GB2302144A

GB2302144A - Externally pressurised air bearing spindle

Info

Publication number: GB2302144A
Application number: GB9611247A
Authority: GB
Inventors: Yoshio Fujikawa; Shoji Fujii
Original assignee: NTN Corp; NTN Toyo Bearing Co Ltd
Current assignee: NTN Corp
Priority date: 1995-05-31
Filing date: 1996-05-30
Publication date: 1997-01-08
Anticipated expiration: 2016-05-30
Also published as: FR2734744B1; GB9611247D0; FR2734744A1; DE19621773A1; GB2302144B

Description

2302144 S PE Cl r, I CATION EXTERNALLY PRESSURIZED AIR BEARING SPINDLE

BACKGROUND OF THE INVENTTON

The present Invention relates to an eternally pressurized air bearing spindle device for non-contact support of a main shaft by eternally pressurized air bearings, which cap be utilized as a spindle dcvIce for drilling machines, precise, machine tools electrostatic coating machines. etc.

The eternally pressurized air bearing. which can support a main shaft In a state of non-conract. is characterized by little frIctIon.loss and high accuracy of motion guide. For this reason. It Is used as a main shaft bearing for the high speed spindles or precision spindles of drilling machines. precise machine tools. electrostatic coating machines. etc.

Figs. 6 through 8 show by way of example a conventional. eternally pressurized air bearInc spindle using eternally pressurized air bearing. This eternally pre-q,,.itlrlzed air bearing spindle Is of the air turblite drive type comprising a plurality of recesses 21al formed around an outer periphery of a thrust plate 21a of the main shaft 21, ttirbine nozzles g2 which tangentially opens at positions opposed to recesses 21al. the 1 1 a arrangement being such that compressed alr fed from a turbine air feed port 22a Is blown from the turbine nozzles 22 tangentially against the recesses 21al of the thrust plate 21a to rotate the main shaft 21. And the main shaft 21 drlvexi for rotation In this manner,Is supported In a state of non-contact by first and second bearing portions X and Y to be later described.

The first bearing portion X comprises a cylindrical housing 23 forming an outer wall hereof, and a bearing sleeve 24 fixed In the housing 23 by suitable means. such as shrink fitting, press fitting or adhesion. An Inner surface of the bearing sleeve 24 Is provided with a journal bearing surface 24a opposed to an outer surface of the main shaft 21 with a slight journal bearing clearance defined therebetween, while a rear end of the bearing sleeve 24 Is provided with a thrust bearing surface 24b opposed to a front end surface of the thrust plate 21a of the main shaft 21 with a slight thrust clearance defined thereberween. Further. the bearing raleeve 24 Is provided with a plurality of fine nozzles 24c,24d opened to the Journal bearing surface 24a, and. a plurallty of fine nozzles 24e opened to the thrust bearing surface 24b. The nozzles 24c, 24d, 24e are dIsposed on same respective circumferences of the bearing surface of the bearing sleeve 24.

The second bearIng portion Y.comprises a cylindrical housing 25 coaxially jointed to a rear end of the housing 2 23. and a bearing sleeve 26 fixed In the housing 25 by suitable means, such as shrink fitting. press fitting or adhesion. Further. a front end surface of the bearing sleeve 26 Is provided with a thrust bearing surface 28b opposed to a rear end surface of the thrust plate 21a of the main shaft 21 with a slight thrust bearing. clearance defined therebetween. Further, the hearIng sleeve 26 Is provided with a plurality of fine uozzles 28c opened to the thrust bearing surface 26b and whIch are disposed on the same circumference.

As shown In Fig. 6 (b). the joining between the housing 23 of the first bearing portIon X and the housing 25 of the second bearing portion Y Is effected by putting together the mating surfaces of fixing portions 23a, 25a. inserting joining holts 27 therethrough from the side of the fixing portion 25a to engage Internal threads 23al opened to the fixing portion 23a for tightening.

