Experimental investigation on the friction coefficients for different materials, lubrication conditions and coatings in bolted joints

The present paper investigates the influence of several design parameters on the frictional response of a bolted joint, involving screws from strength grades 8.8 to 12.9, with black oxidization coating or zinc coatings. The experimentation deals with different underhead and nut materials (cast iron, aluminum), lubrication conditions (dry, lubricated, two different threadlockers: Medium and strong), roughness condition in the underhead (cast, machined, painted, underhead washers) and cosidering also the effect of repeated tightenings. The friction coefficients of the actual application, retrieved based on the relevant operating parameters, are needed to support a correct design. The experimentation has been carried out by a purposely developed specimen, consisting of two parts: An instrumented sleeve, with a double array of strain gauges capable of sampling both the axial preload and the underhead torque, and interchangeable underhead washers. The testing apparatus is provided with a device which allows preventing undesired rotation of the washers. The washer material and surface treatment change among the different specimen sets. The tightening torque is applied by means of a 10-120Nm digital torque wrench. The collected data have been processed by the tools of ANoVa. Such tools allowed assessing the significance of each factor, as well as related interactions