B O L T
T O R Q U E
Machinery and Equipment MRO
Proper Bolt Torque in
Power Transmission BY MICHAEL DUNN
hreaded fasteners are one of the most common components in machine design. Such fasteners come in a great variety of shapes, sizes, and materials that serve a wide variety of applications across virtually every industry in the world. In all cases, the performance of bolted joints depends heavily on the torque applied during installation. Below, the basic design and function of bolted joints is explained, best practices for tightening threaded fasteners are provided, and the importance of designing around and applying the proper torque spec to any bolt or screw is emphasized.
Figure 2: Tire-style coupling assembled with radial bolt pattern.
Photo credit: ABB Motors and Mechanical Inc.
Although it appears rather rigid, a bolt acts much like a coiled spring, generating a force as it is stretched or compressed. When torque is applied to the bolt, the threads pull the end of the bolt away from the head, stretching it and generating tension in the bolt. This principle also applies to screws, where the fastener is threaded direct-