How are ball bearings made?

Ball bearings have been used to support the rotary motion for hundreds of years. This article describes the modern process of manufacturing the ball bearings that are widely used today, from ball design to part assembly and packaging.

Ball bearings consist of a row or rows of balls held in a cage to hold them in place, between an inner and outer ring called raceways. There are often additional features, such as seals to protect lubricants or screws to hold the bearing in place. However, this article describes only the main parts of a basic bearing: the raceways, the balls, and the cage.

Most bearings and bearing parts are made of steel because it has a long life and high strength under harsh conditions. The most commonly used steel grade is SAE 52100, chromium steel with 1% carbon and 1.5% chromium alloy. This material is stable at temperatures above 250 degrees Celsius and provides a reliable bearing with long life. Some ball cages are made of polyamide plastic to reduce production costs, but this material is not always suitable for harsh conditions, especially for high-temperature applications.

Raceway design

The inner and outer raceways go through a very similar manufacturing process. They start as steel tubing that is cut to the basic raceway shape by automated machinery, with a small amount of additional material left over to account for deformation during the heating process. The outer rings are stamped with the bearing number and manufacturer’s specifications.

The rings are then placed in a heat treatment oven for hardening and heated to approximately 1550 degrees F (840 C) for a period of time ranging from 20 minutes to several hours, depending on the size of the parts. They are then cooled in oil and hardened in a second oven at approximately 148 degrees Celsius. This process makes the raceways hard and durable.

Since the raceways are now too hard to be machined with cutting tools, they are brought to the desired size with grinding wheels. Each part of the rings must be ground to ensure proper bearing width, radius, raceway position, and geometry. Some bearings, such as angular contact ball bearings, require additional grinding later in the process to ensure that the rings have the correct dimensions.

Ball design

Bearing balls go through a very specific and thorough manufacturing process to produce a ball that is perfectly round and smooth, minimizing friction in the bearing. The balls begin as a wire or rod slug that contains the material necessary to form the finished ball. This wire goes through a process called “cold heading,” named for the lack of heat and the original purpose of putting heads on nails, which is still used today. In this process, the ends of the wire are struck against each other, creating a ball with a small ring around it called a “flash.”

The balls are then tumbled to remove the flash. In this process, the balls are repeatedly fed into the grooves between two cast-iron discs, with one disc rotating and the other stationery. The rough grooves effectively tear away the burr, leaving the ball fairly round and slightly oversized for grinding. Next, the balls are subjected to a similar heat treatment process as the raceways to increase durability before being ground to the proper size and roundness.

Finally, the balls are taken to a lapping machine, which polishes them with soft cast wheels, similar to the tumbling process, and low pressure. A polishing compound is used to perfectly smooth the surface without removing any further material. The balls remain in the lapping machine for 8-10 hours to obtain a perfectly smooth ball.

Construction of the cage

Ball cages are a part of the bearing that can be made from a variety of different materials. The outline of the cage is stamped from a thin sheet of metal and then placed in a mold-like structure called a “die” that bends the cage into its proper shape. The cage can then be removed and is ready for assembly. For plastic cages, a process is known as “injection molding” is used in which molten plastic is injected into the mold and hardens.

Assembly

After all the bearing parts are assembled, the bearing can be mounted. First, the inner ring is inserted into the outer ring. Then the balls are inserted and evenly distributed between the raceways. Finally, the cage is installed to hold the balls in position. Plastic cages are easy to snap into place, while steel cages usually have to be riveted together. The bearing is then coated with a rust inhibitor or other special coatings for specific applications and packaged for shipment.

Ball Bearing Alternatives

Ball bearings are inexpensive to manufacture and have proven themselves for hundreds of years. There are alternatives to these bearings, such as bearings that use magnets or compressed air to prevent contact between the two objects altogether, but these technologies are much more expensive to manufacture and operate.

There are also alternative methods for building ball bearings, such as making “space balls.” Bearing balls can be manufactured in a space shuttle, where molten lumps of steel float freely in zero gravity, forming perfectly perfect spheres. However, this process is also more expensive than grinding and lapping, leaving the time-honored methods of polishing on Earth.

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