Displacement In the radial and thrust directions are surrpressed by compressed air flowing Into the bearing clearances through the nozzles 24c, 24d, 24e, 2Gc of bearings sleeve 24 and 26. This compressed air for the bearIngR Is fed from an alr feed port 28 and flows through the radial air feed passages 29. 30 and axial air feed passages 31, 32 and through the axial air feed passages 31, 32, flowing Into circumferential air feed passages 33, 34. 3.S (see FIgs. 7 and 8) which put the nozzles 24c. 24d, 24e. 26c In circumferential COmmunleatlon with each other, 3 and flowing into the bearing clearances though the nozzles 24c. 24d, 24e, 26c. The compressed air which has flown Into the bearing clearances flows In the bearing clearances to the bearing end. from which It Is discharged out of the spindle directly or through exhaust passages 36.

37. The pressure distribution of the compressed air produced In the bearing clearances Is effective to support the main shaft 21 In state of non-contact with the journal bearing surface 24a and thrust benrIng surfaces 24b, 26b.

As described above. In the conventional eternally pressurized air bearing spindle. the first and second bearIng portions X and Y for non-contact support of the main shaft 21 are constructed of housings 23, 25 and bearing sleeves 24. 28 Integrally fItted together; the main reason Is that If the bearing portion X, Y are each of single cylindrical construction, It would be Impossible to form the circumferential air feed passages 33, 34. 35 which put the noz2les 24c. 24d. 24e. 26c In circumferential communication with each other.

in the field of machine tools described above. recently there tins been a tendency to make multidimensional or accelerate the positioning of this type of ipindle in order to Improve productivity. and the reduction of weight of the spindle, which Is an associated factor. has become an Important problem. For example. In the case of an electrostatic coating machine. there has been a desire to attach the spindle, which has heretofore

4 been mounted on a reciprocating table, to an artletulated robnt In order to provide a more flexible controlled coating operation. Further reduction of the weight of the spindle Is thus a great merit In consideration of the limited load capacity of the robot.

On the other hand, In this type of spindle. since the dimension, shape and material of the main shaft are often determined by the functional need for load capacity, rigldity, elongation due to thermal deformation, wear resistance. etc., the reduction of weight of the spindle will be focused on the reduction of weight of the bearing portions (housing and bearing sleeve). As a means therefor. it may he contemplated to make the bearing portion, portionicularly the housing, of a material of low specific gravity, for e xample. a nonmetallic material. such as ceramic material, synthetic resin, or graphite, or a metallic material. such as aluminum alloy (generally, the housing js made of stainless steel, and the bearing sleeve is made of bronze type alloy). or to make the housinC and bearInC sleeve thIn-walled.

However, In the conventional spindle as described above, the bearing portion Is of the construction In which the housing and the bearing sleeve are Integrally fitted together, and as compared with the single cylindrical construction of the same thickness, IL has structurally low rigidity. Therefore, direct application of said various means for weight reduction can lead to the whole spindle vibration attended by the Increased whirl of the main shaft during operation under the action of exciting force due to the Imbalance of the main shaft caused by the reduced rigidity of the bearing portion (because the materials of low specific gravity have less rigidity than stainless steel and the like and because making the houslng thIn-walled reduces rigidity). with the result that sufficient accuracy of rotation cannot he obtained. Further, If the bearing sleeve Is made thIn-walled. when the journal bearing surface Is machined. the inner surface deforms under the action of the chucking force, so that the journal bearing surface cannot be finished In accurate c Ircular shape, creating the problem of In.SUffiCiCnt bearing accuracy.

Further, the nonmetallic materials mentioned above are.

generally brittle or often lacking In rigidity or strength as compared with metallic materials. causing the fear of deformation by external force or Influences of repeated load- Therefore. If the bearing portion, portionicularly the housing of the conventional spindle Is made of such nonmetallic materials, the following disadvantages are expected.

(1) It 19 feared that the Internally threaded portions of the fixing portion Into which the joining bolts are screwed would be damaged; (2) Since the joining bolts cannot be tightened with sufficient torque. the rigidity of the Joined portions of 6 the two bearing portions is reduced, resultIng In the whole spindle vibration attended by the Increased whIrl of the main shaft during operatIon under the action of exciting force due to the Imbalance or the main shaft. wIth the result that sufficient accuracy of rotation cannot be obtained.

(3) When the journal bearing surface Is machined. th Inner surface deforms under the action of the chucking force, so that the Journal bearing surface cannot be finished In accurate circular shape, creating the problem of bearing accuracy. However. since the chucking force Is radially applied. It does not so much Influence the accuracy of the thrust bearing surface.

e SUMMARY OF THE INVENTION

The present Invention Is 1Atended to solve the problems described above. and provide an eternally pressurized air bearing spindle which Is light In weight and compact.

