Machine Design

Hydrostatic Bearings

Designing hydrostatic bearings is more complex than for self-acting types because of the greater number of components requiring design decisions. The pressure pads in hydrostatic bearings carry the load.

Most hydrostatic systems (thrust or journal bearings) use several evenly spaced pads so nonsymmetrical load distributions can be handled. To estimate performance, pads can be treated separately. Pressure is maintained in the recess by fluid pumped through a flow restrictor. The pressure lifts the rotor until flow out of the recess and over the land equals flow in. A constant gap is maintained for a given recess pressure and bearing load. The gap establishes the volume of fluid pumped through the bearing. An alternative design is to connect a fluid displacement pump (gear or vane type) directly to each pocket without flow restriction.

Pressurizing pump power can be estimated from the product of pad pressure and total flow through the pads. For an estimate of lubricant flow rate, a gap (or film thickness) is assumed -- typical values are 0.001 to 0.002 in.

Design of flow restrictors influences bearing stiffness, pumping power, supply pressure, and lubricant flow. A flow restrictor is necessary to provide a pressure drop between the supply manifold and the pad recesses to ensure pressure requirements in any given pad never exceed supply pressure. A bearing with restrictors is called a "compensated bearing." Action of the restrictors is such that if the thrust load is centered, pressure in all the pads is equal and lower than pump pressure. If the load is off center, the gap decreases somewhat on the loaded side and flow from the pad there decreases. This results in an increase in loaded-pad pressure and a decrease in pad pressure on the opposite side, so the runner automatically seeks a nearly level attitude.

Three compensators are used in hydrostatic bearings -- orifice, capillary, and variable-flow restrictor. The first two are fixed-flow restrictors; the third is a valve which automatically adjusts flow as it senses pressure differentials between pads. The variable-flow restrictor provides a stiffer bearing system, but is more expensive than fixed-flow restrictors. Sizing of the orifice or capillary for optimum performance is an essential part of the design of a compensated hydrostatic bearing.

Hydrostatic bearing design requires adjustment of a number of parameters including pad geometry, restrictor size, supply pressure, and journal bearing clearance to optimize performance.

TAGS: Bearings
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