Information about grease buttons

A grease gun, also known as a grease fitting, is a metal assembly used in mechanical systems to feed lubricants, usually greases, into the bearing under medium or high pressure using a grease gun.

The grease knob consists of standard knob head grease fittings and metric knob head grease fittings. Grease nipples are designed for high flow of lubricant to provide a smooth working surface for easy connection and removal.

Grease buttons can be permanently installed with a threaded or straight push-on ("hammer") arrangement, leaving a pipe connection to which the grease gun connects. The pressure provided by the grease gun forces the small bearing ball in the assembly to move back against the force of the retaining spring. The arrangement is therefore essentially a valve that opens under pressure and allows the lubricant to pass through a channel, thus entering the bearing cavities. When the pressure is released, the bearing returns to its closed position. The bearing ball excludes dirt and acts as a check valve to prevent grease from leaking back out of the fitting. The bearing ball is almost flush with the surface of the hardware, so the amount of dirt carried into the bearing with the grease can be reduced.

The convex shape of the grease knob allows the grease gun to easily stick to the bearing, yet has a tight enough seal to force the grease under pressure to move and penetrate the bearing ball instead of simply leaking out.

Grease nipples are usually made of galvanized steel, stainless steel or brass. 

The grease gun was patented by Oscar U. Zerk in January 1929. Prior to the existence of Zerk fittings (aka grease knobs), bearings were lubricated by various other methods that generally required more maintenance and were often less efficient. For example, in the 19th and early 20th centuries, a typical machine bearing was a slide bearing with a cross-drilled hole to receive oil or grease.

For greased bearings, low lubrication pressure before the grease button can often result in the grease not being able to get down into all the bearing cavities. The grease seal improved this penetration and resulted in more efficient lubrication.

Since the 1920s, the increasing use of sealed bearings in the manufacturing industry has made the use of grease nipples less common. Sealed bearings are factory lubricated for life and are sealed to prevent loss or contamination of lubricant. However, grease nipples are far from obsolete, they are installed in many new machines (e.g. tractors, lawnmowers, industrial equipment, car and truck parts). As long as maintenance is minimal (with occasional lubrication jobs where new grease is put into the bearing), this type of bearing and lubrication setup is cost-effective, simple and long-lasting.

Norton Commando motorcycles (1967-1975) were fitted with a grease knob on the swingarm, but the factory specified heavy gear oil. For Norton Commando motorcycle owners, this has resulted in frequent use of grease (causing rapid wear on the swivel bushings). The same error has occurred on some machine tool models that have Zerk (grease button) type fittings.