According. to the Invention. a houslng Is directly provided with a bearing surface opposed to an outer surface of a main shaft with a slight bearIng clearance defined therehetween. a plurality of nozzles opened to the bearing surface and disposed on the same circumference. and a continuous polygonal air feed passage which puts the plurality of nozzles In circumferential communication with each other, the arrangement being such that compressed air 7 Is allowed to flow Into the bearing clearance through the plurality of nozzles to support the matn shaft tn a state of non-contact with the bearing surface of the housing.

With the above arrangement. the rigidity of the bearing portion Is Improved as compared with the conventional spindle wherein the bearing portion Is of the construction In which the housing and the bearing sleeve are Integrally fitted together. As a result. not only can the accuracy of rotation of the main shaft be Improved but also It becomes possible to apply various means for reduction of weight, such as making the housing constituting the bearing portion of a material of low specific gravity or making It thIn-walled. Thereby. It Is possible to provide an eternally pressurized air bearing spindle of high precision which Is light In weight and compact. Further. the housing constituting the hearing portion can be constructed of a single member. a fact which, as compared with the prior art. leads to reduction of the number of portions. simplification of production steps and cost reduction.

The polygonal air feed passage can be constructed of a plurality of straight passage portions formed from an outer surface of the housing In the predetermined direction. Thereby, air feed passages can be fortned by drilling the housing which Is made of single member.

Further. the housing may be made of a material of low specific gravity. Such material of low specific gravity 8 may be one having a self-lubricatIng property. Further. the bearing surface of the housing may be formed with a lubricating or wear resistant film. Application of such means makes It possible to obtain high durability even under the working condition where contact between Lhe maln shaft and. the bearing surface Is expected.

Further, the present Invention provIdes an eternally pressurized air bearing spindle, a first bearing portion having a journal bearing surface opposed to an outer surface of a main shaft with a slight clearance defined therebetween, and a thrust bearing surface opposed to one end surface of a thrust plate provided on the main shart with a slight clearance defined there between; a second bearing portion having a thrust bearing surface opposed to the other end surface of the thrust plate wIth a slight clearance therebetween:

the first bearing portion and the second bearing portion being put together at mating surfaces of fixing portions formed on members constituting outer walls or the bearing portions. and being coaxially joined to each Other with joining bolts Inserted into the fixing portion of ont. of the bearing portions and threadedly engaged with Internal threaded portions formed on the fixing portion of the other.

wherein at least the member constituting the outer wall of the first bearing portion is made or a material of low specific gravity, such as ceramic material, syntherir 0 resin, or graphite while the Internally threaded portions are made of a metallic material.

The term "at least the member constituting the outer wall" refers, In the case of a bearing portion in which a bearing sleeve Is fixed In an inner surface of a housing. to the arrangement in which the housing alone is made of said nonmetallic material and the arrangement In which both the housing and the bearing sleeve are made of said nonmetallIc material. In the case of a bearing portion (of single cylindrical construction having a housing alone) of the arrangement in which the housing Is directly formed with a journal bearing surface and/or a thrust bearing surface. the housing is made of said nonmetallic material.

Said rIxIng portion may comprise a frange portion Integrally formed on outer end surface of the other of the bearing portions and having the mating surface, and a metallic annular member having the Internal threaded portions and opposed to the mating surface wLLh the flange portion therebetween. Further. a metallic annular or circular member having a seat surface for a head of a joining bolt to be seated thereon may be disposed on the fixing portion of the bearing portion on the side which receives the joining bolt.

By forming at least the member constituting the outer wall or the first bearing portion of a nonmetallic material of low specific gravity. such as ceramic material.

synthetic resin, or graphite, it Is possible to reduce the weight of the bearIng portions and hence the entire spindle. Furthermore. formation of the Internally threaded portions of the fixing portion of a metallic material eliminates the fear of the Internally threaded portions being damaged; thus, the joining bolts can be tightened with sufficient torque, thereby securing the required rIgi.dity for the bearing portions.

