Elastic vs. Plastic Deformation
Speaking of strength, it’s important here to mention that when you tighten or “torque” a fastener in place, what you are doing is tensioning the shaft of the fastener as a very stiff spring. The tiny degree of elastic stretch you apply to a fastener by tightening it produces the powerful clamping force that holds the parts you are bolting together. When you elastically deform metal, the change of length or shape is not permanent. You are stressing the metal within its elastic range. When you apply even more force, and some of the deformation becomes permanent, the process becomes plastic deformation. Normally, threaded fasteners are used within their elastic range, which brings us to…
The Torque Wrench
Because it’s easy for an inexperienced person to over- or under-tighten critical fasteners, service manuals usually specify some way to directly measure installation torque. This generally means using a torque wrench. We don’t want head gaskets to leak or connecting-rod cap bolts to loosen, and we certainly don’t want the Mad Torquer breaking parts with the usual excuse, “Well, I didn’t want ‘er to come loose, did I?” More is not better. Correct torque is better.
But there are other tightening schemes which also allow us to achieve accurate fastener tension. One, practical only when both ends of the fastener are accessible, is to use a micrometer to directly measure how much tightening has stretched the fastener. This gives highly accurate results. Another is to spin in the fastener until firm initial contact is made, then turn the head through a specified angle. The idea here is that, say, a quarter turn on a 1.0mm pitch fastener will stretch it by 1/4 of a millimeter, which is 0.25 x 0.039 inch = 0.00975 inch.
Yet another method, more accurate still because it is based not on friction but on the properties of the material itself, is torque-to-yield. In this system, typically applied to critical fasteners such as head bolts, a torque-sensing wrench is used to tighten the fastener until it begins to yield (that is, the torque peaks and then begins to fall). Engineers know what force corresponds to the onset of yield for a particular fastener size, material, and heat-treat, so this method gives high accuracy, especially in automated assembly. Needless to say, fasteners tightened in this way cannot be reused.
Kevin Cameron
Cycle World
The rest of the article can be found here:
The Nuts and Bolts of Nuts and Bolts
Without threaded fasteners, your favorite motorcycle would just be a bunch of loose parts.
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