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O.K. Here's the deal on torque from someone who's spent a lot of time dealing with the scientific end of it. :00000060:
When you torque a bolt, you're doing one of two things; you're either stretching the bolt itself to supply the clamping load, or you're distorting the threads on the softest piece (bolt or hole) to provide the clamping. Most of the time, the recommended torque is based on the clamping load needed and what is required to keep the bolt from backing out or breaking. Usually the size and grade of the bolt determine this, but you do find custom applications.
Over time, the bolts will succomb to metal fatigue due to the stretching and break anyway, just like a paperclip. The amount of torque required can be augmented by the use of washers (spreading the load over a larger area), lock washers and Loctite (to reduce the possibility of a bolt backing out). However, the use of washers and Loctite should be in the design from the beginning. I wouldn't recommend adding one to your application if it wasn't originally used unless you've elongated the clearance hole in the workpiece being clamped.
Another possible reason to add a lockwasher later in a part's life is when the original threaded hole was threaded by the bolt and if the bolt is long enough to still provide 3 full threads of engagement. Thread-cutting bolts usually don't cut the threads very deep, and therefore the hole's threads are more likely to strip out when re-installing the bolt.
Tightening a nut is basically the same, but the nut's threads should be softer than the bolt/stud if the bolt/stud isn't easy to replace. Most of the time though, the nut will be made of the harder material to provide the maximum clamping load by stretching the bolt or stud.
I said "nut" and "stud" in the same sentence.
Anyway, that's our science lesson for the morning. Everyone is dismissed for recess! :trink40:
