Stress analysis of oil lubricated bearings
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This work covers an investigation of stresses produced in dynamically loaded journal bearings of two different test rigs. The results of a detailed study of all the stresses at different depths produced in a circular geometry representing the bearing shell are discussed. A finite difference program developed by Lloyd and McCallion was modified and used to generate the centreline oil pressure distributions. Finite element stress analysis codes from the work of Hinton  were chosen and modified to enable the program to work in both polar and cartesian form (i.e. for computing the stresses in the radial and tangential directions due to centreline pressure distributions or the stresses in axial direction due to parabolic variation of pressure). Sensitivity of journal bearing performance to some input load parameters) which changes the oil pressure distribution and affects the stresses induced in the bearing shell, are investigated. Numerical simulations of experimental work of other researchers are performed. Different types of stresses induced in the white metal layer were investigated and the one responsible for the failure was determined. Mean and amplitude values of stresses are used in plotting the Modified Goodman Diagram. The stress versus number of cycles to failure (S-N diagram) are plotted. The difference between the time variation of stresses at the point of failure for cases in which the direction of the input load locus is the same as or opposite to the shaft rotation are studied in detail. A crack closure model capable of estimating the stresses at the tip of a surface crack filled with oil is developed and used to relate the fatigue stresses on the bearing shell to the stresses at the tip of a surface crack during its closure. The effects which this model has on the S-N diagram, for a parallel and V-shaped crack assumptions, are studied. Chapter Two presents the hydrodynamic theory of lubrication and attempts to solve the Reynolds' equation including the finite difference method of Lloyd and McCallion which is the basis of all the calculation in this work. Chapter Three explains the modification made to the Lloyd and McCallion program, and the finite element program including how it works in both Cartesian and polar forms. Extrapolation, Interpolation, Automatic mesh generating, and some plotting routines are also discussed. Chapter Four shows how the variation of parameters such as temperature, diametral clearance, bearing land length, and the direction of rotation of inputload affects the pressure distribution and eccentricity locus. It also covers the sensitivity of results to slight variation in input load and discusses the type of stress responsible for the failure of a bearing. In Chapter Five, the results of the numerical simulations of the experimental tests of other workers, performed with the help of the bearing performance program and the finite element program (Chapter Three), are explained. Some stress distributions and scattering of the results on the S-N diagram which persuaded us to study crack closure are discussed. Chapter Six explains the finite element stress analysis of a crack filled with oil under the action of surface stresses (crack closure model). The effects which this model has in increasing the stresses at the tip of a crack, by applying it to the results obtained from numerical simulation of Blundell's and Cyde's work and observing the changes which it brings to the S-N diagram and the sensitivity of the final results to small variation of input load data are discussed. Finally, a brief summary and the conclusions are discussed in Chapter Seven.