then quench in oil: hard but brittle. Temper at about 149 °C (300 °F) and let cool in air: hard and tough.
Tools needed:
In-Line Hardening Furnace with Oil Quench
Draw-Back (Tempering) Oven3
3
Grind and lap the races
Grind and lap the races
The hardened steel is too hard to cut, so grind almost every surface: the fitting surfaces round, the sides flat. Grind the ball track, then lap it for several hours with a very fine abrasive slurry to an almost mirror finish.
Materials for this step:
Lapping Compound1 as requiredTools needed:
Bearing Race Grinding Machine
Race Lapping Machine4
4
Cold head the balls
Cold head the balls
Feed thick wire from a roll into a machine that cuts off a short piece and smashes both ends in toward the middle between two dies, without heating. The ball comes out with a ring of "flash" round its middle.
Materials for this step:
Bearing Steel Wire (52100)1 as requiredTools needed:
Cold Heading Machine5
5
Remove the flash
Remove the flash
Run the balls many times round rough grooves between two cast-iron discs, one turning and one fixed, until the flash is gone and they are nearly round. Leave about 0.008 in (0.2 mm) for finishing.
Tools needed:
Ball Deflashing Machine6
6
Harden, grind and lap the balls
Harden, grind and lap the balls
Heat treat and temper the balls like the races. Grind them in the same kind of disc machine with grinding wheels to within a few ten-thousandths of an inch, then lap them 8–10 hours on cast-iron wheels with lapping compound.
Materials for this step:
Lapping Compound1 as requiredTools needed:
In-Line Hardening Furnace with Oil QuenchDraw-Back (Tempering) Oven
Ball Grinding Machine
Ball Lapping Machine7
7
Stamp the cage
Stamp the cage
Stamp the cage halves from thin steel sheet and bend them to shape in a die. (Plastic cages are injection moulded instead.)
Materials for this step:
Steel Sheet1 as requiredTools needed:
Stamping Press8
8
Assemble
Assemble
Put the inner race inside the outer race, pushed as far to one side as it goes, and drop the balls into the gap opposite. Centre the races and spread the balls evenly, fit the cage (steel cages riveted, plastic ones snapped in) and coat the bearing with rust preventive.
Materials for this step:
Steel Rivets1 as required
Rust Preventive1 as requiredMaterials
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Tools Required
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