Computation and Measurement of Flow Rate and Load Capacities for Graphite-Fed Externally Pressurized Gas Bearings. Interim Report

摘要

High-precision, gas-lubricated bearings for machine tools have been developed. In a few instances, undesirable instabilities develop in the bearings. To study these instabilities, a model for the transient behavior of the lubricating gas in a hydrostatic gas bearing with porous distribution matrix has been described and solved. The model is based on Darcy's law in the matrix and Reynolds' lubrication theory in the film. A surface roughness model is developed for the matrix-film interface. The equations are discretized and solved using a three-dimensional alternating-direction implicit algorithm. Steady-state measurements of load, lubricant flow and bearing distribution are compared to values predicted by the model. In an attempt to explain discrepancies between data and theory, the model is extended to include effects of edge leakage and nonuniformities in the distribution matrix. Lubrication theory is also extended to include the largest neglected terms. A lack of uniformity in the stock matrix material (graphite) is identified as the most likely cause of the discrepancy. Since variations in graphite permeability are random, they cannot be easily incorporated into a design procedure or stability analysis. Thus, further development of an expensive distributed-parameter model to study instabilities cannot be justified. Examination of an approximate lumped-parameter model is recommended. (ERA citation 04:024148)