Abstract: A fastener tensioning method and apparatus for obtaining a desired degree of tension in a fastener. The embodiment of this invention described utilizes the slope of the change in torque per change in rotation gradient to predict a rotation starting point independent of the rotation that occured in the torquing cycle up to the point of establishing a constant gradient. In the constant slope or gradient area of the torque rotation curve, rotation is considered linearly proportional to the tension in the fastener. A prescribed additional rotation is applied to the fastener utilizing the threads as a micrometer device for stretching the bolt to achieve the desired tension or fastener strain level. With appropriate system monitoring such as the total rotation, the maximum torque applied, the gradient slope, etc., the system can be adapted to inspect the fastener for applied tension as well as accurately control tension level.

Abstract: A fastener tensioning system for use with torque producing power tools, such as, nut runners or the like. The system includes apparatus to rotate the fastener and determine when the fastener has been driven to its yield point in tension, and, thereafter, the fastener is backed out a precise amount of rotation to establish a level of tension in the fastener below the yield point. The method of this invention improves upon the conventional turn-of-the-nut method for fastener tensioning; in that, the precise fastener rotation is started from a relatively well-known and well-defined point as opposed to a arbitrary predetermined low level of torque. In addition, the method assures by inspection the proper condition of the bolt and thread prior to making the final tensioning step.