Join Date: Aug 2009
Location: Washington, the state
Carl, when we tighten a threaded fastener, we might be concerned about how hard we squeeze the parts together (clamping force) especially where a gasket is used, and the stretch of the fastener...stretched tight but not beyond the yield point where it does not return to the original size. We really don't care how tight we twist it, but the twisting force, the torque, is the quickly way of tightening the same every time.
Picking a torque spec depends on the diameter of the fastener, the thread pitch, the material, the surface finish, and any lubrication. Friction is part of the torque. Changing the lube , thus reducing the friction, (using antiseize where none was called for, for example) changes the twisting force needed to get the same clamping force or desired amount of stretch. A 20% reduction in the spec'ed torque is a rough rule of thumb for threads where antiseize is used in normally dry threads to avoid over stretching due to the friction reduction. So...use care when antiseize is used where it is not spec'ed.
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