A ZERK is a small fitting on machinery. ZERK fittings work by acting as a one‑way check valve that allows grease to pass through one way, but not the other. Inside the fitting is a ball check suspended by a spring.
The ZERK design, named after Oscar U. Zerk, does not lock the hose coupler of a grease gun, but rather utilizes pressure applied by the grease gun user. In other words, you hold the grease gun on, squeeze the handle, and grease goes into whatever machine you are applying it to.
The patent for the Zerk fitting was granted to Oscar U. Zerk in January 1929, and the assignee was the Alemite Manufacturing Corporation. Before Zerk fittings existed, bearings were lubricated in various other ways that tended to be more maintenance-intensive and often provided less effective lubrication. For example, a typical machinery bearing of the 19th and early 20th centuries was a plain bearing with a cross-drilled hole to receive oil or grease, with no clever fitting at its mouth, or at best a cap or cup. Often lubricant was delivered under no more pressure than gravity or a finger push might provide.
Oscar U. Zerk also invented some panty hose for fatties.