The steady-state and dynamic performance of gas lubricated uniform film porous thrust bearings

Abstract

Air lubricated externally pressurised thrust bearings are commonly used in industry since, even at slow speeds, they have exceedingly low frictional coefficients. Conventional capillary or orifice compensated bearings have, however, low load capacities for the high supply pressures and feed rates required and their operating range is often limited by pneumatic instability of the air film. These disadvantages may be overcome by using a porous pad in place of the combination of a solid pad and compen­sating element, ensuring greater positional accuracy and a smaller tendancy to fail through blockage. The present investigation is concerned with methods of predicting the steady-state and dynamic performance characteristics of a uniform film porous thrust bearing. Analytic predictions are made, based on the laminar flow through the bearing of a perfect gas. A series solution of the pressure distribution throughout the porous pad and air film is presented for the steady operation of the bearing. The performance characteristics are presented in design curves for a range of bearing aspect ratio and supply pressure. The complexities involved in the time dependent situation prevent an analytic solution. In order to obtain theoretical predictions of the bearing dynamic characteristics a complex numerical technique was developed. Theoretical predictions are based on the assumption that the bearing can be modelled as a linear dynamic system. The design of a porous thrust test bearing and associated equipment as described. The experimental results for steady operation are presented as dimen­ sionless performance characteristics as represented by load capacity and mass flow rate. Good agreement is obtained with predictions from the analysis. The experimental results for the forced vibration tests are presented as dimensionless performance character­ istics as represented by dynamic spring force and damping force and it is found that the simple linear model breaks down for large amplitudes of vibration.