Thrust Bearing

The present invention relates to gas bearings and more particularly concerns a bearing assembly having a floating bearing that significantly reduces required dimensional tolerances.

Various types of optical scanning devices, such as a lasar pattern generator, employ a rotating mirror from which a laser beam is reflected in successive angular sweeps across the surface of a medium upon which a pattern is to be generated or which bears a pattern that is to be read by the scanning assembly. Such systems demand extreme precision and stability of the rotating mirror because of the high tolerance requirements for positioning of the laser beam. To attain exceedingly high precision and stability for the rotating mirror position, some prior systems have employed air thrust bearings positioned between and bearing against mutually opposed bearing surfaces on the rotating part. In such systems, a thrust bearing that is fixed to a mounting plate, locates the mirror rotational shaft axis by means of a radial bearing and axially locates the shaft and mirror by a fixed length thrust bearing. The thrust bearing is positioned between mutually spaced and mutually parallel bearing surfaces on the shaft that extend in planes perpendicular to the rotation axis. Any small degree of lack of parallelism of such opposed bearing surfaces introduces significant moments on the shaft and mirror, which moments, even in very small amounts, can cause the reflected laser beam to deflect beyond acceptable limits. Such moments also introduce vibrations which are difficult to damp. The exceeding sensitivity of the system to small amounts of error in thrust surface parallelism requires tolerances in the order of a few ten thousandths of an inch, thus increasing the difficulty and greatly raising the cost of manufacturing the assembly.

Another problem with existing bearings is that air pressures must be closely adjusted and maintained to float the bearing at the designated height. The system is sensitive to loss or significant decrease in supplied air pressure. Should the air pressure supply to such a rotating mirror bearing assembly fail, or should the supplied air pressure decrease below a certain amount, the air bearing, which reacts to the load of the mirror weight (where the mirror is mounted for rotation about a vertical axis), may decrease in air film thickness to a point where the mirror is no longer supported by the air film. Resulting metal to metal contact may effectively destroy the highly precise and expensive mirror. For example, in a system arranged to operate under an eighty pounds per square inch air supply, catastrophic metal to metal contact will occur if the pressure should drop to thirty-five to forty pounds per square inch. Thus it is frequently necessary to employ a reserve air tank that will automatically supply air at the proper pressure upon a sensed decrease in pressure of the normal air supply.

It is an object of the present invention to avoid or minimize above-mentioned problems.

In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a precision bearing assembly includes first and second relatively movable members, of which one has oppositely disposed thrust bearing surfaces. A fixed gas bearing is secured to the second member to provide a gas thrust film at the first surface. A floating gas bearing is positioned adjacent the second surface and is formed by a piston part and a cylinder part having a gas plenum therebetween. In one embodiment the piston part has an opening that is sealed by the first member, or by the bearing means secured to the first member, which forms the cylinder part. The plenum is pressurized to urge the floating bearing toward the second surface to provide a second gas thrust film at the second surface. According to a feature of the invention, the plenum opens laterally and defines a differential piston that is urged toward the second surface. The same gas in the plenum both loads the bearing and provides gas to the gas bearing film so that a more constant height of the supporting gas thrust film is maintained over a greater range of variation of pressure of supplied gas. The arrangement provides a more uniform loading because the differential piston floats and is gas loaded. It also allows the bearing to be designed with looser tolerances than a comparable solid or fixed length bearing. The floating thrust bearing is forced to operate at correct clearances by appropriate adjustment of the applied gas pressure and allows the axle assembly to be loaded without introduction of unwanted moments into the axle and mirror. Thus the rotating mirror attains increased stability during rotation and experiences minimized deflection and vibration.

FIG. 1 is a pictorial illustration showing, in simplified form, a laser beam reflected from a rotating polygonal mirror.

FIG. 2 is an elevation section through a bearing assembly embodying principles of the present invention.

FIG. 3 is an exploded perspective illustration showing the fixed and floating annular bearings.

FIG. 4 illustrates geometry of the differential piston.

The present invention is applicable to systems wherein parts have relative freedom in either rotation or translation. It has been initally mechanized in a rotating system and will be described in such mechanization for purposes of exposition.

Relevant portions of a exemplary laser scanning system having a bearing system of the present invention are illustrated in FIG. 1. Light from a laser 10 is directed to a multi-faceted polygonal mirror 12 to be reflected from successive vertical facets 14, 16, etc., in a pattern of horizontal rays 18 that are reflected downwardly from a fixed plane mirror 19 to sweep a line 21 across a medium 20 that is carried across the sweep line in a direction normal to the plane of the sweep to enable successive lines to be swept along the medium 20. The mirror comprises a reflective polygonal body, having a primary precision lower surface 22 opposite its upper surface 23. Each of the facets 14, 16, etc., is ground with reference to the precision surface 22. All of the reflective facets are precisely planar and precisely perpendicular to the primary precision surface 22. The mirror assembly is mounted for rotation upon a vertical shaft or axle 24 that is driven by a motor 25 (FIG. 2). The assembly of the mirror, axle and bearing assembly are all carried upon a mounting plate 28 having an aperture 30 through which the axle and bearing assembly extend.

