Control of the Clamping Force in Production
The clamping force is the main factor that is needed in controlling a fastened joint. Unfortunately, for large-scale production, no particular method has yet been developed to allow us to do this. Therefore, instead we must control other parameters that influence the clamping force. In this day in age, the most common method of controlling the clamping force is to control the installed torque to which the fastener is tightened.
Controlling the Installed Torque
The reasons for the widespread use of this method are mainly historic. In the past, accurate control of the clamping force was not so critical because products tended to be over engineered (by today's standards), controlling the tightening torque proved to be a satisfactory method of ensuring joints did not fail. Also, it has been a tradition that most quality checks have been made separate from, and after, the assembly operation. The torque is a parameter that quality inspectors can check after the event.
Relationship Between Installed Torque and the Clamping Force
It's important to realize that in a typical joint only about 10% of the energy spent in turning the fastener is actually converted into the clamping force. The other 90% is lost in friction. It's very important to be aware that any changes of friction in the joint have a great effect on the clamping force. For example, if the average coefficient of friction in the joint changes from 0.10 (lightly oiled) to 0.18 (dry). For the same installed torque, the clamping force is reduced by 50%.
It is also important that only the specified fastener is used to form the joint. The frictional losses will vary between bolts and nuts that have different surface finishes. When using threaded fastener to form a joint, the lubrication used must be controlled. It is common to specify that the fastener be installed lightly lubricated.
Also, ensure that the dimensional tolerances of the parts, that form the joint, are in control. If not, again, the friction in the joint will vary, which affects the clamping force. Friction is not just an undesirable property; it has the important function of stopping the fastener vibrating loose in service. In fact, on many applications parts such as "Nyloc" nuts are used to further increase friction.
The key thought is to realize that there is little point controlling the tightening of torque if the rest of the process is not controlled.