The following background information may present examples of specific aspects of the prior art (e.g., without limitation, approaches, facts, or common wisdom) that, while expected to be helpful to further educate the reader as to additional aspects of the prior art, is not to be construed as limiting the present invention, or any embodiments thereof, to anything stated or implied therein or inferred thereupon.
It is known that aerostatic bearings are bearings that use a thin film of pressurized air to provide an exceedingly low friction load-bearing interface between surfaces. The two surfaces do not touch. As they are contact-free, aerostatic bearings avoid the traditional bearing-related problems of friction, wear, and lubricant management, and offer distinct advantages in precision positioning, such as lacking backlash and static friction, as well as in high-speed applications.
The fluid film of the aerostatic bearing is air that flows through the bearing itself to the bearing surface. The design of the aerostatic bearing is such that, although the air constantly escapes from the bearing gap, the pressure between the faces of the aerostatic bearing is enough to support the working loads. Thus, there is a differentiation that has to be made between hydrodynamical bearings, which establish the air cushion through their movement, and hydrostatical bearings, in which the pressure is being externally inserted.
Typically, porous aerostatic bearings utilize porous material such as graphite to allow a uniform air distribution. One major advantage is the lack of metal-on-metal contact. Graphite has a natural lubricity. Graphite can be tuned such that the aerostatic bearing will meet requirements of fluid flow, stiffness, load capacity, and size. The permeability of a porous material is the key to determine the performance of the porous aerostatic bearings.
It is also known that aerostatic bearings, are widely used for high speed and high precision applications. Pressurized fluid is fed through a restrictor (orifice, porous media or other flow throttling devices) into the gap between the bearing and load. The load may include a rotary shaft. The pressurized fluid creates a high-pressurized fluid film to support the load. The advantage of static bearings is that the bearing and load are constantly separated by the fluid film, such that the devices equipped with static bearings run smoothly during startup, shutdown and routine operations with exceedingly low friction. The disadvantage is the need of external supply of pressurized fluids.
There are two types of static bearings available: hydrostatic bearings and aerostatic bearings; the hydrostatic use liquids and aerostatic use gases. Due to the difference in viscosity and density of the lubricating media, the hydrostatic bearings and aerostatic bearings are designed and constructed differently. The liquid with higher density and higher viscosity, such as oil, leads to thicker films, that is, larger bearing clearance. In contrast, the clearance of the aerostatic bearing is very small, often less than 1/10 of the hydrostatic bearings. Obviously, if the hydrostatic bearings are fed by gases or aerostatic bearings by liquid, none of them will work properly or will have the designed loading capacity with the technology known to the public.
It has been disclosed by a patent that oil radial hydrostatic bearings are used to satisfy the radial load while gas thrust bearings are employed to improve the axial position accuracy. Obviously, the present invention is different; it feeds one bearing with vapor, liquid and even their mixture to improve the loading capacity and to retain both axial and radial precision at the same time.
Other proposals have involved aerostatic bearing devices. The problem with these bearings is that they do not provide both enhanced radial load bearing and axial precision for the load. Even though the above cited aerostatic bearings meets some of the needs of the market, a method for increasing load capacity on a porous aerostatic bearing through use of a two-phase fluid; whereby the liquid portion of the two-phase fluid increases a radial load capacity of the aerostatic bearing; and the gas portion of the two-phase liquid enhances the accuracy and speed of the aerostatic bearing is still desired.