WO2003026844A2

WO2003026844A2 - Improved bearing arrangement for a rotary stage

Info

Publication number: WO2003026844A2
Application number: PCT/US2002/020420
Authority: WO
Inventors: George Mauro
Original assignee: George Mauro
Priority date: 2001-09-27
Filing date: 2002-06-27
Publication date: 2003-04-03
Also published as: AU2002316432A1; WO2003026844A3

Abstract

A rotary stage (40) having a rotary table mounted by bearings (54, 56) defining a rotary table axis of rotation, the rotary table being pre-loaded axially and radially by the bearings engaging a frusto-conical surface (50) encompassing the rotary stage.

Description

[001] IMPROVED BEARING ARRANGEMENT

FOR A ROTARY STAGE

[002] FIELD OF THE INVENTION

[003] This invention relates to an improved bearing system for a rotary stage.

[004] BACKGROUND OF THE INVENTION

[005] Prior art rotational stages usually consist of a rotor pivoted by two bearings mounted along the axis of a stationary shaft. Wobble is introduced to the rotating element due to the normal out-of-roundness of the balls in even the best of ball bearings. Typical out-of-round/out-of-size ball-to-ball in a high grade bearing is about 10 -in. This results in a true-center wobble of 4 arc-seconds using a worm gear of 1 inch diameter. This trans-axial wobble is usually acceptable. But, the axial wobble is a result of the compounding of the errors of both bearings, taken over the distance between the bearings. A stage which has a low profile will suffer greater axial error/wobble due to close proximity of the bearings.

[006] The Applicant's own invention, the subject of U.S. Patent Application

No. 09/791 ,073 filed February 22, 2001 , provides a means to overcome the problems associated with the prior art and to provide a device which decreases the angular wobble of the rotor four to eight fold, i.e. decreases the error from 20 arc sec. down to 1 arc sec. or better, while converting random wobble to smooth consistent deviation and repeatability with the error noted.

[007] According to Serial No. 09/791 ,073, there is provided a rotary stage comprising a rotary table mounted by an adjustable axial preloaded bearing on a shaft defining a rotary table axis of rotation and supported on a base plate by an optically polished surface which may be formed on the rotor, on the base plate or on both of these. When a single said polished surface is utilized, the corresponding contacting surface may comprise three low friction (e.g. Teflon™) pads symmetrically disposed about said axis. The mating polished surfaces, when used, may have air under pressure supplied between them to form an air bearing. The bearing on the shaft may, for example, be a ball, air, roller, single point bearing. The rotary table may have an optically flat polished upper surface and may incorporate a worm gear for engagement with a worm to control rotation of the rotary table about the axis. [008] Preferably Serial No. 09/791 ,073 provides a rotary stage having a rotary table mounted by a bearing on a shaft defining a rotary table axis of rotation and supported on a base plate by an optically polished surface, the bearing being preloaded axially by an axially adjustable split ring in engagement with an annular chamfer on a race of the bearing to center the bearing relative to the rotary table axis.

[009] OBJECT OF THE INVENTION

[010] It is an object of the present invention to provide a rotary stage with an improved and simplified bearing system which can accommodate a larger opening along the axis of the rotary table than that of the Applicant's prior system without any increase of angular wobble while providing the smooth consistent and repeatable deviation of applicants Serial No. 09/791 ,073.

[011] SUMMARY OF THE INVENTION

[012] According to the invention, there is provided a rotary stage having a rotary table defining a axis of rotation and supported on a base, the stage being pre-loaded axially and radially relative to the base by three bearings in engagement with a frusto-conical surface encompassing the rotary stage.

[013] Preferable the bearings are equi-spaced about and equi-distant from the axis, the rotary table is supported on the base by three pads in engagement with an optically polished surface on the rotary table and equi-spaced about and equi-distant from table axis.

[014] Also preferably the frusto-conical surface is defined by the rotary table.

More specifically the present invention preferably provides a rotary stage comprising a rotary table supported on a body for rotation about an axis of rotation, and three preload bearings, the bearings being pre-loaded radially of the axis and axially, without play between the rotary table and the body, by the use of a frusto-conical surface encompassing the rotary stage and in engagement with the three preload bearings, the three preload bearings each being equi-spaced about the axis and equi-distant from the axis. In this more specific form the rotary table may be supported on a base of the body by support bearings, equi-spaced about the axis and equi-distant from the axis or by parallel optically flat surfaces, roller bearings, ball bearings, an air bearing or a magnetic repulsion bearing. The pre-load bearings may also comprise such alternatives.

