How to Use a Jam Nut to Stop Self Loosening on Heavy Hex Bolts
How To Stop Bolts From Loosening
Strong building fasteners like heavy hex bolts and heavy hex screws can come loose with frequent vibration. In many cases, using two nuts will help to lessen or completely stop this phenomenon. For the tightest, most secure fit you, opt for a jam nut and place it properly.
What Is A Jam Nut?
Find the hardware solution you're looking for here.
A heavy hex jam nut is basically a thinner than normal heavy hex nut that, when attached in a certain way, will result in a bolt that is locked tight and resistant to even the heaviest vibrations.
How Does A Jam Nut Work?
Here's where it can get complicated. Using two nuts to achieve a locking effect is something that has been done for over a century. But simply putting two nuts onto the bolt is not the key - it’s the specific process used that creates the locking mechanism.
Order is important. You always want to put the thin (or jam) nut on first against the joist. Tighten it to between 25 and 50 percent of the total tightening torque and only that amount, otherwise you will negate the locking.
While holding the jam nut still so that it will not rotate, tighten the standard heavy hex bolt to its full torque. What will happen is this - when the thick nut comes in contact with the jam nut and begins to tighten, the pressure on the jam nut will be relieved. The bolt threads that were touching the bottom flanks of the jam nut will slowly move until they are touching the top of the flanks.
So you have a fully tightened nut on the outside (thick) with the threads engaged against the bottom flanks of that nut. Then, between the joint and the thick nut is the jam, where the bolt threads are touching the top flanks. As the bolt vibrates and the threads turn slightly, those two will jam against each other and the bolt will not be able to move.
It's important to follow the process exactly in order to achieve the lock. Just placing two nuts on is not enough to hold heavy hex bolts in tightly. But with a heavy hex jam nut tightened just so and a standard heavy hex nut tightened on top, the locking mechanism will work and your fastener will be held tightly no matter what.
About the Author
Started my career in the fastener world in 1969 at, Parker Kalon Corp. a NJ based screw manufacturer located in Clifton, NJ working in inventory control, scheduling secondary production and concluding there in purchasing. In 1971 I accepted a sales position at Star Stainless Screw Co., Totowa, NJ working in inside sales and later as an outside salesman, having a successful career at Star I had the desire with a friend to start our own fastener distribution company in 1980 named: Divspec, Kenilworth, NJ. This was a successful adventure but ended in 1985 with me starting Melfast in August 1985 and have stayed competitive and successful to date. Melfast serves the OEM market with approximately 400 accounts nationally.
