Although many methods exist to join two or more parts together, the ease of assembly and disassembly provided by threaded fasteners make them the ideal choice for many applications. The object of a threaded fastener, such as a bolt, is to clamp parts together with a tension greater than the external forces trying to separate them. The bolt then remains under constant stress and is immune from fatigue under normal circumstances.
Why then, I hear you ask don’t we just tighten our bolts as much as we possibly can? No chance of anything coming apart then.
Unfortunately, it isn’t that simple. If the initial tension is too high, the tightening process may cause bolt failure. If that bolt is holding a wheel onto a Formula 1 car the consequences are obvious. Equally, if the tension is too low, varying loads act on the bolt and it will also quickly fail.
There is therefore an optimum tension and the most reliable way of ensuring this is by specifying and controlling the tightening torque.
Torque is any force or system of forces that tends to cause rotation about an axis. Imagine someone tightening a bolt using a socket attached to a meter long bar. If they apply 10 kg of force (kgf) perpendicular to the bar they will produce a torque of 10 kgf·m at the axis or the centre of the bolt.
Under the S.I. system of measurement, force is expressed in Newtons (N) rather than kgf. The conversion between kgf and N is x 9.807 so the person is applying 98.07 Newton metres (N·m) of torque.
However, it must also be remembered that any surface treatment of a bolt can have an impact on torque measurement. Untreated bolts or nuts require different calculations than bolts or nuts treated with zinc, cadmium or phosphate. It should also be noted that when a threaded fastener is tightened, the induced tension results in friction under the head of the bolt and in the threads. As much as 50% of the applied torque is expended in overcoming friction between the bolt head and the abutting surface and another 30% to 40% is lost to friction in the thread.
Further information on torque measurement and calculations can be found at www.norbar.com
- By Philip Brodey