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Found this on the web quite a while ago. It was in response to a question about bolt tightening methods.This is the reponse from an automotive engineer. I don't remember where or who. Also attached is an article from motor magazine on proper torque wrench use.

Bolt Torque
Some assemblies are not critical and can be fastened together by just tightening a bolt with so much torque. Other joints need to be fastened together to a given load so it can withstand the service loads and external loads as well. External loads can be generated by such phenomena as temperature change, either hot or cold or both. The loads can be varied by using dissimilar materials together during a temperature change. A thermostat can be made of dissimilar metals and is called a bi-metal temperature switch because as it is heated or cooled the movement caused by forces generated by the bi-metal either breaks or makes contact in an electric circuit. In a joint of different materials the materials will move relative to each other by the difference of their coefficients of thermal expansion. These joints require a clamping load that is more precise than most joints require. A bolt has to stretch to be utilized as a spring, and a bolt is a very stiff spring, it has to be tightened in a manner that will give repeatable clamping load and be fairly evenly distributed around the joint if it has multiple bolts. As a joint is moved by temperature forces the parts scrub against each other. The parts are being pressed together with a large force from the bolt and are generally under high pressure. The shearing forces generated by the temperature excursions are resisted by the shear load in the joint. These can be calculated by shear load equals friction times the normal load times the number of fasteners in the joint. I’ve broken some more ice, and now we have to talk about one of the biggest variables that affects the proper tightening of a joint...friction...

When you tighten with a torque wrench, you are in effect measuring the amount of friction being generated by the mating surfaces in the joint and as the clamping load is increased so is the torque. You can see that there is some relationship between torque and clamping load. This relationship is determined by the amount of friction in the joint as well as the clamping load being developed... The really big problem is that you cannot measure or predict what the absolute value of the friction is in the joint. If we could , there would be no need for tightening procedures other than torque. We can however, minimize the effect of friction by using lubricants, and zillions have been tried with varying degrees of success. among these have been grease, chicken fat, oleo, mayonnaise, graphite, teflon, plastics and encapsulated formulas that smash and liquify under pressure. however sophisticated they have all fallen short of the goal of absolute friction control. What can we do if we can’t control this continuous variable thing called friction?? The simplest is to utilize the fastener itself to obtain the clamping load by rotating a given angle from some starting point called a threshold. The threshold is usually a low level of torque chosen because the clamping load variation is less at low torque. the bolt is then turned to an angle and the result is an acceptable level of clamping load. And you say ....why? And I say, because now that we are turning through an angle we no longer care about friction and its effect. Remember the thread around the bolt is a continuous spiral and if one turn is straightened out you have an inclined plane that has the height of one pitch of the thread in 360 degrees or one turn.

If we turn the bolt ¼ turn then the threads will engage the mating threads and stretch the bolt ¼ of the value of the pitch, almost. The stretch is actually somewhat less than the ¼ pitch because the joint compresses and uses some of the pitch value, so if the pitch was 0.060" then ¼ pitch would be 0.015" and the joint might use up 0.005" and the bolt would only stretch 0.010". This 0.010" stretch represents a finite clamping load and will be repeatable every time the bolt is stretched that amount. This simple fact is the basis for the tightening procedure we call ‘torque-angle ‘and it is also the basis of the sophisticated tightening procedure called ‘LRM ‘ (logarithmic rate method) used to tighten some fasteners during final assembly.

Torque extension calaculator, unit converters, etc from Belknap Torque products.
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