In the industrial sector, it’s not uncommon to have uncertainty regarding the integrity of a bolted assembly. A common check performed to assess whether the nut has been tightened to the correct torque, or if loosening has occurred, is the measurement of residual torque. In this article, we will explore the principle of this method and its limitations.
What is residual torque?
The residual torque method involves measuring the torque required to turn an already tightened bolt or nut by a small angle, typically 2 to 5 degrees, in the tightening direction. This method is the standard approach used by most industries to assess the torque value of a previously tightened fastener. The advantage of this approach is that you do not need to retighten the fastener after the verification. On the contrary, measuring the torque in the loosening direction is called breakaway torque and is lower than the residual torque for a newly tightened fastener. When loosening the bolt/nut, a lower torque than the tightening torque is required because the threading extension torque component aids in loosening. A commonly used analogy to explain this phenomenon is that it is easier to push an object downhill than uphill. The term residual torque is a misnomer because there is little or no torque present in a tightened fastener. The measured torque is a function of the preload in the fastener, the thread geometry, and the friction conditions present.
How to perform a residual torque measurement?
Normally, the movement of the fastener is felt by the person performing the test. Although not widely used, there are audit torque wrenches that measure the torque value after a few degrees of rotation. Using such a wrench eliminates the human uncertainty involved in the process. Alternatively, the fastener and the joint can be marked, and the head of the nut or bolt is turned until movement is noted. A problem that can arise on longer bolts is that if the nut is only turned a small amount, the friction grip between the nut and the surface is broken without overcoming the friction grip between the threads, leading to a significant measurement error. Several factors influence the reliability of the method to determine that the measured torque is a true representation of the actual value.
The longer the time, the less accurate the measurement
A common reason for performing residual torque measurements is that a concern has been raised about the integrity of the bolted assembly. This may be due to a loosening problem observed on one or more joints, or perhaps due to fatigue failures of the bolts. The longer the time between the residual torque measurement and the installation date, the greater the uncertainty that the measured torque is a true representation of the bolt’s preload. Over time, on most joints, especially those without applied grease, friction tends to increase. This increase in friction results in a higher torque requirement to turn the fastener. Consequently, this can mask that the bolt preload has decreased and is below acceptable limits.
Alternatives to residual torque measurement
The limitations of this approach prompt us to adopt more reliable methods and tools to control the tightening of a bolted assembly. Ultrasonic tightening tension measurements provide crucial information on the presence of a significant structural integrity issue. In general, this is the primary alternative for performing such checks over time to decide what actions, if any, are necessary on the bolted assembly.
