Nearly all lubricant applications via a grease gun involve a small device called a grease nipple or grease fitting, located on the body of the vehicle or equipment being lubricated. A grease nipple is a small metal fixture that functions as a valve cap, sitting over a grease hole on the exterior of a rotary bearing or similar moving part.
These grease fittings are designed in such a way that the nozzle of a grease gun will fit snugly over the nipple. When lubricant is forced through the grease gun under pressure, a sprung ball bearing blocking an aperture in the grease nipple is depressed, allowing the grease to flow through the hole in the fitting. This hole feeds directly into a channel, which in turn goes straight to the part that requires lubrication.
Grease fittings can either be threaded into the grease hole on the exterior of the part being lubricated or less commonly held in place through friction via a tap-in method. The fact that the ball bearing blocking the aperture of a grease nipple is spring-loaded means that it functions as a basic check valve and will return to a closed position once the flow of pressurised lubricating grease stops. In turn, this prevents dirt, moisture and other unwanted impurities from entering the main rotary bearing and potentially interfering with its smooth operation.
There are several common formats for grease fittings, with the most common being a standard-sized convex shape that fits snugly against the typical concave end of a grease gun nozzle. This combination allows the grease gun to be attached easily from various angles and still make an air-tight connection, meaning fewer instances of lubricant oozing out around the outsides of the grease nipple rather than being forced into the voids as intended.
Kits featuring a variety of grease nipple sizes and grease gun fittings are widely available too. These are a sensible option for professional mechanics in auto workshops, for example, where there’s an increased chance of encountering a nonstandard grease fitting in day-to-day maintenance.