I don’t know how I thought about fasteners before- just didn’t, I guess. But after reading (Max)Pete’s PDF on fasteners, I began to think about fasteners as “friction enhancers”, and started to think more about the friction between piece pulled together rather than the bolt itself. I have included it here (hopefully with his blessing).
Primarily there is the static friction holding two or more pieces together; this is the goal. There is the enhanced friction due to the bolts; these are what we torque. And this is accomplished by the friction in the bolt threads themselves (also static) derived in part by the elasticity of the material. Mentally unwind the threads and see a long, thin incline plane that is essentially a friction plate. The surface area is probably greater than you think, but still astonishingly small.
But what do the bolts need to do? They need to exert enough pressure to counteract any shear force on the main objects; that’s why I think of them as friction enhancers. Aside from that, the bolt must have enough internal friction in the threads so that it doesn’t slip; even with vibration. And operate within its elastic boundaries; not be tightened so tight as to fatigue the metal. But I think this is a range most seasoned mechanics can do by “feel” for non-critical bolts (heads etc. excluded).
Think of a posted speed sign on a curve. You can take the curve faster or slower. But if you take it at, or near, the posted speed you will likely get through unscathed. Still, there is a range of speeds that can get you through the turn.
I have always cleaned my nuts and bolts; usually with WD40 or PB Blaster. I will also flush a bolt hole out. But Gary’s tip about chasing threads was a Homer Simpson “doh” moment for me. I will begin this practice immediately. The cleaner the threads, the more surface area for the friction/ elastic tension between the threads.
Think of dirty threads like gravel or oil on a curve. The posted speed may is fine for a clean road. But throw some gravel in the mix and the posted speed (torque spec) goes out the window. I am thankful for torque values in my manuals. But they are a guideline, not a god.
