Friction and Clamping Force

Use of a torque wrench to tighten fasteners will help prevent overtensioning. Overtensioning causes permanent stretching of the fasteners, which can result in breakage of components or fasteners. When torqueing a fastener, typically 80 to 90 percent of the turning force is used to overcome thread, cap, and nut face friction; only 10 to 20 percent results in capscrew or bolt clamping force. About 40 to 50 percent of the turning force is needed to overcome the friction between the underside of the capscrew head or nut and the washer. Another 30 to 40 percent is needed to overcome the friction between the threads of the capscrew and the threaded hole or the friction between the threads of the nut and bolt.

All metals are elastic to some extent, which means they can be stretched and compressed to a certain point. This elastic, spring-like property is what provides the clamping force when a bolt is threaded into a tapped hole or when a nut is tightened. As the bolt is stretched, clamping force is created due to bolt tension. Like a spring, the more a bolt is stretched, the tighter it becomes. However, a bolt can be stretched too far, which will result in shear. At this point, the bolt can no longer safely clamp the load it was designed to support.

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