If we asked a handful of maintenance engineers what exactly they are trying to achieve when they tighten a bolt, it’s likely that we’d receive a wide range of answers.
Strictly speaking, when we tighten bolts, we’re trying to stretch the bolt enough so that the elastic tension within it clamps the assembly together. That’s right – it’s one that’s easier done than said!
In a perfect world we would always recommend measuring the tension in the bolt, as opposed to the torque, because the tension is what we really need to know. In the real world, torque is the simplest way of achieving control in threaded fasteners but it is only an indicator of the tension.
Unfortunately, there is no cheap, easy, reliable way to directly measure tension in a bolt but application of known torque gives sufficient control in the majority of situations.
It is widely accepted that the majority of torque we put into a threaded fastener goes into overcoming friction in the assembly as only between 10 and 15 per cent goes into stretching the bolt to create tension.
Interestingly though, a very slight change in friction can disrupt the torque vs load relationship so if you reduce friction by applying a lubricant, it will result in more tension for a given torque compared with the dry condition. This can be seen very easily using the http://www.norbar.com/Home/Torque-Tension-Calculator on the Norbar website.
It’s very important that maintenance engineers understand this relationship. Torque combined with angle tightening methods can reduce the uncertainty in the tightening process by reducing the reliance on torque control alone.
For every 360 degree rotation of the bolt, the nut advances down the thread by a fixed number of millimetres on a given bolt. This is referred to as the thread pitch. If you bed the assembly together using a low torque value, every degree of rotation after this will be used to stretch the bolt. This almost eliminates the effect that variations in friction have on the tightening process compared with torque control used on its own.
Something to be aware of in torque and angle tightening is that you cannot change the bolt type to a bolt with a different thread pitch as it would invalidate the angle required.
The method is relevant to every fastener type though, so engineers need to keep re-evaluating in every situation to determine the best torque-angle combination.
An electronic torque and angle wrench measures both applied torque and angular rotation. This one tool eliminates the need for using an angle gauge or protractor together with the torque wrench. Ultimately it is also more accurate, faster and will be able to record your results for the purpose of process monitoring, control and traceability.