How Are Bearings Made?
There are four major parts to a standard ball bearing: the outer race, the rolling balls, the inner race, and the cage.
Races
Both races are made in almost the same way. Since they are both rings of steel, the process starts with steel tubing of an appropriate size. Automatic machines similar to lathes use cutting tools to cut the basic shape of the race, leaving all of the dimensions slightly too large. The reason for leaving them too large is that the races must be heat treated before being finished, and the steel usually warps during this process. They can be machined back to their finished size after heat treating.
Surprisingly, the rolling balls start out as thick steel wire. Then, in a cold heading process, the wire is cut into small pieces smashed between two steel dies. The result is a ball that looks like the planet Saturn, with a ring around its middle called "flash."
degrees Fahrenheit (843 degrees Celsius) for up to several hours (depending on the size of the parts), then dipped into an oil bath to cool them and make them very hard. This hardening also makes them brittle, so the next step is to temper them. This is done by heating them in a second oven to about 300 degrees Fahrenheit (148.8 degrees Celsius),
and then letting them cool in air. This whole heat treatment process makes parts which are both hard and tough.
with grinding wheels. These are a lot like what you would find in any shop for sharpening drill bits and tools, except that several different kinds and shapes are needed to finish the races. Almost every place on the race is finished by grinding, which leaves a very smooth, accurate surface. The surfaces where the bearing fits into the machine must be very round, and the sides must be flat. The surface that the balls roll on is ground first, and then lapped. This means that a very fine abrasive slurry is used to polish the races for several hours to get almost a mirror finish. At this point, the races are finished, and ready to be put together with the balls.
Balls
The bulge around the middle of the rolling balls is removed in a machining process. The balls are placed in rough grooves between two cast iron discs. One disc rotates while the other one is stationary; the friction removes the flash. From here, the balls are heat treated, ground, and lapped, which leaves the balls with a very smooth finish.
heated before being smashed, and that the original use for the process was to put the heads on nails (which is still how that is done). At any rate, the balls now look like the planet Saturn, with a ring around the middle called "flash."
are left oversize so that they can be ground to their finished size after heat treatment. The amount of steel left for finishing is not much; only about 8/1000 of an inch (.02 centimetre), which is about as thick as two sheets of paper.
The four parts of a finished ball bearing: inner race, outer race, cage, and ball.
Cage
Assembly
Quality Control
Bearing making is a very precise business. Tests are run on samples of the steel coming to the factory to make sure that it has the right amounts of the alloy metals in it. Hardness and toughness tests are also done at several stages of the heat treating process. There are also many inspections along the way to make sure that sizes and shapes are correct. The surface of the balls and where they roll on the races must be exceptionally smooth. The balls can't be out of round more than 25 millionths of an inch, even for an inexpensive bearing.High-speedor precision bearings are allowed onlyfive-millionthsof an inch.