Are loose bolts annoying the crap out of you? It’s time to tighten those threads on the nuts to tighten the bolt. What happens is when you tighten a bolt using a wrench, a torque is experienced by the bolt. And the torque applied makes the nut move around and slide up in the threads of the nut. And because of such motion, the surfaces of nut and bolt come closer to each other. The dimensions of the wrench depend upon the circumference of the bolt. When you tighten the grip on the bolt, the bolt stretches or expands in all directions like a stiff spring. This action results in increasing the clamping force. Consider bolt tightening calculations before tightening.

Tightening Preload

It doesn’t matter if you tighten the nut or its head, the torque applied to it is what matters. Now another terminology to be considered is preload. Preload is the tensile force developed when a bolt is tightened in a fastener. The tension created in bolt results in causing a compressive force within the bolted joints. This compressive force is known as clamp force. If proper preload is experienced by the bolted joint, then no clamping force is created or developed. Issues like fatigue failure or joint separation are not experienced by the bolted joints if clamp force isn’t developed. Sometimes joints self-loosen themselves because of the constant vibrations. The to and fro movement makes the nut move from its original position. And temperature fluctuation might also be responsible for loosening of joints.

There are basically six methods to limit the fastener’s preload:

  • Angle control
  • Torque control
  • Yield control
  • Bolt stretch
  • Heat tightening
  • Using tension indicating procedures

Controlling the fastener’s preload by limiting the torque being applied to it is one of the most popular techniques. The basic procedure is to tighten the bolt up to a benchmarked preload. The preload can be determined by using the formula which states the relationship shared by the bolt tension and the torque. The preload can also be determined with the help of predefined tables. They key is to determine bolt tightening toque calculation correctly.

Torque Control

The bolt on being tightened causes stress on the shank. The torsional stress acts along with elongation strain due to which threads experience torque. Usually, in 75% of the cases, the tightening torque determined from the tables doesn’t consider the torsional stress. It only considers the direct stress being applied to the threads of the nut. Sometimes, when the high friction is encountered, the torsional stress combines with the direct stress and which finally results in fatigue failure. A better approach to avoid this situation would be to determine the torsional stress along with the direct stress. It will give you equivalent stress which will give us a proportion of yield of nearly 90%.

These torque controlling fasteners are used in situations where vibration causes bolt loosening. The torsional stress acting on the bolt shank prevails the torque from loosening the bolted joints. Thus the preload acting on the bolt reduces significantly. The trick is to determine the correct torque values for the fastener i.e. bolt tightening calculations should be on point.


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