How Are Ball Bearings Made?
Ball bearings have been used to facilitate rotary movement for hundreds of years. They were first patented by Philip Vaughan to support a carriage axle in 1794, and have since been improved upon and varied to support a multitude of rotary applications. This article will describe the modern process used to create the ball bearings widely used today, from the construction of the balls to part assembly and packaging.
Ball bearings comprise a row or multiple rows of balls, which are held in a cage to keep them in place, between an inner and outer ring called raceways. There are often added features, such as seals to protect lubricants or screws to hold the bearing in place, however this article will just review the main parts of a basic bearing: the raceways, balls, and the cage.
Most bearings and bearing parts are made of steel, due to its durability and strength under strenuous conditions. The most common type of steel used is SAE 52100, a chrome steel containing 1% carbon and 1.5% chrome alloy. This material is stable in temperatures over 250 degrees Fahrenheit and provides a dependable bearing with long operating life. Some ball cages are constructed from polyamide plastics to reduce production costs, but this material is not always suitable for harsh conditions, especially high temperature applications.
The inner and outer raceways undergo a very similar manufacturing process. They begin as steel tubing, which is cut to the basic shape of the raceway by automatic machines, leaving a small amount of extra material to account for warping during the heating process. The outer rings are stamped with the bearing number and manufacturer information.
The rings are then placed in a heat treating furnace for hardening, and heated to around 1550 degrees F (840 C) for any time frame ranging from 20 minutes to several hours, depending on the size of the parts. They are then cooled in oil and tempered in a second oven at around 300 degrees F (148 C). This process makes the raceways both hard and durable.
The raceways are finished using grinding wheels, as they are now too hard to cut with cutting tools, to reach desired dimensions. Every part of the rings must be ground to ensure proper bearing width, radius, race location and geometry. Some bearings, such as angular contact bearings, require additional grinding later on in the process to make sure the rings are the proper dimensions.
Bearing balls go through a very specific and thorough manufacturing process to create a ball that is perfectly round and smooth, minimizing friction within the bearing. The balls start out as a wire or rod slug that contains the necessary material required to form the finished ball. This wire goes through a process called “cold heading,” named for the lack of heat and original purpose of putting heads on nails, which is still used today. In this process, the wire ends are smashed towards each other, forming 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 disks, with one disk rotating and the other stationary. The rough grooves effectively tear off the flash, and leave the ball fairly round and slightly oversized to allow for grinding. Next, the balls undergo a heat-treating process similar to that of the raceways to increase durability, before they are ground down to the proper size and roundness.
Finally, the balls are moved to a lapping machine, which polishes them using soft cast iron disks, similar to the tumbling process, and low amounts of pressure. A polishing paste is used to make the surface perfectly smooth without further removing material. Balls remain in the lapping machine for 8-10 hours to produce a completely smooth ball.
Ball cages are a part of the bearing that can be made of a variety of different materials. For steel or metal cages, the outline of the cage is stamped out of a thin sheet of metal, and then placed into a mold-like structure called a “die,” which bends the cage into its proper shape. The cage can then be removed and is ready for assembly. For plastic cages, a process known as “injection molding” is used, in which melted plastic is injected into the mold and left to harden.
With all of the bearing parts constructed, the bearing is ready to be assembled. First, the inner ring is placed inside the outer ring. Then the balls are inserted and spaced evenly between the raceways. Finally, the cage is installed to keep the balls in place. Plastic cages are easily snapped in, while steel cages typically need to be riveted together. The bearing is then coated with a rust preservative, or other special finishes for specific applications, and packaged for shipping.
Ball Bearing Alternatives
Ball bearings are inexpensive to produce and have been trusted in their job for hundreds of years. There are alternatives to these bearings, such as bearings that utilize magnets or compressed air to prevent the two objects from contact altogether, however these technologies are far more expensive to produce and operate.
Alternative processes to ball bearing construction exist as well, such as making “space balls.” Bearing balls can be created on a space shuttle, where melted blobs of steel are left to float freely in zero gravity, forming completely perfect spheres. This process, however, is also more expensive than grinding and lapping, leaving us with the old and trusted methods of polishing on Earth.