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Prevailing torque compensation strategies

A prevalent phenomenon within the automotive and aerospace industries is prevailing torque during fastening applications. Prevailing torque is defined as the driving torque needed to overcome friction in a threaded fastener.

This value reveals how much friction is left over when operators continue to turn the bolt with a torque wrench. Prevailing torque is often inevitable, and a part of the fastening process. However, it does still need to be addressed and overcome during tightening. In this article, we’ll discuss prevailing torque compensation strategies.

torque-compensation-strategies-chart

Challenges that prevailing torque creates

Though it’s a normal part of the tightening process, operators need to have intuitive, powerful resources to avoid complications that can occur when prevailing torque goes undetected or ignored. Without reliable compensation strategies, prevailing torque can stop the tightening process in its tracks, or it can cause operators to prematurely move on to the next tightening sequence without sufficiently completing the one before. It can lead to damaged threads, missing components, and incorrect torque data and results. To compensate for prevailing torque, we suggest using the PVT compensate strategy on the Power Focus 6000 (PF6000) controller and incorporating the use of torque wrenches that have the power to overcome prevailing torque and provide a properly seated fastener.

torque-compensation-strategy-issues

PVT compensate – a Power Focus 6000 tightening strategy

PVT compensate is a basic tightening strategy that comes on our PF6000 tightening controller. By compensating for the prevailing torque in a joint, it ensures consistent clamp load despite variations in the level of prevailing torque. PVT compensate will calculate the prevailing torque during the rundown portion of the fastening and then drive the target torque plus the prevailing torque. By doing this, operators are always providing a consistent level of torque to the clamp load, as opposed to applying an undefined amount of torque to friction between the threads of the fastener and the workpiece.

PVT compensate is most commonly specified in aerospace applications with prevailing torque fasteners. However, it can be used in any application where fluctuations in friction and prevailing torque in the joint are causing assembly issues.

Torque wrenches with the power to overcome prevailing torque

Another option in compensating for prevailing torque is to use a torque wrench that has enough power to account for and overcome the prevailing torque. We recommend the use of our STwrench or our Tensor STR Electric Nutrunner. The STwrench is a part of our Quality Assurance – QA Platform 4.0 product line. It’s more than your standard transducerized handheld wrench, and it integrates fully with the PF6000 controller platform. This all-purpose wrench is lightweight but powerful, able to perform a wide variety of quality control tests such as residual torque and joint analyses, which include joint behavior and stiffness checks.

The Tensor STR Electric Nutrunner is another manual torque wrench option that can overcome prevailing torque with its patented and proven TurboTight technology. Also fully-integrated with the PF6000 platform, the Tensor STR provides operators with essential feedback during the tightening process, and it handles angle-controlled tightening with very high precision, thanks to its integrated gyroscope with the TrueAngle function. This function eliminates operator influence.

Helping you avoid serious production issues

Without sufficient compensation strategies, prevailing torque can cause a handful of serious issues during production. It can also negatively affect power tools that cannot properly account for or overcome prevailing torque. Possible effects on power tools can include a reduction in rundown RPM because of added friction, increased torque reaction on operators, and a reduction in tool life. With all of this in mind, we recommend the use of our Power Focus 6000 PVT compensate strategy in conjunction with a manual torque wrench that has the power to overcome prevailing torque, such as our STwrench or Tensor STR.

Questions? Contact Atlas Copco today.