The influence of lubrication on the design of yield tightened joints

Abstract

This paper presents the results of a series of tests in which high strength fasteners, socket head set screws (SHCS), were tightened to yield as a means of generating maximum clamping force. The tightening tests were performed with the aid of a microprocessor controlled torque wrench and a torque–tension testing rig. Load cells fitted to the test rig provided an accurate measurement of the joint clamp load and the faster thread and underhead torque distribution under a range of lubrication conditions. Measurements were made during tightening to yield, under static load condition after yield tightening, and during loosening. The results obtained established that the lubrication conditions dramatically affect the magnitude of the maximum clamp load achieved on the joint and the torque distribution on the fastener. The results also enabled an estimate to be made of the coefficient of friction on the various mating surfaces during the tightening and loosening of the fastener. It was shown that the design of bolted joints must take account of the influence of friction if the joints are to operate effectively in service.