On Tuesday, August 2, 2016 at 6:26:31 AM UTC-7, Discus 44 wrote:
I have followed this post with quite some interest. I believe there is some additional information that has been missed however. The bolted joint in question uses M12 x 100 bolts with Nylon lock nuts. The actual joint couples a C channel and a square tube. The bolted joint is subject to transverse vibration forces which translates to shear once the joint has loosened.. If the "locknut" nylon has become dry and hardened it may loosen as one is driving down the road and eventually separate. Then the bolt will fall out. Because both elements in the joint are "flexible" as in the square tube being hollow and the C channel can flex or distort some (called fretting) the eventual result is loosening of the fasteners. The design of the joint does include threads in the shear plane which is always a problem. Increasing the length of the fastener and adding washers to eliminate the shear in threads may help. Some of the posts have suggested using a "spacer" which I believe would have to be installed inside the square tube to act as a method to keep the joint from distorting and the prevailing joint torque from loosening as a result of the less flexible / compressible joint, however this would require drilling out the tube and potentially weakening the tube. One would have to probably weld this new spacer and of course it is a galvanized finish which has it's own hazards. And what diameter the spacer? If I have misunderstood the intent of the spacer idea, sorry. OK, so my answer is without significant engineering study, and experiments that could lead to an accident, I suggest checking the existing bolts and nuts for tightness and replacing them with at least 10.9 strength and new "locknuts". The rated torque value for this bolt is 105 N-m or 77 Ft/lbs. Due to the forces translated to this joint this joint will loosen as the miles pile up. Checking the joint periodically should become part of the pre-towing checklist. Keeping a spare couple of bolts and nuts in the kit is also not a bad idea. I have seen on my own trailer the effect of the bolt necking as it was slipping around in the joint and I replaced it with new bolts and locknuts. Phew! I almost had the same experience as Dave. Just for more information If you look at Bolt Science.com for some interesting ways bolts fail. http://www.boltscience.com/

There is one simple solution to bolted joints losing their preload tension (which is what torquing accomplishes): use Belleville spring washers in place or in addition to the flat washers. The washers maintain constant preload tension over a wide range of conditions.

Tom