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How To Use Lock Nuts

LOCK NUTS – Good Vibrations! More Like Shake Rattle & Roll

One of the constant dilemmas we have when working with fasteners, is how to ensure the bolt will stay in place when there is the element of vibration involved.

Let me introduce you to two alternatives; one being a good old faithful – 150 years of recognition in the industry must speak for itself, and the other, a more innovative approach.

The Lock Nut, often referred to as the Jam Nut combines 2 nuts to secure the bolt and has been used effectively for over a century. The art of the construction of the nuts is essential to produce good results, and is often the reason for ineffective locking.


The two nuts consist of a regular sized hex nut and a smaller nut, either a half nut or lock nut is ideal. When combining these nuts, the smaller nut is to be placed first on the bolt and only secured to approximately 25 – 50% of locking tension. The second larger nut is then added whilst holding the first nut in place with a spanner. It is important to not tension the first nut to full torque or the whole application will fail.

As you continue to tighten the top nut, a transfer of pressure occurs, resulting in the bolt thread connecting with the bottom nut, as tightening continues, the threads are literally jammed together – hence the name “Jam Nut”. What you have achieved is the bolt connecting with the bottom of the small nut and the top of the large nut; the bolt is now securely held in position with the ability to resist vibration. Be careful not to over tighten the top nut, as the increased pressure may result in stripping the thread on the bolt.


Having said all of this – I decided to put this into practice and came up with a surprising problem. Although we are told to hold the small nut with a spanner when applying the second nut, a standard spanner is too thick to hold the small nut in place without impeding on the large nut. I thought this probably only applied to small bolts, however the same problem arose with larger bolt and nut sizes. Therefore my conclusion is you need to check your spanner “depth” or file it down in order to follow the procedure recommended. Maybe someone has a suggestion as to how to get around this problem – please feel free to send in feedback on this.

I spoke earlier about a more innovative approach to tackle vibration problems, this is the Nyloc Nut. A layer of nylon is inserted in a hex nut so when tightening, the nylon “squeezes” into the thread of the bolt. Imagine a standard hex nut and bolt – you have 2 threaded non flexible components relying on the grip they achieve when tightened. Two rigid elements literally ground together to try and form a bond. The nylon, being a softer more adaptable material, can adjust itself to “the groove” and give a variant to the pull and take of a hex nut and bolt. Vibration is basically absorbed.

The diagrams will give you a good understanding of what I have explained but as always, our team at Bolts and Industrial are here to talk you through your project and requirements. No matter how trivial your question, we specialise in looking after all our customers both great and small.



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