[015] Preferably the three pre-load bearings are supported by the body and are surfaces defined by pads in contact with the frusto-conical surface defined by the rotary table, one of the three preload bearings being resiliently adjustable to provide the radial and axial pre-load and to center said rotary table on said axis without play therebetween.

[016] The preload bearings and support bearings may alternatively be balls or rollers and the preloading and centering of the rotary table is by an axially adjustable arrangement connecting first and second parts of the body and disposed axially of one another.

[017] Alternatively each preload bearing may be affixed to a support bearing with the support bearings engaging a surface encompassing the rotary stage adjacent the frusto-conical surface, the preload bearings being centered on said axis by a resilient device, i.e. a leaf spring, located between the body and one of the preload bearings.

[018] BRIEF DESCRIPTION OF THE DRAWINGS

[019] The invention will now be described, by way of example, with reference to the accompanying drawings, in which: [020] Fig. 1 is a diagrammatic cross-sectional elevation of a rotary stage including an adjustable axially preloaded bearing according to U. S. Patent Application

No. 09/791 ,073; [021] Fig. 2 is an exploded view of a first embodiment of a rotary stage of the present invention; [022] Fig. 3 is a plan view of the rotary stage of Fig. 2 when assembled; [023] Fig. 4 is a section on section line 4-4 of Fig. 3, similar sections being the basis the cross-section locations of Figs. 1 , 4, 6, 8, 9 and 10; [024] Fig. 5 is a further illustration, shown assembled, of the rotary stage of Fig. 2;

[025] Fig. 6 is a diagrammatic cross-sectional elevation of a second embodiment of the present invention; [026] Fig. 7 is a detailed section of a pad 54;

[027] Fig. 8 is a diagrammatic cross-sectional elevation of a third embodiment of the present invention; [028] Fig. 9 is a diagrammatic cross-sectional elevation of a fourth embodiment of the present invention; and [029] Fig. 10 is a diagrammatic cross-sectional elevation of a fifth embodiment of the present invention.

[030] DESCRIPTION OF PREFERRED EMBODIMENTS

[031] The invention of the Applicant's prior application Serial No. 09/791 ,073 is disclosed in detail by the accompanying Fig. 1 and comprises a single radial ball bearing 6 substantially centrally axially located on a rotary table supporting rotor 18, and a three point support pad arrangement 26 on either the underside of the rotor 18 or the upper side of base 2. The use of optical surfacing permits the achievement of: a) superior flatness (1/4 wave, etc.) beyond the capability of machine grinding and b) a smooth bearing surface. The rotary table supporting rotor 18 is fast within the outer race 10 of ball bearing 6 and supports a rotary table 20 for rotation with rotor 18 by a worm 22 which engages worm gear teeth on the outer periphery of the rotor 18.

[032] A split ring 28 closely, but freely, surrounds shaft 4 above the bearing 6.

This split ring 28 engages an annular frusto-conical chamfer 30 in the upper end surface of an inner race 8 of bearing 6 adjacent the shaft 4. Axial downward pressure is exerted on the split ring 28 by a nut 32, threaded onto the shaft 4, by way of a cushion spring 34 (O-ring, bellville washer, etc.) and a washer 36, to move the inner race 8 downwardly relative to the outer race 10 to preload the bearing 6 in an axial direction thereby accurately centering the bearing 6, ring 18 and rotary table 20 relative to the shaft 4 and thus, axis 24.

[033] The present invention omits the preloaded bearing 6 in favor of the of the construction illustrated in Figs. 2-10 and otherwise described herein. Referring to Figs. 2-5 a circular rotary stage or table 40(hereinafter, rotary table) defines a central opening 42 and, on an outer cylindrical circumferential V groove 44 having an upper surface 46 parallel to the flat upper surface 48 of the rotary table 40 and a frusto-conical lower surface 50 disposed at 45° to the surface 46.

