How To

How Grease Fittings Work

Grease fittings, mostly known as Zerk fittings, are metal fittings installed on mechanical systems to feed lubricants into a bearing under moderate to high pressure without overflow or leaks, by using a grease gun. They’ve been used since the 1920s, and they’re far from obsolete. This type of lubrication and bearing setup is simple, cost-effective, and long-lasting. Although the process of grease fittings may sound a bit complicated, below we’ll try to explain a bit more about it so it can help you better understand.

What are grease fittings?

Grease fittings were patented by Oscar U. Zerk in 1929, hence the name Zerk fittings, and the assignee was the Alemite Manufacturing Corporation. Ball check valves to accept grease under pressure from a grease gun already existed, but Zerk’s fitting was an improved style and less vulnerable to dirt.

Simply put, grease fittings help you inject grease lubricant into wear points on different kinds of machinery including cars, trucks, industrial plant equipment, and others. It was introduced as an improved method of lubricating moving parts, and they come in various types including flush mount grease fittings, ball-joint, button head joint fittings, hydraulic couplers, safety fittings, and others. They can be manufactured using stainless steel, brass, and Monel materials.

How do grease fittings work?

A grease fitting is a small nipple that threads into a hole. The nipple also has a ball bearing in the tip to keep the grease in and contaminants out. The nipple connector on the grease fitting is what the grease gun attaches to. Inside the nipple, there’s a small captive ball suspended by a spring, and as grease is pushed into the fitting, pressure is created against the ball that compresses the spring. It functions as a valve that opens under pressure and allows grease to pass through the fitting.

When the grease flow stops, the ball returns to its original position, closing the fitting. In case you notice grease coming out of the nipple connector after greasing, there’s probably a particle preventing the valve from forming a proper seal. Note that in this situation, you’re not supposed to apply grease since it can force the particle into the bearing causing wear and damage. In this case, the grease fitting should be removed, and replaced with a new one.     

Grease gun

A grease gun is a common garage or workshop tool used for lubrication. The standard grease gun creates the right angle to help you press grease into the grease fitting or nipple and usually can hold an entire tube of grease. There are 4 types of grease guns:

  • Hand-powered, where the grease is forced by the back-pressure from hand-cranking the trigger mechanism.
  • Hand-powered, which includes no trigger mechanism and the grease is forced through by the built-up back-pressure.
  • Air-powered, which uses compressed air that is directed to the gun by hoses.
  • Electric, electric motor drives a high-pressure grease pump.

Safety tips for lubing a Zerk

First, make sure that the fitting is clean before you attach the grease gun. You wouldn’t want to get dirt into the fitting.

Second, pump in the grease until you hear or feel increased resistance, and stop.

Third, you should have a lubrication diagram for reference when you are servicing equipment that you’re not familiar with.

Grease fitting threads and how much grease should be put on a fitting?

There are different styles of grease fittings of which 2 are most common, a 1/4”-28 thread and a 1/8” NPT thread. There are also other, less common grease fitting threads, and there are 3 measurements to determine grease fitting threads including whether it is straight or tapered, the number of threads per inch (thread pitch), and thread diameter.

You should also know how much grease should be put on a fitting. Remember, the amount of grease put in a fitting will depend on the size of the part being attached. As a general rule, you can apply grease until you begin to feel resistance while pumping, and remember not to apply until it seeps out of the bearing seals, or other components.

In closing

A grease fitting is a simple technique that if you know how to use correctly, you won’t have any problems. Once you properly learn how to use a grease gun, you can start using grease fitting on the machinery you tend to apply it to. Also, there are many types of grease guns, so make sure you choose the one that it’s easiest for you to operate with.    

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