Hydrostatic bearings support heavy loads

When one mechanical component in a machine tool must move relative to another, a bearing is often used to reduce friction between the two components. Examples include a spindle rotating in its housing and a carriage sliding along a guide way to provide linear positioning. The bearings are required to support the load, such as cutting forces and machine component weight, and provide low friction in the direction of motion. They must also provide accurate, repeatable motion and high stiffness.

Some commonly used bearings involve rolling elements—such as balls, cylindrical rollers or tapered rollers—to separate the moving components. These rolling-element bearings have many desirable characteristics, including low cost and compact design, but they also have limited load capacity, low damping capabilities and may require a mechanism for recirculating the rolling elements through the contact zone. In addition, any geometric errors in the rollers or races may increase errors in the motions of the moving components.

When accurate, high-load-capacity, high-damping, low-friction bearings are required, noncontact bearings, such as hydrostatic bearings, are the most desirable. The principle of operation is simple—hydrostatic bearings support loads on a thin film of oil, which separates the moving components.

All images courtesy S. Smith

Figure 1. A hydrostatic bearing with insufficient pressure to lift the load.

Figure 1 illustrates a bearing supporting a table. The bearing body (green) has a pocket between it and the nonmoving guide way (blue). At the moment shown, the oil pressure (red) is not enough to support the load (black), and the bearing and the table are in contact. As the pressure is increased, the force between the two components (oil pressure × pocket area) also increases.

When the pressure is great enough to overcome the load, the bearing lifts off the guide way and a small amount of oil begins to flow from the pocket out through the small gap between the bearing and the guide (Figure 2 below). Because the thin film of oil in the small gap prevents mechanical contact, the only force opposing sliding motion between the bearing and the guide way is fluid friction. This low-friction condition is maintained as long as oil flows.