Preload from Tightening and Removal Torque

Abstract

This paper presents a novel approach to determine preload in a bolted joint at installation or point of use. This approach uses both tightening torque and removal torque measurements. This work reveals preload is proportional to tightening torque minus removal torque and that taking this torque difference subtracts out friction. Tests with bolts, nuts, lock nuts, inserts and locking inserts are provided to assess this approach, and the results show good agreement with measured preload. The tests reveal the uncertainty in preload determined from this torque difference is less than the uncertainty in preload from tightening torque alone. A useful application of this work is to obtain a desired preload with nothing more than fastener thread pitch and a torque wrench to measure tightening torque and removal torque. This application is illustrated with data. The significance of this application is that preload is determined without any previously acquired torque–tension data as needed with tightening torque alone to achieve preload. In addition, this paper presents analysis and test data for quantifying self-loosening and primary locking in threaded fasteners from tightening and removal torque measurements.