Further, by fixing the metallic annular member on the outer surface of' the bearing portion In advance and processing the journal bearing surface while chucking an outer surface or the annular member. It Is possible to prevent deformation of the bearing portion under the action of the chucking force., thus, the journal bearing surface can be accurately machined.

Further. the metaille annular or etreular member having the seat surface for the head of the Joining bolt to be seated thereon Is disposed on the fixing portion of the bearing portion on the side which receives the joining bolt. whereby the scat surface portion can be prevented from being damaged under the action of the tightening force for the Joining bolt. Particularly, the disposition of such annular member ensures hat the tightening force for the Joining bolt Is uniformly applied circumferentially of this member, so that the joining bolt can be tightened with greater torque and hence the rlgidtry of the bearing portion c-an he further Improved.

r 1 BRIEF DESCRIPTION OF TRE DRAWINGS

Fig. 1 Is a longitudinal sectional view showing a spindle according to an embodiment of the Invention; Fig. 2 Is a cross sectional view taken along the line A-A In Fig. 1; Fig. 3 Is a cross sectional view taken along Lhe line B-B In Fig. 1; Fig. 4 Is a longitudinal sectional view showing a spIndle according to a second embodiment of the Invention (Fig. 4 (a) and Fig. 4 (b) show different sections); Fig. 5 Is a longitudinal sectional view according to a third embodiment of the Invention; Fig. 8 Is a longitudinal sectional view of a conventional spindle (Fig. 6 (a) and Fig. 6 (b) show different sections); Fig. 7 Is a. cross sectlonal View taken along the line A-A In Fig. 6; and Fig 8 Is a cross sectional view taken along the line B-B In Fig. 6; DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. I shows a Iong1tud1nal section taken along the axis of an eternally pressurized air bearing spindle according to a first embodiment of the Invention. The eter'nally pressurized air bearing spindle In L11IS 12 rembodiment comprises a main shaft 1 to perform rotative motion, and housIngs 2 and 3 as the bearing portion for non-contact supporting of the main shaft 1. The housIngs 2 and 3 as bearing portion are each of a single member. not having a bearing sleeve usually found In a conventional spindle. The driving system for the main shaft 1 Is, for example, an That is, an outer periphery main shaft 1 Is formed with and turbine nozzles 4 which tangentially open at posItIons opposed to the recesses lal Is fixed to surface of the housing 3, so that compressed air through a turbIne air feed port 5 and turbine air passage 6 opened to the housing 3 is tangentially aralnst the recesses lal air turbine driving system. of a thrust plate la of the a plurality of recesses Ial a an Inner fed In f eed of the thrust plate la to rotate the main shart i. And Lhe main shaft 1 thus rotated Is supported in a state of non-contact by the housings 2 and 3 as the bearing portion.

An inner surface of the housing 2 Is formed with a journal bearing surface 2& opposed to an outer surface of the inain shaft 1 with a slight journal bearing clearance defined therebetween, and the rear end thereof Is formed with a thrust bearing surface 2b opposed to the front end surface of the thrust plate la of the main shaft 1 with a slight thrust bearing clearance. A plurality of fine nozzles 2c, 2d are opened to the thrust bearing surface 2a, whi le a plurality of fine nozzles 2e are opened to the 13 journal bearing surface 2b. The nozzles 2c, 2d, 2e are the same In number (in this embodiment, each being six nozzles) and are respectively disposed on the same circumference. Further. the housing 2 19 provided with radial air feed passages 2f. 3g. and axial alr feed passages 2h which put the nozzles 2c, 2d. 2e of the same cross section In communication with each other and. as shown In Fig. 2 (cross-section A-A In FIC. 1), It Is also provided with a contliluo4s polygonal air feed passage 21 which Is approximately polygonal as a whole and which puts the nozzles 2c In circumferential communication with each other. In this embodiment, Lhe polygonal air reed passage 21 Is approximately triangular, wherein the three straight passage portions 211. 212. 213 forming the! respective sides are each associated with two nozzles 2c; 'thus. the six (In all) nozzles 2c as a whole are put in circumferential communication with each other. This polygonal air feed passage 21 communicates with an axial air feed passage 2J opened to the rear end nf the housing 2.

