towards a fundamental understanding of the tribological behaviour of cam-roller follower contacts
Shivam Alakhramsing is a PhD student in the Department of Mechanics of Solids, Surfaces & Systems (MS3). His supervisor is dr.ir. M.B. de Rooij from the Faculty of Engineering Technology (ET).
Cam-roller followers are part of the valve train mechanisms found in internal combustion engines. Valve train friction losses may contribute up to 10% of the total engine friction losses. The lubricated cam-roller follower unit consists of two lubricated contacts, namely the cam-roller contact and roller-pin contact. While the former is a non-conformal contact, the latter is a conformal contact. The traction induced by the cam-roller contact drives the roller. It is important to study friction between the cam and roller as it influences the engine efficiency and the onset of wear – and thus ultimately lifetime – of components. Understanding lubrication conditions in the cam-roller contact is complicated due to transient variations of the geometry, force and friction in the roller-pin contact. A very important variable, affecting the cam-roller lubrication performance, is the frictional force (which resists the motion of the roller) at the roller-pin contact. The lubricated cam-roller contact studied in this thesis is highly loaded with contact forces up to 16 kN. Important lubrication performance indicators of the cam-roller contact are the pressure, film thickness and frictional force.
This thesis focuses on the factors influencing the aforementioned lubrication performance indicators. Relying on the aforementioned, the factors studied in this work are:
- The influence of axial shape of the roller:The axial shape of the roller and its influence on the generated film thickness, pressure and friction in the cam-roller contact. This was done by means of a smooth surface finite element method (FEM)-based finite line contact elastohydrodynamic lubrication (EHL) model. The model shows the effects of parameters describing the axial shape of the roller, velocity, load and material properties on the lubrication performance indicators.
- The influence of elastic deformation in the roller-pin contact:In order to better quantify the friction, film thickness and pressure in the roller-pin contact a similar smooth surface FEM-based EHL model was developed for the roller-pin contact. The model demonstrates the importance of allowing for elastic deformation of the conformal roller-pin contact and its effect on film thickness, pressure and friction coefficient.
- The influence of roller-pin contact friction in the high slide-to-roll ratio (SRR) domain of the cam-roller contact:At increasing levels of friction in the roller-pin contact the amount of sliding at the cam-roller contact increases, in turn affecting the film thickness, pressure and cam-roller contact friction coefficient. In order to analyse the effects of higher values of roller-pin friction coefficient on the cam-roller lubrication performance, an infinite line contact mixed-thermo-elastohydrodynamic (TEHL) model was developed for the cam-roller contact. The mixed-lubrication model is based on the load-sharing concept and uses real surface roughness measurement data as input. The complete model is based on a smooth surface FEM-based EHL line contact model and a boundary element method (BEM)-based dry rough normal contact solver. The model exposes the influence of roller-pin friction coefficient on non-Newtonian, thermal and surface roughness effects in the cam-roller contact.
- The influence of roller-pin contact friction in the low SRR domain of the cam-roller contact:In order to improve mixed friction predictions under low sliding velocities, the dependence of boundary layer friction on sliding velocity was incorporated into the previously developed infinite line contact mixed lubrication model. This was done by means of a BEM-based dry rough tangential contact solver which was added to the previously developed model. The model shows how the shear stress-slip relationship influences the macroscopic frictional force in the cam-roller contact, under low sliding velocity operating conditions.