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What is a Well Nut?

By Lori Kilchermann
Updated May 17, 2024
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A well nut, or jack nut as they are often called, is a fastener used to not only affix one component to another, but to also limit vibration. Made of several types of rubber to suit a variety of uses, the well nut has a wide flange at the top of a barrel-like body with a nut, commonly made of brass, molded into the bottom. The well nut is placed into a pre-drilled hole or a machined opening until the flange is flush with the surface of the hole. A bolt is threaded into the well nut and tightens the embedded nut, causing the body to swell and fill the hole to create a seal. The tightened well nut has now created a watertight seal in the hole that will cushion the attached component due to the rubber body of the nut.

When used in a vibration dampening mode, the well nut is typically mounted onto a leg or mounting bracket of a machine and placed into a blind hole. The mounting bolts are then tightened, causing the rubber body of the jack nut to expand and effectively fill the hole and tighten the nut in place. The mounting bolt inside of the rubber nut is allowed to move within the rubber, thus dampening any vibration. This mounting style is often used with fragile instruments such as compasses and critical gauges and has even been used as engine mounts and motor mounts in all types of vehicles, from motorcycles to automobiles.

The attaching nut used inside of the well nut is commonly made of brass or stainless steel. This prevents rust and corrosion from occurring prematurely when used as a type of water plug. Often, plumbers will use a jack nut to stop the flow of water through a pipe. In this application, the well nut is inserted into the pipe, often while water is continuing to flow through the pipe; as the nut is tightened, the nut expands against the pipe walls and the water is effectively sealed within the pipe. The nut is easily removed in the event that the flow needs to be restored through the pipe.

Other uses for the well nut are to insulate electrical wiring as it passes through a steel firewall in a vehicle and to create a drain or outlet in a metal or plastic container. It is wise to check the compatibility of the rubber with the intended use of the nut and be sure that there are no issues. When used in a vibration dampening application, it is best to occasionally recheck the nut to assure it has not loosened over time.

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