Design and Performance of an Automatic Control System for Fastener Tightening

Methods of controlling the tightness of bolted assemblies by presetting the torque or bolt rotation are reviewed. It is shown that these produce either inaccurate clamp loads or excessive elongation of the fastener. As a result the tightening operation is often the weakest link in obtaining a reliable assembly.

The development of a system of tightening control that overcomes these problems is described. It operates by sensing the gradient of the torque-rotation characteristic, thus detecting the yield point of the fastener. In principle it requires no preset control torques or rotation angles. Comparative results show that the system increases bolt clamp loads and reduces their scatter very significantly.