[034] The rotary table 40 is supported in a body 52 by two sets 54, 56 of three equi-spaced Teflon™ or turcite pads (preload and support bearings) equi distant from axis 57 of the rotary table 40, the contact surfaces 54' of the pads 54 (preload bearings) engaging the frusto-conical surface 50 and the contact surfaces 56' of the pads 56 (support bearings) defining plane normal to axis 57. The pads 54 are supported in the body 52 by, i.e. set screws, with one pad adjustable to provide desired preloading, in both the axial and radial directions, of the pads 54, 56 by virtue of the pads 54 engaging the frusto-conical surface 50 and the pads 56 engaging the underside 60 of the rotary table 40. The set of pads 56 correspond to the pads of Application Serial No. 09/791 ,073 and are supported on surface 51 of the base 58 of the body 52 in supporting contact with a flat optically polished underside 60 of the rotary table 40. Machine turned or precision ground surfaces will yield a lesser degree of angular consistency, but give the same arc-second repeatability.

[035] It will be appreciated that the arrangement of pads 56 may be reversed with the pads 56 mounted on the underside of the rotary table and the upper surface 51 of the base being optically flat. In an alternative arrangement, the pads 56 may be omitted in favor of both surfaces 60 and 51 being parallel optically flat surfaces which may have air under pressure supplied between them to form an air bearing.

[036] As shown in Figs. 2-5, two of the pads of the pad set 54 are held in place by set screws 62 with a third of the pads 54 angled downward at 45degrees to meet the frusto-conical surface 50 while the other two pads of this set extend normal to the axis of the rotary table into contact with the frusto-conical surface. The angled pad 54 is adjustable by a set screw 53 to adjust spring pressure of coil spring 66 to bias the pads 54, 56 to center the rotary table on the axis 57 and to remove axial and radial play between the rotary table 40 and body 52.

[037] The frusto-conical surface is diamond ground and the pads 54, 56 are machined in place to fit the pad support surfaces of the rotary table 40 and may be temporarily removed for assembly.

[038] In alternative constructions, all of the pads 54 extend horizontally into the body 52 with surfaces 54' formed at an angle to effectively engage the frusto- conical surface 50.

[039] In the second embodiment as illustrated in Fig. 6, all three pads 54 are angled downwardly at 45° to the surface 50 whereby the end surfaces 64 of these pads are normal to their length. One of the pads 54 is spring loaded by a coil spring 66 adjustable by its set screw 53 to pretension the pads 54, 56 against the surfaces 50 and 60 respectively. Here the pads 56 are fixedly mounted in the base 68 of body 52. The pads 54 may be removable for assembly and disassembly.

[040] A typical pad 54 is illustrated in Fig. 7 and comprises a low friction tip 70, i.e. Teflon™ or turcite, supported on a shaft encased in a metal outer shell 72. The tip may be as shown, or i.e. the same diameter as the shaft.

[041] Fig. 8 illustrates a third embodiment in which the pads 54, 56 are replaced by six roller bearings 74 (alternatively ball bearings could be used) located in the same layout as the pads 54, 56. The bearings are held in their relative positions by cages (not shown) and are pre-stressed by a pressure plate 76 forming part of the body 52, having a frusto-conical surface 78 complimentary to the surface 50 and being affixed to the base 58 of the body 52 by tensioning tie-down screws 80. In this embodiment the upper surface 46 of V-groove 55 is omitted in favor of surface 78. The base 58 of the body 52 defines a bearing race 82 for the lower three bearings 74 with the underside 60 of the rotary table 40 defining a complimentary surface thereto. Surfaces 50, 78, 60 and 82 are diamond turned to form the bearing raceways. [042] Fig. 9 illustrates a fourth embodiment in which pads 54, 56 are replaced by air bearing jets 84, 86 with jets 84 held in place by set screws 88. The jets 84, 86, of which there may be more than 3 though still equi-spaced, are arranged to supply air under pressure between the surfaces 84' of jets 84 and the frusto- conical surface 50 and between the base 58 and the undersurface of the rotary table 40 to provide air bearings in place of pad to rotary table contact. The jets may be supplied with pressurized air by way of external supply lines 90 and/or internal manifolds.

[043] Fig. 10 illustrates a fifth embodiment in which pads 54 and 56 are combined and are located between the surface 46 and the frusto-conical surface 50 with the pads 56 mounted on the pads 54 for contact of their surfaces 54' with surface 46 and the surfaces 56' of pads 56 contacting the frusto-conical surface 50. In this embodiment the rotary table 40 is centered and play removed by a resilient device, i.e., a leaf spring 92 disposed between one of the pads 56 and an inner surface of the body 52 to which it is affixed.

