Diaphragm preload air bearing

Details Diaphragm preload air bearing Diaphragm preload air bearing

1999-02-01

2001-11-13

Marmor, Charles A. (Department: 3681)

Bearings

Linear bearing

Plain bearings

C384S012000, C384S040000

Reexamination Certificate

active

06315450

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bearings. More particularly, the present invention relates to a preload hydrostatic bearing with a diaphragm for improved performance.
2. Description of the Related Art
X-Y stage systems are typically used in machine tools and other applications where two-dimensional precision movement is required to position an object supported on the stage. A typical X-Y stage system includes a pair of parallel-spaced guide rails and a stage with at least one fixed air bearing at one end and a corresponding preload air bearing at the other end. The fixed and preload air bearings ride along respective guide rails to move the stage therealong. The preload air bearing provides a constant force to the fixed air bearing and maintains a constant air gap or flying height in the fixed air bearing.
Because it is difficult for guide rails of stage systems to be perfectly uniform, a preload air bearing must compensate for variations in the guide rails, due to thermal growth or other causes, while providing a constant force to the fixed air bearing. Conventional air bearings utilize mechanical preload devices including combinations of ball bearings, conical bearing seats and spring washers, such as Belleville washers, to compensate for rail variations. Examples of these air bearings may be found in U.S. Pat. No. 4,191,385, issued Mar. 3, 1980 to Fox et al. and U.S. Pat. No. 4,882,847, issued Nov. 28, 1989 to Hemmelgarn et al.
FIG. 1
illustrates one such prior art preload air bearing
100
. Bearing
100
includes a pad
102
having a bearing surface
103
. Pad
102
is coupled to a cap
104
. Bearing pad
102
is made of a porous material, such as graphite. In the alternative, pad
102
may have a plurality of orifices formed therein. Cap
104
has an internal space for receiving a compressed gas, such as air, from an external source. The compressed gas flows through cap
104
and pad
102
to create an air film between bearing surface
103
and a rail surface (not shown) on which bearing
100
rides. A ball
106
which is received in a seat
144
supports bearing cap
104
. A spring washer
148
, or stack of spring washers, supports seat
144
and ball
106
. Washer
148
is secured on a boss
150
at one end of a preload pin
146
. The arrangement of ball
106
, seat
144
and spring washer
148
allows bearing cap
104
and pad
102
to tilt and accommodate slight variations in the rail surface. The air film gap may be altered by adjusting the position of preload pin
146
.
One problem with conventional air bearings, however, is their inability to supply a constant preload. A small change in the uniformity of the guide rails can significantly alter the amount of force developed in the bearing, changing the bearing flying height, which can cause instability and possibly derail the stage. These bearings are also less stiff, and the stage, therefore, is more prone to yaw. In addition, these mechanical preload devices generate a great deal of friction between the spring washers, conical bearing seat and ball bearing, which results in motion loss. Other associated problems include dynamic oscillations, such as pneumatic hammer instability, hysteresis and non-linearity.
One solution includes replacing the spring washers with an air cylinder, which would ensure a constant preload and eliminate the friction associated with the washers. This preload air bearing, however, still requires a ball bearing pivot, another source of friction, to compensate for any non-uniformity in the guide rails. In addition, such an air bearing may be difficult to implement due to packaging constraints. Thus, it would be advantageous to provide a preload hydrostatic bearing with a simple design that is capable of providing a constant force with minimum hysteresis to a fixed hydrostatic bearing despite variations in the guide rails.
SUMMARY OF THE INVENTION
The present invention addresses these problems by providing a preload hydrostatic bearing with a single diaphragm. The diaphragm replaces the various mechanical preload devices, such as a ball bearing, bearing seat and spring washers. The size and thickness of the diaphragm are optimized to minimize the axial and bending stiffnesses and to maximize the radial stiffness of the diaphragm. Because the diaphragm has low axial and bending stiffnesses, the diaphragm can accommodate variations in the surface of a guide rail while generating little or no friction, thereby improving the performance of the preload hydrostatic bearing.
In accordance with one aspect of the invention, a preload hydrostatic bearing includes a pad, a diaphragm and an adjustable member. The pad includes a bearing structure, an inlet manifold, and a plurality of orifices. The orifices direct a fluid or gas, such as air, from the inlet manifold toward the bearing surface. The diaphragm is mounted on the pad and includes a center portion. The adjustable member, which extends in an axial direction, is coupled proximate one end to the center portion of the diaphragm. The diaphragm transfers a preload in the axial direction to the member.
In accordance with another aspect of the invention, a hydrostatic bearing stage system includes a pair of guide rails and a stage movable therealong. The guide rails include a first rail and a second rail. The stage has a first end proximate the first rail and a second end proximate the second rail. The system further includes a first hydrostatic bearing mounted on the first end of the stage and a preload hydrostatic bearing mounted on the second end of the stage. The preload hydrostatic bearing is similar to that described above.
In accordance with still another aspect of the invention, a method of bearing a structure on a surface includes directing a pressurized fluid onto the surface from the structure and flexibly coupling an axial member by a diaphragm to the structure. The method further includes adjusting an effective length of the axial member. This adjusting sets an amount of preload applied to the structure to urge the structure towards the surface.


REFERENCES:
patent: 3597020 (1971-08-01), Thomas
patent: 3597021 (1971-08-01), Thomas
patent: 3744858 (1973-07-01), Weichsel
patent: 3758175 (1973-09-01), Van Roojen
patent: 3799628 (1974-03-01), Van Gaasbeek et al.
patent: 4113325 (1978-09-01), Miller
patent: 4191385 (1980-03-01), Fox et al.
patent: 4351574 (1982-09-01), Furukawa et al.
patent: 4378134 (1983-03-01), Eddy
patent: 4643590 (1987-02-01), Olasz
patent: 4719705 (1988-01-01), Laganza et al.
patent: 4802774 (1989-02-01), Pesikov
patent: 4882847 (1989-11-01), Hemmelgarn et al.
patent: 4946293 (1990-08-01), Helms
patent: 5257461 (1993-11-01), Raleigh et al.
patent: 5648690 (1997-07-01), Hinds
patent: 2 397 266 (1978-06-01), None

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