When bearings are discussed, people often call them ball bearings. In truth, there are several other types of bearings that don’t even use balls. The most common of these is a roller bearing. It is also a misnomer when people refer to the balls in bearings as ball bearings. The balls are just a component of a ball bearing, which may also contain an outer race, an inner race and a retainer (cage). This discussion is all about the balls used in ball bearings.
Have you ever wondered how the balls are made? Although they look simple and are an uncomplicated shape, the production process and requirements are a bit more challenging. Balls can be made from a variety of materials, including (but not limited to) tool steel, stainless, brass/bronze, titanium, Stellite, Teflon, and even ceramics such as aluminum oxide and silicon nitride. The material is chosen based on the application of the bearing.
Regardless of the material, the manufacturing process is similar for metal balls. We will focus our discussion on through-hardened steel balls.
Perhaps surprisingly, balls begin life as a piece of wire. The steel wire is typically annealed coil stock of a diameter appropriate for the final ball diameter being manufactured. The wire is fed into a machine that cuts off a measured piece and smashes both ends toward the middle. This is called cold heading because no heat is used in this process, which leaves a bulge or flash around the middle of the ball.
The next step in the process is the deflashing operation. The balls are placed in rough grooves between two cast-iron disks. One disk rotates while the other is stationary. After deflashing, the balls are soft ground. This is a process similar to deflashing, but grinding stones are used to improve the precision. Balls remain oversize so that they can be ground to their finished size after heat treatment.
Heat treatment is often performed in a rotary-retort furnace, depending on the specification requirements. If an application, such as aerospace or automotive, requires AMS 2750 temperature-uniformity survey (TUS) requirements, heat treatment will generally be performed in a batch furnace. TUSs are very difficult to do in a rotary-retort furnace.
Small parts, particularly precision parts such as bearing balls, require good heat-treat control. Load sizes are determined by the surface area of the balls based on the maximum surface area that can be quenched in a load. Soak times are determined not by the diameter of the balls in the load but by the packing height of the balls in the basket. The hardening process typically involves the following:
- Austenitizing (hardening)
- Subcooling (deep-freezing)
- Tempering (based on final hardness requirement)
Following heat treatment, the balls undergo several finishing operations. Descaling is followed by hard grinding, which is a slow and meticulous process that produces diameter tolerances as close as +/-0.0001 inch. Lapping and other finishing operations follow, most of which are proprietary in nature.
These are particularly important for high-precision balls, but all balls need to be sized very precisely. Why? Ball bearings are designed with the assumption that all of the balls in the bearing are carrying an equal load, which means they must be the same size. If, for example, one or two balls were slightly larger, the load would be carried only by the larger balls, and the others would be free spinning. This would result in premature bearing failure.
Balls are manufactured in accordance with ABMA (American Bearing Manufacturers Association) standards. The ABMA grade is designated by a number, which indicates a specific combination of dimensional form and surface roughness. Grades 3 to 50 are considered “precision” balls, and grades 50 and 100 are semi-precision. The normal range of sphericity for precision balls is +/-0.000003 for grade 3 and +/-0.00005 for grade 50. From here, the grading and specifying requirements get even trickier and are better left for a more detailed treatment of the subject.
Bearing ball manufacture was undertaken in space aboard the shuttle. Molten blobs of steel were released into the zero-gravity atmosphere. Because a sphere has the lowest surface area of any form, the molten blobs form perfect spheres while they cool and possibly harden (depending on the material). Since space travel is expensive, it’s likely that bearing balls will continue to be made using the tried-and-true manufacturing method discussed here for years to come.