The radial air passages 2T. 2g and polygonal air feed passage 21 can each be formed by performing a drilling operation from the outer surface of Lhe housing 2 In the associated direction to the predetermined position. Similarly. the axial air feed passages 2h, 2J can be formed by performing a drilling operation which proceeds from the end surface of the housing 2 axially to the 14 predetermined position. Thus. the Individual air feed passages can be formed by performing a drilling operation on the h ousing 2 composed of a single member and. if necessary. closing the opening In the outer surface side or end surface side by sealing means 7. such as a sunk plug or resin molding.

The front end of the housing 3 Is provided with a thrust bearing surface 3b opposed to the rear end stirface of the Lhrust plate la with a slight thrust bearing clearance defined therebetween. A plurality of fine nozzles 3 care opened to, the thrust bearing surface Sh. The nozzles 3c are disposed on the same circumference. Further, the housing 3 Is provided with an air feed port 3d connected to an unlilustrated compressed air source. and an axial air feed passage 3e communicating with the axial air feed passage 2j. and. as shown In Fig- 3 (crosssection B-B In Fig. 1). It Is also provided with a continuous polygonal air feed passage 3f which Is approximately polygonal as a whole and which puts the n02zles 3c In circumferential communication with each other. In this embodiment. the polygonal air feed passage 3f Is approximately triangular. wherein the three straight passage portions 3fl, 3f2, 3f3 forming the respective sides are each associated with two nozzles 3c; thus, the six (In all) nozzles 3c as a whole are put in circumferential communication with each other. This polygonal air feed passage 3f communicates with the nxial air feed port 3d through the axial air feed passage 3g.

The polygonal air feed passage 3f can be formed by performing a drilling operation from the outer surface of the housing 3'-In the directions of the passage. portions 3fl, 3f2, 3f3 to the predetermined positions. Thus, the Individual air feed passages can be formed by performing a drilling operation on the housing 3 composed of a single member and, If necessary, closing the opening In tbe outer surface side by seallng means 8. such as a sunk plug or resin molding.

As described above, the housings 2 and 3 are coaxially joined to each other with jointing means. such as bolts, with their end surfaces put together after the main shaft 1 and turbine nozzle 4 have been assembled.

When compressed air Is fed from the air feed port 3d of the housing 3. It flows through said various air feed passages and nozzles into the journal and thrust bearing clearances and then It Is discharged from the bearing end directly or via exhaust passages 9 and 10 Into the outside of the spindle. The pressure distribution. of compressed air produced In the bearing clearances is effective to support the main shaft I In a state of non-contact to the Journal bearIng surface 2a and thrust hearing surfaces 2b and 3b.

As has been described so far. the eternally pressurized alr-bearlng spindle In this embodiment has the arrangement In which the nozzles 2c. 2d, 2e. 3c opened to 16 the bearing surfaces 2a, 2b, 3b are put irL elreumferenLial communication with each other by the polygonal air feed passages 21. 3f; therefore, all air feed passages can be opened to the housings 2, 3 each composed or a single member by performing a drilling operation. Therefore, there Is no need for the bearing portions to be of the construction In which housing and the bearing sleeve are Integrally fitted together. as In the conventional spindle, (as described above, the reason for employing the construction In which the housing and the bearing sleeve are Integ rally fitted together, in the conventional sPindle, Is that It is necessary to form the circumferential groove which puts Lhe nozzles lit circumferential communication with each other) and It can be constructed of housings 2. 3 each composed of a single member. whereby the rigidity of the hearing portions can be Improved as- compared with the conventional spindle. As a result of the rigidity of the bearing portions (the housIncs 2. 3) being Improved. not only can the accuracy of rotation of the main shaft be Improved buE also lic becomes possible to apply various means for reduction of weight, such as making the housings 2. 3 of a material of low specific gravity or making it thin-walled.

When the housings 2, 3 are to be made of a material 6f low specific gravity to achieve reduction of weight. it is advisable to use an aluminum alloy, synthetic resin. graphite or the like as such material of low specific 17 1 gravity. Among other things. when the durabillLy Is " taken into consideration which is associated with the case where the main shaft 1 and the bearing surfaces (2a. 2b. 3b) come In contact with each other under overload or the like. it is advisable Lo use a synthetic resIn having a selflubricating property, graphite or the like. As for resins having a self-lubricating property, mention may be made of fluorine type resins. polyamide type resins, poly1mide Lype resins, ketone type resins. such as polyetherathylketone(PEEK).- and polyphenylenesulfide(PPS) type resins. Alternatively. the bearing surfaces 2a, 2b. 3b of the housings 2. 3 may be formed with films containing a solid lubricant, such as fluorine type resin, molybdenum bisulfide. or hexagon.system baron nitride. or a wear resistant material, such as S1, SIC. or TIC, by a suitable method, such as resin coating, dispersion plating or vapor deposition.

