Torque performance in the field

What is torque?

By definition, torque is a twisting force, but as it pertains to fasteners, it is the amount of twist we put into that fastener to achieve clamping force with that fastener. Proper torque is the twisting force required to accurately apply the desired clamping force, working within the limitations of the fastener and the materials to be fastened.

What is the reason behind proper torque?

What are we trying to hold together? Different materials call for different torque. Higher loads require higher torque. There is a design parameter that calls for the correct amount of clamping force. The expected performance of equipment is best met when specifications are maintained. Under-torqued conditions normally lead to mechanical looseness. An over-torqued condition can lead to distortion, fastener fatigue, parts fatigue, and structural failure or broken components.

How to store torque devices

Most technicians have been told throughout their career to store a torque wrench at a “0” load. This, in most cases, is incorrect. If the torque wrench is the spring-loaded type, with a rotating handle to set the spring tension, the tool manufacturer normally indicates a specific setting to properly store the device. If left at “0”, the spring is allowed to expand and contract with temperature changes, changes in humidity, or vibration from transport. By having some tension on the spring, those changes are reduced to a minimum effect on the tool’s ability to achieve accurate torque. Newer torque wrenches, like a digital one or the Pall-Pull wrenches, do not have this issue and can be left at the set point for the last use.

How to prepare for proper torque

  1. What is the torque spec? Most equipment manufacturers will give specifications for fastener torque. If not, standard torque charts are available almost anywhere. Specifications are given for different thread diameter, thread pitch, and quality (or grade) of the fastener.
  2. To lubricate or not? Adding any sort of lubrication to the fastener drastically changes the coefficient of friction required to apply that twisting force, which in turn changes the clamping force of the fastener. Most manuals will include a note as to what kind of lubrication is to be used if any.
  3. Proper tooling: Having a tool capable of applying accurate torque is at the heart of the operation. Adding a longer handle to a smaller torque device is not a good way to maintain accuracy. Always work within the design limitations of the tools.
  4. Proper techniques: Torque should always be applied in a smooth, repeatable manner. Jerking motions on the end of the handle yield much higher loads. Applying the old “double click” trick vastly changes the torque on the second click and should not be done. If you want to verify that something is properly torqued, let everything relax, and apply force smoothly. If the fastener does not turn anymore when the device indicates the desired torque, then it was properly torqued

What to inspect for proper torque

Over time, things can affect how the clamping force changes in a fastener. Heat, vibration, load cycles—all of these can reduce the ability of a joint to maintain a working condition. In some instances, regular checks are required, with intervals that open as the joint is constantly found to be properly torqued. The first time a joint is found to be loose, the interval goes back to short-term checks to prevent failure. Sometimes, the simplest indicators work the best. Cracked paint marks, bolt tabs, or even safety wire can give quick inspection points, and ensure that everyone did their part to #Keepitrunning.

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by Diana Pereda