The Influence of Material, Hardness, Roughness and Surface Treatment on the Frictional Characteristics of the Underhead Contact in Socket-Head Screws

The present paper aims at determining the effect of several design and manufacturing parameters on the frictional response of the underhead contact in a bolted joint comprising a socket-head M6 class 12.9 screw. The rationale of the research moves from the need to understand the frictional behavior of screw joints used in machine tools as a means to join roller monoguides with the moving carriages of three-axis tooling units. The experimentation takes into account different underhead materials (Steel, Cast Iron, Aluminum), different roughness levels (Ra=0.8, 1.6, 3.2), different surface treatments (No treatment, oxidised, DLC), as well as repeated tightening operations. The experimentation has been run by a specifically developed specimen, consisting of two parts: an instrumented sleeve, equipped with a double array of strain gauges capable of sampling both the axial preload and the underhead torque, and interchangeable underhead washers, with an anti-rotation device, whose material and surface treatment are suitably changed among the different sets. The tightening torque is recorded by a digital torque wrench. The collected data are processed by ANoVa tools, to investigate the significance of each factor, as well as related interactions. The friction coefficients associated with the different operating parameters are a useful tool to support the design task of bolted joints.