A bit on threads

 Since bolts and nuts were invented, many ways to keep a tight threaded fastener tight have been invented. There are several styles of lock washers and then there is anaerobic locking compound and safety wire, with of course the ultimate redneck locking device, the welder.

First; why do tight threaded fasteners become loose. The primary culprit is vibration. When you consider that a thread is an inclined plane wrapped around a shaft, and the nut is the mirror image, think when vibration moves the nut ever so slightly around the bolt. The inclined plane causes the nut to move away from the load. After many slight moves the nut is loose. 
Another thread loosener is HEAT. if the nut and bolt is heated there will be minor differences in the total expansion of the two due to thickness and perhaps slightly different materials. Same result.

So... how do we keep our threaded fasteners tight? In some low vibration cases simple prevailing torque will do the job. These cases are rare. There are a number of lock washers such as the toothed, the split, and so forth. These dig into the metal of both sides and resist rotation of the items. Ever torque a bolt with a split lock washer and when removed it had gouged out part of the washer and the item bolted? This is how they lock.
Another method of locking threads is a "Staked" nut. These have either been squeezed a bit to make the threads a bit out of round and they resist side to side motion in vibration. Some have a dent in one flat, same method. Some have 3 or so dents on one end of the nut, same method.
Yet another method is an inserted nut. These have a plastic insert that is not threaded and must be force threaded when installed, same method. Both the staked nuts and inserted nuts lose most to nearly all locking effect after removal and re-installation so a new nut should be used at every installation.
Monomer anaerobic locking compounds are a monomer and when they enter the tread space during installation the lack of oxygen and the presence of a bit of trace copper in the steel cause the monomer to cross link into a hard polymer. Most are an acrylic monomer and become something like plexiglass inside the joint. By totally filling the thread tolerance gaps they prevent side to side movement and therefore are very vibration resistant. These do have a critical limit in that temp limits their use. Loctited a bolt with the Red permanent compound? The bolt will break before it turns, but if you heat to say 450F as long as the heat is present no lock and out it comes.

The granddaddy of locking devices is an aerospace device, soft stainless steel wire called Safety Lock Wire. The bolt heads and or nuts have small holes in the corners of the flats and you lace the wire through each bolt, twist it and lace through the next bolt and so forth. Very time consuming but very very effective, just like that blacksmith made locking device, the cotter and later the cotter pin.Cotter pins are very effective when run through a castellated nut, although one has align the slots with the hole and sometimes that is a problem.

Before you laugh, a weld bead along one flat is a very effective locking device and can be easily cut in most cases with a cutoff wheel in a grinder. If nothing else is working this is effective.

Written by Jeff Reinhardt