Tightening system

Abstract

A tightening and control system is disclosed for automatically tightening a fastener to a predetermined tightened condition in a workpiece, where the fastener is of the type which exhibits more than one installation region on a graph of two tightening characteristics during a complete tightening cycle. Typical of such fasteners are thread tapping fasteners, such as thread-cutting or thread forming types, which first generate a mating thread in a workpiece material and thereafter are tightened in the workpiece to a predetermined load condition. Such fasteners typically exhibit a curve of two related input tightening characteristics, such as torque and rotation, having a thread forming region characterized by a generally linear portion followed by a marked drop off in the slope, an intermediate region having several possible characteristic shapes, and a tightening region having a somewhat similar shape to the thread forming region. Apparatus is generally disclosed for determining the gradient of the torque-rotation curve through which the assembly is being tightened, determining when a first condition indicative of having formed the thread has occurred, and thereafter determining when the tightening region has been reached. The gradient information is utilized to determine when tightening should be discontinued at the predetermined tightened condition. Typically, this predetermined tightened condition may be the yield point of the assembly. Alternatively, the instantaneous torque value at the first condition is obtained and stored, and tightening is discontinued when the instantaneous torque bears some predetermined relationship to the instantaneous torque value at the first condition.