Tightening system for blind fasteners

Abstract

A tightening and control system is disclosed for automatically tightening a blind fastener assembly to a desired tightened condition in a workpiece, where the assembly 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 assemblies are the types which first form an outwardly flared bulb adjacent the blindside surface of a workpiece and thereafter are tightened against the blindside surface to a predetermined clamp load condition. Such fasteners typically exhibit a curve of two related input tightening characteristics, such as torque and rotation, torque and time, or force and stroke. Each curve has a bulb forming region characterized by a generally linear portion followed by a marked drop off in the slope, an intermediate region, and a tightening region having a somewhat similar shape to the bulb forming region. Apparatus is generally disclosed for determining the gradient of the curve through which the assembly is being tightened, determining when a first condition indicative of having formed the bulb has occurred, and thereafter determining when the tightening should be discontinued at the desired tightened condition. In a preferred embodiment, the gradient is used to determine the occurrence of the first condition. The instantaneous force or torque value at this first condition is obtained and stored, and tightening is discontinued when the instantaneous force or torque bears some predetermined relationship to the corresponding force or torque value at the first condition.