The function of a threaded fastener, such as a screw, is to secure two parts together. It achieves this by exerting a clamping force that compresses the two parts together.
When installed, a screw acts very much like a stretched spring. This stretching of the screw occurs due to the threads pulling the shaft away from the head of the screw. The more the screw is tightened, the more it stretches, and the higher the resulting clamping force will be.
Elastic deformation and yield point
You may have studied the stress-strain behavior of a beam as part of a basic materials science or statics class.
As you pull two ends of a beam away from the center, the beam stretches. When you release the force, the beam returns to its original length, much like a spring. This is called elastic deformation.
At equilibrium, the clamping force equals the tensile stress applied to the fastener.
Above a certain amount of force, the beam deforms permanently and won’t return to its original length. This point is called yield point, or yield strength, and is typically defined as a permanent deformation of 0.2% of the original length. 
Figure 2: the stress-strain relationship for a beam is the same as for a threaded fastener.
The shaft of a screw is a beam, and the threads are the mechanism by which the pulling force can be exercised on the shaft (beam) once the screw is seated.
Like a beam, the screw can undergo elastic deformation, permanent deformation, or complete failure, depending on the amount of pulling force applied.
Clamp force specifications in threaded fasteners
Generally speaking, the clamping force should be equal or greater than the highest expected load that the junction might experience on the field. This ensures that the screw never extends beyond its installed elongation, which could otherwise cause yield deformation or fatigue failure.
All threaded fasteners come with a force rating that specifies the maximum clamping force that they should ever bear when installed, which will always be below the yield-point (yield-strength) so that it retains its properties when removed and re-tightened.
However, when working with threaded fasteners in an assembly process, we are given a torque specification, not a clamping force specification. Let’s see why…
Torque specifications
For any given threaded junction, there is an approximate relationship between the torque applied and the resulting clamping force. The reason we use torque as a proxy for clamping force is that clamping force is extremely difficult and impractical to measure directly, in the context of a production line.
The relationship between torque and clamping force is unique for any given junction, and depends on a number of factors such as the construction materials and the properties of the fastener (thread pitch, diameter, prevailing torque, …). Even between identical parts, the same torque can generate a different amount of clamping force and it is the responsibility of the design engineer to ensure that the torque specification always results in a sufficient amount of clamping force.
Refer to the table below for an example of the relationship between torque applied and clamping force generated:
In general, high precision and repeatability in the torque applied, achieved for instance by using a high-precision electric screwdriver by Kolver, can limit the variability of the resulting clamping torque.
However, variability in the prevailing torque caused by Loctite patches, helicoil inserts, locking nuts, or simply from friction and lot-to-lot variability in the tapped holes and fasteners, can result in significant differences in the clamping force generated by the same amount of torque. In these cases, applying a consistent and precise amount of torque may not be sufficient.
In our next article, we’ll show how you can eliminate the effect of prevailing torque on the generated clamping force by using the prevailing torque compensation function of Kolver’s K-DUCER series.