Grease fitting

A grease fitting, grease nipple, Zerk fitting, or Alemite fitting is a metal fitting used in mechanical systems to feed lubricants, usually lubricating grease, into a bearing under moderate to high pressure using a grease gun.

Grease fittings are permanently installed by a threaded connection, leaving a nipple connection that a grease gun attaches to. The pressure supplied by the grease gun forces a small captive bearing ball in the fitting to move back against the force of its retaining spring. The arrangement is thus essentially a valve that opens under pressure to allow lubricant to pass through a channel and be forced into the voids of the bearing. When the pressure ceases, the ball returns to its closed position. The ball excludes dirt and functions as a check valve to prevent grease escaping back out of the fitting. The ball is almost flush with the surface of the fitting so that it can be wiped clean to reduce the amount of debris carried with the grease into the bearing. The convex shape of the fitting allows the concave tip of the grease gun to seal against the fitting easily from many angles, yet with a sufficiently tight seal to force the pressured grease to move the ball and enter the fitting, rather than simply oozing past this temporary annular (ring-shaped) seal. Grease fittings are commonly made from zinc-plated steel, stainless steel, or brass.

The patent for the Zerk fitting was granted to Oscar U. Zerk in January 1929, and the assignee was the Alemite Manufacturing Corporation (thus the eponymous names for the fittings). Alemite was already marketing ball check valves to accept grease supplied under pressure from a grease gun, such as for car and truck chassis lubrication points, both for OEM installations and for aftermarket upgrade kits which would screw in as replacements for stock grease cups. But Zerk's fitting was an improved style, less vulnerable to dirt and more forgiving of angled approach. Today, many companies make these grease fittings.

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