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What is a Check Nut?

By Christy Bieber
Updated May 17, 2024
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A check nut is used to ensure that a nut, once installed, will stay in place. Check nuts, also called lock nuts, are usually thin and used in instances where there is a high probability of the first nut shaking or vibrating loose. These nuts are designed and used to prevent the first nut from becoming loose and potentially causing damage or injury when using a device. Lock nuts, or check nuts, were designed to replace lock washers, which did not work as well at preventing loosening over time from movement and vibration.

The modern day lock nut is typically made to look like the normal castle nut. There is, however, a nylon inset on the inside portion of the check nut. The nylon inset is the key component of the nut.

When properly installed, the nylon inset on the check nut grips the threads. This allows the nut to take a firmer hold on the bolt or stud it is being applied to. This feature ensures the nut will stay firmly in place and not shake loose due to movement and vibration.

Check nuts will only work as they are supposed to when they are properly installed. To install a check nut properly, it is necessary to have the stud or bolt the nut is being applied to and one or two correct sized wrenches. Next, the user must take the nut in hand.

To install, the user must slide the check nut over the shaft of the bolt or stud and turn it in a clockwise direction until the nylon inset on the inside of the nut catches the first thread. It will be clear when it has done this because the user will feel some resistance and will no longer be able to turn it by hand.

Once the user feel this resistance, he must grab the correct size wrench and turn the nut the rest of the way down the shaft of the stud or bolt with the wrench. This will ensure a tight fit. It must be turned until the lock nut sits just against the other nut. If needed, the second wrench can be used to hold the other end of the shaft of the stud or bolt. A tight fit against the shaft and against the first nut will ensure that the lock nut will perform its key function of keeping the other nut from loosening over time due to movement and vibration.

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Discussion Comments
By Mammmood — On Sep 05, 2011

@David09 - Unfortunately things like that have already happened. I don’t know if entire wings have fallen off but in one incident a wing panel flew off, and cracks in the wing fasteners were implicated.

Nobody was hurt in the incident I am thinking of, but I’m with you; you can’t have less than one hundred percent safety with airplanes.

By David09 — On Sep 04, 2011

Simple things like check nuts and barrel nuts can be life savers in some situations. Some time ago I heard that the FAA found cracks in the barrel nuts of one airplane's wings.

If the barrel nut is cracked, it could to lead to potential damage to the wing and that’s the last thing that you need in flight. I suppose in a worst case scenario the entire wing would fall off, but I think a lot of other cracking would have to take place for that to occur.

Nonetheless, nothing less than one hundred percent safety would satisfy me when getting on an airplane. That's what makes the airplane maintenance so vital in my opinion.

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