Floating and Bouncing on Oil: The Static and Dynamic Characteristics of Hydrostatic Thrust Bearings

IN A HYDROSTATIC thrust bearing the two flat metal bearing surfaces are held apart by a film of oil two or three thousandths of an inch thick, the oil film being maintained from a pressurized supply. In this way coefficients of friction between the bearing surfaces can be reduced to very low values, usually less than 0.00005. The Mount Palomar telescope weighing nearly 500 tons is supported on three hydrostatic pads and the whole system can be rotated by a 1/12 h.p. clock motor. In machine tool practice the use of hydrostatic slideways enables the moving member to move along straight lines to a tolerance of a few millionths of an inch. The author has been concerned with the design of hydrostatic thrust bearings for machine tool applications. In this case it is not enough to design a bearing merely to lift a given weight, but the bearing must be designed so that its stiffness (i.e. load per unit deflection, as for a simple spring) is adequate and so that the damping ratio of the oil film is nearly critical. The Mount Palomar bearings have theoretically no stiffness at all, and would be quite unsuitable for machine tool applications, or for any other application where the amount of deflection under load is important or where there is a possibility of vibrattion occurring. In this article the author will show how the stiffness and damping characteristics of hydrostatic thrust bearings can easily be found.