Tribological properties of connecting rod high strength screws improved by surface peening treatments

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.