Tribological Properties of Connecting Rod High Strength Screws Improved by Surface Peening Treatments

Abstract

Bolted joints are highly loaded components and serious issues may arise from improper fastening and in particular from too high or too low preload. Friction at the underhead plays an important role, as it significantly affects the achievable preload for fixed and controlled tightening torque. In addition, multiple tightening is usually performed on connecting rod screws, which may be a further source of friction increment. This study investigates the effect of two surface treatments, shot-peening and deep-rolling, on the tribological properties upon bolt fastening. This topic was tackled experimentally and the campaign involved MJ9 X 1 4 g grade 13.9 36 NiCrMo connecting rod screws, in both lubricated and dry conditions. The results, processed by statistical tools, indicate that deep-rolling does not affect friction, whereas shot-peening yields significant benefits. As an effect of the generation of dimples and multiple contacts, it is able to lower (up to 25%) the bearing frictional coefficient in lubricated conditions, also making the friction level independent of the number of re-tightenings. For a dry surface, an even higher friction decrease (up to 30%) is achieved. Without lubrication, the friction coefficient keeps increasing for the incremented number of tightenings, but the increase rate is lowered with respect to the untreated surface.

Research on the relationship between the tightening torque and the preload of composite bolted structures

“…Croccolo et al 27 studied the lubricating properties of acrylic anaerobic adhesives for thread locking. Croccolo et al 28 studied the effect of shot peening and rolling on the friction coefficient of the bolt head. Stefano et al 29 studied the effect of process parameters on the residual torque of the bolt, including the torsional stiffness of the bolt and the connected parts.…”

Effect of tightening process parameters on the T-F curve of bolts in composite structures

“…The medium-strength threadlocker significantly reduced the coefficient of friction, but the high-strength threadlocker had no lubricating effect. The literature 10 investigated the effect of shot peening and roller pressing or rolling on the friction coefficient of the bolt head. The literature 11 investigates the effect of process parameters on the residual torque of the bolt, including the torsional stiffness of the bolt and the part being connected and the coefficient of friction between the support surface and the threads.…”