[044] Although not illustrated an alternative construction, to all embodiments described, of the present invention is the mounting of the rotary table 40 externally about the outside of the body 52 (somewhat as in 09/791 ,073 with the V-groove formed in an outer cylindrical surface of the body 52, the preload bearings 54 supported in the rotary table still to engage the V-groove formed frusto-conical surface 50 and with the support bearings or alternative supports disposed between the surfaces of the body and rotary table normal to the axis of rotation of the rotary table.

Reference numerals

2 base 53 set screws

4 shaft 54 set of pads (preload bearings)

6 ball bearing 54' surfaces of pads 54

8 inner race 56 set of pads (support bearings)

10 outer race 56' surfaces of pads 56

12 ball bearing guiding grooves 57 axis

14 ball bearings 58 base

18 table supporting rotor 60 underside

20 rotary table 62 set screws

22 worm 64 end surfaces

24 axis 66 coil spring

28 split 68 base of body 52

30 ring 70 tip

32 nut 72 shell

34 spring 74 roller bearings

36 washer 76 pressure plate

40 rotary table 78 frusto-conical surface

42 opening 80 screws

44 V-groove 82 race

46 upper surface 84 air jets

48 flat upper surface 86 air jets

50 frusto-conical surface 88 set screws

51 surface 90 supply lines

52 body 92 leaf spring

94 inner surface

Claims

I CLAIM

1. A rotary stage having a rotary table defining a axis of rotation and supported on a base, the table being pre-loaded axially and radially relative to the base by three bearings in engagement with a frusto-conical surface encompassing the rotary stage.

2. The rotary stage of claim 1 , wherein the bearings are equi-spaced about and equi-distant from the axis.

3. The rotary stage of claim 1 , wherein the table is supported on the base by three pads in engagement with an optically flat surface on the rotary table.

4. The rotary stage of claim 3, wherein the pads are equi-spaced about and equi-distant from table axis.

5. The rotary stage of claim 1 , wherein the frusto-conical surface is defined by the rotary table.

6. A rotary stage comprising a rotary table supported on a body for rotation about an axis of rotation and three preload bearings, the preload bearings being pre-loaded radially of the axis and axially, without play between the rotary table and the body, by the use of a frusto-conical surface encompassing the rotary stage and in engagement with the three preload bearings, the three preload bearings each being equi-spaced about the axis and equi-distant from the axis;

7. The rotary stage of claim 6, wherein the rotary table is supported on a base of the body by support bearings in contact with an optically flat surface, equi- spaced about the axis and equi-distant from the axis.

8. The rotary stage of claim 6, wherein the table is supported on a base of the body by one of parallel optically flat surfaces, roller bearings, ball bearings, an air bearing and a magnetic repulsion bearing.

9. The rotary stage of claim 6, wherein the three preload bearings are supported by the body and are surfaces defined by pads in contact with the frusto- conical surface defined by the rotary table, one of the three preload bearings being resiliently adjustable to provide the radial and axial pre-load and to center said rotary table on said axis without play therebetween.

10. The rotary stage of claim 7, wherein there are three support bearings are mounted to the base of the body, the three bearings having surfaces, defining a plane, to support the table.

11. The rotary stage of claim 10, wherein the surfaces of the support bearings contact a optically flat surface of the rotary table to provide the support.

12. The rotary stage of claim 6, wherein the preload bearings and support bearings are one of balls and rollers and the preloading and centering of the rotary table is by an axially adjustable arrangement connecting first and second parts of the body and disposed axially of one another.

13. The rotary stage of claim 7, wherein at least the support bearings together are provided by an air bearing.

14. The rotary stage of claim 7, wherein at least the preload bearings together are provided by an air bearing.

15. The rotary stage of claim 13, wherein both the support and preload bearings are provided by air bearings.

16. The rotary stage of claim 6, wherein each preload bearing is affixed to a different one of the three support bearings with the support bearings engaging a surface encompassing the rotary stage adjacent the frusto-conical surface, the preload bearings being centered on said axis by a resilient device engaging one of the preload bearings.

17. The rotary stage of claim 16, wherein the resilient device is a leaf spring.