This invention relates especially to gas or air bearings, and more specifically to method and apparatus to provide uninterrupted liquid thin film lubrication of at least one surface of an air bearing.
Over the past several years, as a replacement for more commonly known ball bearing assemblies, the design of fluid dynamic bearings has progressed. In a typical ball bearing, the ball bearings are supported between a pair of races which allow relative rotation of the inner and outer pieces. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing inconsistencies. Moreover, resistance to operating shock and vibration is poor because of small contact area and low damping. It is these issues which have led to the search for a replacement bearing assembly such as fluid dynamic bearings (FDB).
In a fluid dynamic bearing, a lubricating fluid such as a gas, or a liquid or air provides a bearing surface between a fixed member and a rotating member, or two relatively rotating members. The most common currently used fluid dynamic bearings include oil or ferromagnetic fluids. Such fluid dynamic bearings spread the bearing interface over a large continuous surface area in comparison with a ball bearing assembly which comprises a series of point interfaces defined by the ball and the race in which it rolls. This enlarged surface area is desirable because the increased bearing surface area reduces wobble or run-out between the relatively rotating members. Further, improved shock resistance and ruggedness is achieved with a fluid dynamic bearing. Also, the use of fluid in the interface area imparts damping effects to the bearing which helps to further reduce non-repeatable run-out.
Development has now been moved on to the use of gas as a fluid in the fluid dynamic bearing. Such gas fluid dynamic bearings have unsurpassed utility as bearings at very high rotational velocities, where highly concentric (low run-out) rotation is required. They are used in gyroscopes, turbochargers, and medical and dental equipment.
However, one difficulty in execution of gas bearings is the problem of startup and touchdown, when the surfaces are in contact. Contact results in wear, and wear particles and wear tracks in a tight bearing with submicron tolerances typical of gas bearings can cause catastrophic failures.
In most execution of such gas bearings, wear of the bearing surfaces is minimized by choosing a material, or pair of materials, that can rub against one another with minimum wear. The problem with this approach is that such materials are almost exclusively very hard and difficult to fabricate. Typical examples of such materials are ceramics, and ceramic-metal composites. Using these materials can make the cost of fabrication become the majority of the cost of the bearings. Another approach has been to apply lubricating or hard coating to the mating surfaces. This adds complexity, and may not be feasible, depending on the geometry of the bearing.
Liquid lubricants are well known to reduce wear between rubbing surfaces dramatically, often by many orders of magnitude. Even a layer as thin as a few molecules can be effective to reduce wear, and liquid or gelled liquid lubricants are used in most bearings other than gas bearings. Liquid lubricants have not been extensively used in gas bearings due to the complexity and difficulty of ensuring that the correct, minute amount of fluid is always present throughout the life of the bearing. Excess fluid can fill up the gap, causing the bearing to stick or malfunction. Too little fluid, or fluid loss, can lead to lubricant starvation and increased wear. Therefore, to overcome these problems, it is important to find a way to controllably apply or dispense a liquid lubricant to be used to lubricate at least one surface of a gas fluid dynamic bearing.
According to the present invention, a method and apparatus is provided for allowing a molecularly thin film to be established on a surface of the gas fluid dynamic bearing. It is further achieved in this invention that the film can be controllably replenished so that the problem of liquid lubricant starvation is overcome.
In one embodiment of the invention, a suitable non-sludging lubricant of low surface tension is held in a porous reservoir within the stationary portion of the bearing. This fluid migrates out of the reservoir to coat the contiguous bearing surfaces.
Alternatively, the lubricant may be held in a porous reservoir within the rotating portion of the bearing; due to centrifugal force, as the rotating portion spins, the fluid is spun out and coats the opposite non-rotating surface.
In a further alternative, the reservoir may be replaced by a singular reservoir such as a simple hole, depression, cavity or groove filled with lubricant.
In another alternative approach, the liquid will migrate by capillary force through natural surface asperities or roughness by texturing the surface to promote capillary migration; in yet another alternative, the distribution of the lubricant is accomplished by evaporation and condensation.
In yet another embodiment, the reservoir, which is as described above, is located in a region of the bearing that reaches a relatively higher operating temperature than the wear surfaces of the bearing. The transport of the lubricant then occurs by evaporation at the warmer reservoir, and condensation at the cooler bearing surface.
In yet another embodiment, the reservoir is located in a region of the bearing that contains gas at a relatively lower pressure than the gas above the wear surfaces of the bearing; once again, transport of the lubricant from the reservoir to the bearing surface occurs by evaporation from the reservoir and condensation at higher pressures.