Air bearing, in particular for the shaft of a motor spindle

Details Air bearing, in particular for the shaft of a motor spindle Air bearing, in particular for the shaft of a motor spindle

2000-03-28

2001-12-11

Footland, Lenard A. (Department: 3682)

Bearings

Rotary bearing

Fluid bearing

C384S121000, C384S907100, C384S912000

Reexamination Certificate

active

06328475

ABSTRACT:

This invention relates to an air bearing for a rotor adapted to rotate at (high peripheral speed, in particular for the shaft of a motor spindle for driving a machining tool, with provision being made for an air gap between a bearing surface of the rotor and a mating bearing surface of a stationary bearing element, said mating bearing surface having several bearing pockets communicating via bores in the bearing element with a source of compressed air.
Air bearings of the type referred to are known from DE 21 24 918 A1. They are used primarily for supporting shafts rotating at very high rotational frequency and accordingly high peripheral speed in order to be able to cope with the frictional resistance of the bearing and the resultant heat development in the bearing. With air bearings of this type, the rotor is supported by air cushions that are produced with the aid of air supplied under pressure to the air gap between the bearing surface and the mating bearing surface and which prevent contact between the bearing surface and the mating bearing surface. Contactless bearing of the rotor is guaranteed not only while it is rotating but also when it is stationary. Air bearings are highly sensitive to overloading during operation, i.e., with the rotator rotating at high speed. If, with the rotor running at high speed, contact is made between the bearing surface and the mating bearing surface due to overstepping of the maximum bearing load defined by the load-carrying capacity of the air cushions, the friction resulting from the high relative speed between the bearing surface and the mating bearing surface will instantly cause such intense heating and deformation of the metallic bearing materials that the bearing will seize and be completely destroyed. This sensitivity to overloading poses considerable problems, particularly when air bearings are used for supporting the shafts of high-speed motor spindles for driving machining tools, since an error in controlling the motor spindle movement can result in overstepping of the permissible bearing load and hence in the destruction of the motor spindle bearing.
According to W. J. Bartz et al., Luftlagerungen, Expert Verlag, 1982, ISBN 3-8169-0992-2, pages 22 and 99, the materials suitable for manufacturing air-lubricated plain bearings are metals, ceramic materials and plastics. The use of stainless steels to counter the risk of corrosion is considered possible, but these materials are said as a rule to have poor sliding properties. A ceramic material regarded to have good antifrictional and machining properties is metal-impregnated sintered carbon, a porous sintered graphite impregnated with metals such as white metal, lead bronze, antimony or silver. Tests conducted with copper-impregnated manufactured carbon on the bearing side and steel on the shaft side are also mentioned in connection with improving the antifrictional properties of an airbearinged, hand-held turbine. In this case the bearing material was found to have a greasing tendency, which resulted in clogging of the air bearing nozzles. Antimony- or silver-containing manufactured carbons proved to have better properties as bearing materials but their increased wear in the presence of oil were a disadvantage. By contrast, experiments with ferrotitanide as the bushing material while steel was retained for the shaft material produced significantly better results in terms of wear behavior.
From U.S. Pat. No. 3,645,500 there is known an aerostatically supported roller designed for high speeds and low loads, being comprised of a sleeve made of resin-impregnated carbon graphite. The sleeve is supported on an inner sleeve made of porous carbon graphite, which is carried by a hub provided with channels for the supply of compressed gas. The compressed gas directed through the porous inner sleeve produces a gas film which carries the rotary outer sleeve and centers it relative to the inner sleeve. On failure of the gas film due to overloading or interruption of the gas supply, the sleeve slides on the inner sleeve, with the antifrictional properties observed as this occurred having been good.
It is an object of the present invention to provide an air bearing of the type referred to, which is suitable for a high peripheral speed, is notable for its good load-carrying capacity and long service life and suffers no impairment to its function as a result of temporary overloading. Furthermore, it is desirable that the air bearing lend itself to economical production.
According to the present invention, this object is accomplished in that the rotor is made of a corrosion-resistant steel alloy having hardened bearing surfaces while the bearing element with the mating bearing surface is made of a densely compacted, air-impervious graphite material.
The mating of materials as disclosed in the present invention is suitable in particular measure for air bearings, acted upon from outside, of shafts rotating at very high peripheral speed such as are used on motor spindles for the machining of workpieces. The antifriction conditions needing to be satisfied in such applications are characterized primarily by a very high energy density because in the start-up and run-out phase the shaft is aerostatically supported, and failure of the air film by overloading of the bearing generally occurs at full shaft speed. In this connection it has been shown that, owing to the mating of materials as disclosed in this invention, the frictional energy released upon contact between the shaft and the bearing element can be limited and controlled, in spite of the high peripheral speed, so that the bearing surface and mating bearing surface are not changed in any way that adversely affects the bearing's function. Even at peripheral speeds of the bearing surface of 10,000 ms
−1
, the metallic bearing surface of the rotor is maintained essentially unchanged by any contact and is only blackened somewhat by graphite dust. The mating bearing surface of graphite may be exposed to minor material abrasion, and small score marks may also form there, but these have no adverse effect on the function of the bearing. Even after plural contacts and an accordingly higher amount of material abrasion from the mating bearing surface, the support function of the bearing will be maintained as long as the bearing's air gap does not become too wide in relation to the quantity of supplied air. Where axial bearings are concerned, a considerable increase of bearing clearance is admissible because the appropriate air gap will invariably form on the carrying side. A further significant advantage of the bearing configuration of the present invention resides in that the bearing can be renewed with little effort if it is damaged by overly frequent or excessively long overloading. And because there is no seizing of the bearing, a damaged bearing can be dismantled without difficulty. As a rule, the bearing surface of the rotor turns out to be undamaged or the damage is slight enough to rectify by lapping. The mating bearing surface on the bearing element made of graphite is restored to working order by renewing the component in question. A further advantage has proven to be the fact that the bearing pockets and the very narrow throttle bores can be formed directly in the bearing element, thereby obviating the need to insert separate nozzle elements.


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Patent Abstracts of Japan, vol. 1996, No. 8, Aug. 30, 1996 & JP 08-105443 (Nippon Carbon), Apr. 23, 1996.
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W. J. Bartz et al, Luftlagerungen, Expert Verlag, 1982, ISBN 3-8169-0992-2, pp. 22 a

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