A grease fitting is a small component that is designed to allow lubricating substances to be applied to bearings, joints and other moving parts. Ported orifices located in each grease fitting can allow pressurized lubricants to pass into a bearing but not escape. This design can also stop unwanted contaminants from entering a bearing or joint, which is a secondary but important function. Prior to the invention of the modern grease fitting, lubricating substances were typically pressed into place by hand or poured in, which can be more time consuming, less efficient and messier than application with a grease gun.
The first grease fittings, which were developed around the year 1916, used screw-on connectors. These early fittings were developed to allow the quick application of lubricating grease to all manner of bearings and other components. In order to accomplish this, a device known as a grease gun was also developed. The grease gun could be screwed on to a fitting and then used to fill a bearing with a lubricating substance very quickly. Later grease fitting designs performed this same function but were much smaller and did not require the screw-on connection.
Modern grease fittings are very similar to the original designs and typically consist of a small metal housing that has a cylindrical channel inside it. The channel contains a spring loaded ball that can keep grease in and contaminants out under normal circumstances. When an operator presses a grease gun onto a fitting and activates it, the pressure of the grease can overcome the spring tension and fill the bearing. This method of application is typically more effective than older techniques of packing bearings by hand, since the pressure tends to force the grease into hard to reach voids.
There are many different types of bearings and other components that can contain grease fittings, though automobiles are one common location that these parts are found. Some automobiles use sealed bearings that do not require any lubrication, while others have one grease fitting in every bearing and bushing in the suspension and steering systems. It is also common to find grease fittings in universal joints, drive shafts and sometimes even speedometer housings or cables. In some cases a vehicle will ship from the factory with plugs instead of fittings. Before the first lubrication service can take place on one of these vehicles, a new grease fitting must be installed in place of each plug.