Tribology of threaded joints: Sustainable lubricants

The first part of this chapter emphasizes the importance of tribology in threaded fasteners. In particular, the main equations relating the tightening torque to the achievable axial preload in the screw are proposed. Contributions due to friction are discussed and analytically introduced. The following sections are focused on the main factors, including material, surface finishing, lubrication, and repeated tightenings, which usually affect the tribological response of bolted joints. The second part of the chapter tackles the experimental measurement of friction coefficients by suitable tools. In particular, the use of instrumented bushings or specifically developed testing rigs is discussed. Related formulas, geometry, and recommended layouts are also provided. The third and final part of the chapter presents recent research studies related to the effect of suitable lubrication on reducing friction and wear. Significant reduction is observed with multiple tightening and loosening cycles, which is followed by a beneficial effect of shot-peening that reduces friction even in dry conditions. Design guidelines are provided for suitable choice of peening parameters. The effect of joint system tribology on self-loosening is presented.