The alternative design bolts incorporate a design feature,which is intended to indirectly indicate tension.The most common type of this bolt is the "twist-off bolt"(tension control bolts).An assembly tool holds this bolt from the nut end.An inner spindle on the tool grips a spline section connected to the main portion of the bolt.An outer spindle on the tool turns the nut and tightens the fastener.When the design torque has been reached,the reaction forces on the spline snap it off.
This type of torque control system allows for a quick reference for inspectiong if the fastener had been installed with a minimum amount of torque.The amount of tension actually developed in each fastener is not directly determined.There are other types of fasteners,some of highly elaborate designs,which work in a similar fashion.
