limitations are placed on fasteners to ensure a safe and secure joint. Heavy hex bolts will come with a tested proof stress and yield strength, and two distinct figures represent a threshold that is important to overall fit.
Experts will tell design engineers and workers that bolts of any kind need to be tightened to their yield strength, and in some cases beyond that point. That level of tension ensures a more permanent fit and helps to avoid loosening and vibrations.
Why Tighten To That Level?
Heavy hex bolts experience a slight deformity when tightening beyond a certain limit. That may sound like a bad move, when in fact that deformity creates a better fit for the fastener.
As the bolt stretches, the external threads also stretch increasing the pitch of those threads. On the other side, the threads of the corresponding nuts also change, decreasing in pitch. Those converse deformities actually set the bolt in tighter, resulting in a more dependable joint.
This practice is not recommended in all applications. It should only be used in stationary or static joints where the bolt is permanently installed. In general a bolt will not be reused once it has been stretched in the method outlined above.
It is recommended that joints where the bolt is not permanently installed be tightened to the following levels:
Grade 2 fasteners tightened to 40,000psi
Grade 5 fasteners tightened to 60,000psi
Grade 8 fasteners tightened to 90,000psi
It is useful to note that when this method of tightening is practiced you may be able to use smaller heavy hex bolts and still achieve higher levels of strength. This guideline can actually save a significant amount of money over certain applications, given the fact that proper tightening procedures will allow the fasteners to provide the maximum amount of strength.
Assemblers should be trained to tighten heavy hex bolts to their yield strength, and in static, permanent applications even beyond that point. The deforming process provides a tighter fit and ultimately creates a stronger joint. Make a note that torque levels are not the same as clamping load or recommended tension levels.