Engineers design joints to withstand a certain amount of clamp load, but clamp load is not easy to measure when assembling a joint, so torque is specified. The assembler knows they have a “good” joint when the proper torque is achieved, indicated by an audible click of the wrench, or a green light.
Now consider what happens if too much torque is applied, and the joint starts to yield. If the bolt is the weaker joint member, it will begin to stretch, necking down in the threaded portion, eventually breaking before it ever reaches the prescribed torque. In this case, the assembler stops his work and raises a red flag – something is wrong and the problem is addressed.
In the second scenario, we again apply too much torque and the joint starts to fail, but the nut or tapped hole begins to yield instead of the bolt. As the internal threads start to fail, friction and heat are created, and galling or seizing of the threads may occur. In some instances, we may achieve the click or green light, indicating that we have a “good” joint and the assembler moves on to the next job, not realizing that yielding has occurred. Once the product is put to use, the service loads may be enough to cause a catastrophic failure of the already compromised joint.
So, which is stronger – the nut or the bolt? As a rule of thumb, always make sure that the nut (or nut member) is stronger than the bolt!
Check out our thread engagement length calculator on www.bossard.com to make sure that your joints are designed properly.