Axle 24 includes a fixed annular radially extending flange 32 (FIGS. 2, 3) to which the mirror 12 is fixedly secured by means of a plurality of bolts 34, with the precision mirror surface 22 being mounted directly to the upwardly facing surface of flange 32.

A fixed annular gas thrust bearing or pad 40 has an outwardly projecting circumferentially projecting shoulder 42 that is bolted to an upper surface of the supporting or mounting plate 28 by means of a plurality of bolts such as the bolt shown at 44. The fixed bearing 40 includes an outer cylindrical surface 46 that bears against the bore of aperture 30 of the mounting plate and is formed with a pair of axially spaced circumferential gas manifold channels 48, 50, that completely circumscribe the bearing. Each channel opens radially outwardly for communication with a respective one of a pair of gas supply conduits 52, 54 formed in the mounting plate 28. O-ring seals 56, 58, 60 are carried by the fixed bearing 40 on opposite sides of the manifold channels 48, 50 to seal these to the gas conduits 52, 54. A plurality of circumferentially spaced passages 62 extends vertically through the body of the air bearing 40, from the upper manifold channel 48, to the mirror surface 22 which is immediately adjacent the upper surface of the pad or bearing 40.

Fixed bearing 40 has an axial bore formed with a first section 70 of relatively small diameter and a second section 72 of relatively larger diameter. The fixed gas bearing bore 72 is radially spaced from and concentric with a section 74 of the axle 24 that is below the first flange 32. Mounted within the annular space between the axle section 74 and the fixed bearing bore sections 70, 72, is an annular floating bearing or pad 80 having an inner bore 82 circumscribing and adjacent the outer surface of axle section 74 and having first and second radially outwardly, mutally axially spaced annular legs 84, 86, which define between them a plenum 88. The outer end of floating bearing leg 84 bears against the small diameter bore surface 70 of the fixed bearing and is sealed against the bore 70 by means of an 0-ring 90 carried by the floating bearing. The outer end of larger radius leg 86 bears against the larger diameter bore section 72 of the fixed bearing, and is sealed against the bore section by a second O-ring 92 carried by the floating bearing.

The stepped diameter of the bore of fixed bearing 40, provides a downwardly facing shoulder 94 which overlies but is slightly spaced from the upwardly facing surface of the outermost portion of the longer floating bearing leg 86.

Fixed bearing 40 is formed with a conduit 98 extending from the second gas channel 50 to the interior of plenum 88, and groups of circumferentially spaced conduits 100, 102 provide gas flow from the interior of the plenum 88 to the radial bearing surface of bore 82 and to the lower surface of the floating bearing 80. The assembly is held together by a means of an axle cap 110 bolted to the lower end of the axle section 74 by means of a plurality of bolts such as bolt 112. A gas, such as air under pressure, is supplied from a pressure source (not shown) to the conduits 52, 54 to flow gas to the manifold channels 48, 50 and thence, by means of conduits 62, to the first thrust bearing formed between the lower precision ground surface of the mirror and the upper surface of the fixed air bearing. Gas from the lower manifold chamber 50 flows into the plenum 88 and through the conduits 100 to provide a radial gas film at the radial bearing surface 82. The floating bearing 80 provides a lower thrust bearing between the lower face of the floating annular bearing and the upper surface of the axle cap 110 by means of gas supplied from plenum 88 via the conduits 102. The same gas in plenum 88 is supplied through conduits 102 to the floating bearing gas film so that the gas pressure at the bearing surface varies in direct relation to the gas pressure within the plenum.

The floating bearing 80 is driven downwardly toward the axle cap by a piston and cylinder arrangement formed by the floating and fixed bearings. One bearing forms a piston and the other forms a cylinder. The floating bearing forms a differential piston of which the geometry is illustrated in FIG. 4. The radial extent of the shorter leg 84 (as measured from the main body of the bearing) is indicated by the distance A and the radial extent of the longer leg 86 is indicated by the distance B. Accordingly, the differential piston has a net or effective piston area in planes perpendicular to the rotation axis comprising an annular area having a radial extent indicated at C, where C is equal to B-A. The differential piston is driven axially of the shaft (downwardly) by pressure of gas in the plenum 88 which has a longitudinally slidable sealing relation with the bore sections 70 and 72 of the fixed thrust bearing. The use of a differential piston allows the floating gas bearing to be made reasonably strong and of a fairly large size without producing excessively large forces. Because the configuration of the differential piston provides a net internal area in a plane that faces the mirror surface 22, pressure within the laterally opening plenum 88 forces the differential piston downwardly with a total force proportional to the net area of the differential piston. This downward force is reacted against the mounting plate through the fixed bearing 40 at its downwardly facing shoulder 94. The arrangement enables proportioning the parts to provide a desired amount of piston force to increase the bearing thrust loading. For example, with a gas supply pressure of 80 pounds per square inch and a floating annular bearing of approximately two inches in diameter, the parts are readily proportioned to provide a net area of the differential piston of between 0.5 and 0.6 square inches, whereby a net downward bearing loading force of about 45 pounds is exerted by the differential piston. Thus, the floating bearing is formed, in effect, by a piston part (the differential piston) and a cylinder part. The cylinder part is, in effect, formed by and fixed to the mounting plate and fixed bearing.

