What happens as a bolt is tightened?
When using a traditional wrench to tighten a bolt, the torque applied to the nut causes it to slide up the inclined plane of the threads. This relative motion between the nut and the bolt attempts to reduce the distance between the bearing surfaces of the bolt and nut. This dimension is the grip length of the bolted joint. When the joint members within the grip resist, the bolt begins to stretch like a stiff spring, developing tension and simultaneously compressing the components together creating the all-important clamp force.