Elastohydrodynamic lubrication of big end bearings under steady and cyclic loads
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Lubrication of the big-end bearing is re-examined under elastohydrodynamic assumptions. All current models were found to be in someway deficient, motivating the development of a new consistent schema. Uniform axial film-thickness assumptions and parabolic axial pressure profiles are combined with curved-beam and planar Finite Element housings to produce a single dimensional EHL model. Body-forces due to con-rod motion were found to be a necessary part of the elasticity implementation. The role of discretisation and surface displacement interpolation errors are investigated under steady load conditions. Under dynamic load, ring, housing and previous experimental works are compared. Increased dynamic journal action from housing distorsion was found to lead to film collapses not present in equivalent rigid bearing analyses; these collapses are likened to vapour cavitation. Correlation of dynamic film-thickness measurements with the elastic solutions are generally improved over rigid predictions. With regard to minimum film thickness, inertial 'ring' solutions gave similar values to housing solutions with and without gas loading; this facilitates non-dimensionalisation. Two separate minimum-film regime were subsequently identified: one in the con-rod neck and a second, at higher load, in the cap. The first condition sees thicker minimum films than the rigid bearing; the second, thinner films with an increased sensitivity to load. Non-dimensionalisation of this transition along with bearing flexibility and load enabled new tribological measures to be developed; the influence of elastic geometry on minimum film thickness is sufficiently well portrayed to make these useful design tools.