Abstract

A meter for determining stiffness or torque gradient including, in one embodiment, a deflectable coupling connected between an output shaft and a power input shaft, sensing means for developing two separate series of signals representative of the displacement of the input and output shafts respectively, a comparator for determining from the two series of signals a function of the stiffness or torque gradient and, in another embodiment, including a single shaft having an input end and an output end, transducer means associated with the shaft for developing a signal representative of the torque being applied through the shaft, sensing means for developing a signal representative of the displacement of the shaft, and comparator means for determining from the two signals a function of the stiffness or torque gradient. Either embodiment of the stiffness meter may be used for tightening a fastener by including an additional circuit responsive to the comparator output for sequentially storing information representative of the maximum stiffness developed up to any given point during the tightening operation and for producing a stop signal when the instantaneous stiffness has dropped to a predetermined ratio of the maximum stiffness.