The gas bearing assembly comprises two bearing parts of which thrust bearing part 40 is fixed and the other bearing part 80 is a floating piston capable of relatively large vertical displacement. This arrangement provides a bearing that, in effect, is automatically adjustable in length along the axle as the distance between the two opposed bearing surfaces of the mirror and end cap varies. A compact longitudinal arrangement is provided by effectively mounting the floating thrust bearing within the bore of the fixed thrust bearing, making the two thrust bearing parts mutually concentric and relatively axially extensible. The arrangement permits much looser tolerances and less precision of parallelism between the opposed facing surfaces of the mirror and axle end cap, because the floating bearing is formed by the piston that can shift axially by relatively large amounts as the distance between the opposed bearing surfaces changes. The bearing can accommodate very large changes in dimensions caused by tolerance build-up or temperature changes without effect operating height (gas bearing film thickness). As the floating bearing shifts axially, the constant pressure in the plenum 88 provides a constant load on the bearing and maintains the desired clearances at the bearing surfaces to ensure the proper thickness of the gas bearing films.

The weight of the mirror itself provides a first thrust load on the bearing. This thurst load is augmented by the additional load provided by action of the differential piston. Thus in an exemplary arrangement, wherein the mirror weight provides a thrust load of about eight pounds, the load exerted by the mirror is considerably less than the added load (about forty-five pounds) exerted by the differential piston. A stiffer, more stable bearing is achieved by the added load. The described arrangement allows a very large load (as much as five times the mirror weight) to be applied to the thrust bearing, thereby decreasing the gas thrust film thickness and providing a stiffer bearing in a small space.

An unexpected and surprising advantage results from operation of the described bearing. This advantage is that the system exhibits significantly increased tolerance to changes in applied gas pressure. In a previously employed one piece thrust bearing supplied with air pressure of 80 pounds per square inch, a drop in the pressure to about 35 to 40 pounds per square inch, will decrease the gas film supporting characteristics to a point where the bearing is no longer able to support the weight of the mirror. Should this occur, metal to metal contact will ensue, destroying the mirror. With the floating bearing arrangement described herein, however, the bearing is loaded with the very same gas which provides gas to the bearing film through the orifices. Thus both load and bearing height vary with variation of pressure of supplied gas. As the supplied pressure decreases, bearing film height ordinarily tends to decrease. However, the load on the bearing also decreases because of the fixed piston area and because the added load component caused by the differential piston 80 is directly related to the magnitude of the pressure. Such decrease in load tends to counteract the tendency to decrease bearing film height. Similarly, as supplied gas pressure increases, the load on the bearing increases and the load carrying capacity of the bearing also increases because of increased pressure to the bearing surface through conduits 102. This means that when the piston is designed for air bearing size, conduit diameter and lift coefficient it can tolerate a large variation of air pressure without affecting the operating height. It has been found that, with the described arrangement, supplied gas pressure can drop from a normal 80 pounds per square inch to as low as nearly 20 pounds per square inch before occurence of metal to metal contact.

Many variations of configuration and application of the described bearing assembly can be made without departing from principles of the invention. The arrangement described above has the two bearing parts, fixed part and floating part positioned on the same side of the rotatable mirror. Nevertheless the invention can also be mechanized with the two bearing parts on opposite sides of the mirror between two mutually spaced arms of a fixed support structure. In another modification the floating bearing piston can be made to react directly against this fixed support instead of against the fixed bearing.

The floating bearing arrangement decreases required tolerances, as previously described, and also avoids moments exerted upon the axle and mirror due to uneven bearing loading. The floating bearing provides equalized thrust loading that is distributed along the bearing surfaces with much greater uniformity. Even with significantly looser tolerances, the floating bearing is constrained to operate at the appropriate clearances and desired gas thrust film thicknesses because of the applied gas pressure on the differential piston. Because axial thrust load is a function of gas pressure in the plenum, and bearing height is also a function of pressure of the same gas that provides the thrust load, the bearing clearance will vary less over a wider range of varying gas pressures. The net result is that, in use with a rotating system of greatly eased tolerances, the mirror may rotate at high speed with maximum stability and minimum deflection, vibration and wobble.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.