Grease fitting

Grease nipple on a bearing

A grease fitting, grease nipple, Zerk fitting, or Alemite fitting is a metal fitting used in mechanical systems to feed lubricants, usually lubricating grease, under moderate to high pressure, into a bearing using a grease gun. The fitting is permanently installed by a threaded connection, leaving a nipple connection that the grease gun attaches to. The pressure supplied by the grease gun forces a small captive bearing ball in the nipple 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 intrusion and functions as a check valve to prevent grease escaping back out of the nipple. The ball is almost flush with the surface of the nipple so 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 nipple easily from many angles, yet with a sufficiently tight seal to force the pressured greased to move the ball and enter the fitting, rather than simply oozing past this temporary annular (ring-shaped) seal. Grease nipples are commonly made from zinc-plated steel, stainless steel, or brass.

Besides grease, oil can be fed this way as well.

The patent for the Zerk fitting was granted to Oscar U. Zerk in January 1929, and the assignee was the Alemite Manufacturing Corporation^[1] (thus the eponymous names for the fittings). Today many companies make these grease fittings.

Before Zerk fittings existed, bearings were lubricated in various other ways that tended to be more maintenance-intensive and often to provide 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. Often lubricant was delivered under no more pressure than gravity or a finger push might provide. For example, oil was gravity-fed into the hole, or grease was pushed in. Grease guns to feed the grease with higher pressure existed,^[1] but they lacked any fitting to make clean, sealed contact easily, and they were less widely used than today.

The oil hole or grease hole was usually covered with a cap of some kind to keep dirt out, from a simple plug or screw to a spring-loaded hinged cap. The standard mode of maintenance was to have each machine's operator, or a dedicated oiler, go around adding a small squirt of oil or blob of grease to each and every hole on a frequent basis.

Typical frequencies for oiling were one oil squirt (to every bearing) for each day of use, or in some cases each week. Grease is essentially oil held in a viscous gel or cream whose viscosity is used to hold the oil over time in places where oil alone cannot be supplied constantly enough, and would quickly drip away without the viscous suspension. This trait makes it better for bearings that cannot practically receive new oil on a near-constant basis. Thus a typical frequency for greasing might be monthly, yearly, or every several years.

If the lube schedule was faithfully and conscientiously adhered to, the lubrication quality could be very good. Large marine engines and stationary engines to power whole factories were likeliest to get such top-quality care. But locomotives, rail cars, agricultural implements, and automobiles were less certain of good care, and lubrication quality without constant lubing tended to be intermittent, from mediocre to bad. Attempts to improve upon the simple oil hole concept included putting a small reservoir of oil above the hole which would slowly drip oil into it. An example is the Lunkenheimer oiler seen in the nearby image. Such oilers often incorporated one or more of the following features:

• a wick (to hold the oil and allow it to be drawn down slowly by capillary action, just as an oil-lamp wick or candle wick fed oil or melted wax to its flame
• a fine mesh screen to act as a filter to keep dirt from getting down the hole
• a glass vessel rather than a metal one, which allowed an operator or oiler to see at a glance how low the oil level was

A typical engine oiler
This is one made by Lunkenheimer

For greased bearings, the low pressures of lubrication before the Zerk fitting could often result in a failure of the grease to travel all the way down into all the voids of the bearing. The Zerk fitting improved this penetration, yielding more effective lubrication.

Since the 1920s, the ever-growing dissemination of sealed bearings throughout the manufacturing industries has made the use of grease fittings less common. Sealed bearings are those which are lubricated for life at the factory, and are sealed hermetically enough that the lubricant will not be lost or dirtied. However, grease fittings are far from obsolete, and much new machinery is built with them every year (e.g., tractors, lawnmowers, industrial plant, and still a few car and truck parts), because as long as maintenance is even minimally attended to (via occasional lube jobs where new grease is pumped into the bearing), this type of bearing and lubrication setup is cost-effective, simple, and long-lasting. However, total neglect of maintenance shortens lifespan.

Thus grease nipples are rarely found on today's consumer goods, appliances, or cars, because maintenance-free products have more sales appeal to the general public. But they are still commonly used on industrial, agricultural, and mining equipment where shaft diameters exceed 20 millimeters, as well as electric motors larger than 5 kilowatts. One of the reasons for this is that commercial users have plans and enforcement for lubricating schedules, so the expense of sealed bearings may be unnecessary. For example, in ship or power plant MRO, the management will ensure and document that an employee does the lubrication rounds as needed, whereas individual consumers often ignore maintenance needs. In commercial use, as long as the lubrication rounds are being added to the workload of an already-needed employee, thus not expanding the payroll, this is the less expensive route.

Zerk fittings are commonly found on marine engines. Besides providing fresh lubrication, pumping grease into a Zerk fitting on a boat motor or lower unit expels moisture that would otherwise cause corrosion.^[2]

References

1. ^ ^{a b} Lubricant fitting. US Patent 1697217
2. ^ Marine Engine Digest