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.
Highlights
The connecting rod may be regarded as one of the most critical components in an engine
The results of the two experimental campaigns in dry conditions are collected in Tables 3 and 4 with regard to bearing and total friction coefficients
The results retrieved in dry conditions are collected in the bar graphs in Figure 3a,b with reference to bearing and total friction, respectively
Summary
The connecting rod may be regarded as one of the most critical components in an engine. It has the role of transferring loads from the piston to the crankshaft and often operates under high fatigue as well as inertial loads during service. The primary reasons for rod breakage were mainly related to fatigue cycling that triggered crack initiation in zones with too high stress concentrations owing to poor design [6]. As an effect of this too high clamping, and of poor design leading to a too high stress concentration, a strong tensile stress was induced on the rod in the neighborhood of the bolt hole.
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