Friction and wear in rolling and sliding contacts

Abstract: Mechanical products are always expected to perform better, last longer, be more environmentally friendly and preferably cost less. Rolling and sliding contacts are found in many mechanical products, and friction and wear in the contacts have a direct impact on the products’ ability to meet these demands. Friction directly influences efficiency; low friction is often wanted to minimize power loss and fuel consumption. Wear generally shortens the lifetime, leading to more frequent service stops and increased costs. Increased demands on the products also means increased demands on the contacts in the products; contacts must work with higher loads and less friction, and they must last longer. The combination of increased demands and the high levels of sophistication of many products puts the spotlight on the contacts, and it becomes perhaps more important than ever to be able to predict and optimize their tribological performance. Simulation of systems with rolling and sliding contacts is a useful tool to understand the contact conditions and to help optimize their performance.This thesis is focused on friction and wear of boundary-lubricated, non-conformal rolling and sliding contacts. It presents a 3D brush model for transient friction in rolling and sliding contacts that can handle rough surfaces, varying surface velocities and varying normal load. Friction is simulated in interference mesh gears, cam mechanisms, a system with a roller between two planes and a system with a contact between two discs. Friction is also studied experimentally in interference mesh gears and in the contact between two discs. A wear model based on a generalized form of Archard’s wear law and the single-point observation method is used to simulate wear in the contact between the rocker arm pad and valve bridge in a cam mechanism of a diesel engine.The results show that the friction model can be used to simulate friction in both motion- controlled and force-controlled systems. The model can be used for both detailed contact studies and studies of the overall behaviour of systems with rolling and sliding contacts.Simulations and experiments show that the efficiency in interference mesh gears decreases significantly depending on the combination of mesh force pressing the gears together and the load on the output shaft. It is also seen that the torque loss varies heavily during a gear mesh depending on the position of the gear teeth and the number of contact points. The results from the simulations are consistent with the experimental results.The simulations of the cam-roller contact show that the creep in the contact is low except at high cam speeds when there is a period with high creep when the contact is close to the tip of the camshaft. The simulations of the rocker arm pad and valve bridge show that the contact radii of the wear pad and the position of the centre of the wear pad radii have a strong influence on the amount of wear. The simulations also show that the change of surface shapes due to wear can worsen contact conditions with high normal pressures.The simulations and experiments of force-controlled systems show that contacts can have a strong influence on a system’s behaviour. The contact acts as a spring damper system and can cause oscillations of the system. Simulations show that the oscillations could, at least in part, be explained by the surface roughness. Simulations also show that the creep in the contact is influenced by the contact stiffness and that the contact stiffness is lower for rough surfaces than for smooth. The experiments also show that the creep is higher for a lubricated contact than a dry contact.

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