Hydrostatic thrust bearings

In full-film lubrication, the term "hydrostatic" is commonly used to describe all systems where the load is transmitted through a lubricant film and pressure is maintained by a pump. In practice, the term hybrid is rarely used, and the term hydrostatic bearing (or thrust bearing) is used interchangeably to describe a mechanism whose surfaces are either stationary or moving. However, this does not mean that its performance (lift, flow, etc.) is independent of speed: as we shall see, speed sometimes plays a decisive role.

Although the lubricating fluid may be a gas or a liquid, in this presentation we shall confine ourselves to the case of Newtonian liquids. For gases, please refer to the article Thrust and aerodynamic bearings. in this treatise.

Compared with other types of support, the essential difference is that there is never any contact between the two surfaces, since an external pump allows pressurized liquid to be introduced into the contact zone, thus ensuring the permanent existence of a lubricating film, even at zero speed. This has the following advantages:

• absence of wear ;

• a very low friction coefficient;

• no stick-slip ;

• very high stiffness, enabling precise positioning despite major load fluctuations;

• the absence of stress concentrations, as the pressure in the cell is essentially constant and the load is supported by a large surface area;

• Formal defects of the surfaces involved are of less importance than in the hydrodynamic regime, since the pressure in the cell is a function of the overall flow rate, i.e. of the film thickness distribution and not of the thickness at a single point;

• thermal problems within the lubricant film are very often secondary, as we are dealing with forced flow at high flow rates; thus, the assumption of an isothermal flow regime is justified.

The main disadvantages of hydrostatic systems are their cost, as they require a pump, filters, pressure regulators, etc., and their reliability, as the slightest incident in the supply system can lead to surface destruction.

Because of the large environment required by a hydrostatic device, the cost of its manufacture and the lack of standardization, a hydrostatic system is used when a hydrodynamic bearing or bearing is not suitable for correct operation. The advantages listed above show that the field of application for hydrostatic thrust bearings is vast. Here are a few particular applications:

• telescopes and large...