Fig. 4 shows a longitudinal section taken along the axis of an eternally pressurized air hearing -plyadle according to a second embodiment of the Invention (Fig. 4 (a) and Fig. 4 (b) show different longitudinal sec. tions). In addition. substantially the same members or portions as those shown In Figs. 6 through 8 are denoted hy the same reference characters and a repetitive descriptlon thereof is omitted.

The points In which the eternally pressurized air bearing spindle of ChIs embodiment differs from the 18 conventional arrangement shown In Figs. 6 through 8 are that a housing 23' which constitutes an outcr wall of a first bearing portion X and a housing 25' which constitutes an outer wall of a second bearing portion Y are made of a nonmetallic material of low specific gravity, such as ceramic material, synthetic resin, or graphite and that a metallic annular member 41 having Internal threads 41a Is fixed on an outer surface of the housing 2 3'. Bearl.ng sleeves 24 and 26 are made of a metallic material. such as a bronze type alloy, but it may be made of a nonmetallic material as mentioned above.

The outer end surface of the end of the housIng 23 Is Integrally formed with a flange 42 having a mating surface 42a. and the annular. member 41 Is fixed to the side opposite to the mating surrace 42a of the flange 42 by suitable means, such as force fitting or adhesion. The annular member 41 Is made of a metallic material, such as steel, stainless steel, or aluminum alloy and Is provided with Internal threads 41a corresponding to the joining bolts 27. Further. the flange 42 Is formed with a plurality of boltholes 42b extending therethrough which are In phase with the Internal threads 41a or the annular member 41. The fixing portion 43 of the first bearing portion X is composed of such flange 42 and annular member 41.

The fixing portion 45 of the second benrIng portion Y Includes. as In the conventional arrangement. a matIng 19 surface 45a fitted to the mating surface 42a of the fixing portion 43 of the first bearing portion X, a plurality of boltholes 45b extending therethrough In phase wiLli- tile boltholes 42b In the fixing portion 43 and Internal threads 41a, and counterbares 45c to receive the heads 27a of the joining bolts 27. In this embodIment, a circular flat washer 45d is disposed on the bottota of each counterbore 45c.

The jolnlng between the housing 23' of' the rirst bearing portloin X and the housing 25' of the second bearing portion Y Is effected by putting together the mating surfaces 42a. 45a of the fixing portions 43, 45. Inserting the joining bolts 27 Into the boltholes 45b, 42b from the side of the fixing portion 45 to engage Lhe Internal threads 41a for tightening. Since the Internal threads 41a of the fixing portion 43 are provided in the metallIc annular member 41. there Is no fear or the Internal threads 41a being damaged even If the housings 23' and 25' are made of a nonmetallic material as mentioned above for reduction of weight. Therefore, the Joining bolts 27 can be tightened with sufficient torque. thereby securing the required rigidity for the main bearing portion. Further. since flat washers 46d are disposed In the counterbores 45c, It Is possible to prevent the heads 27a of the joining bolts 27 from damaging the counterbores 45c. Further. deformation of the housing 23' under the action of chucking force can be prevented by fixing the metallic annular member 41 to the outer surface of the housing 23' In advance (before the journal beaHng surface 24a Is processed) and machining the journal bearingsurface 24a while- ehueklng the outer surface of the annular member 41; therefore. the journal bearing surface 24a can be accurately machined.

In a third embodiment shown In Fig. S. a metallic annular member 46 having a seat surface for the head 27a of a jolninir bolt 27 to be seated thereon Is disposed In the fixing portion of the second bearing portion Y. The annular member 46 Is made of a metallic material, such as.steel. stainless sLeel. or aluminum alloy, and Is provided with boltholes 46a corresponding to the joining bolts 27. The annular member 46 Is disposed on a step 45e formed on the outer end surfane of the end of the fixing portion 45.

When the joining bolts 27 are tightened with the thickwalled portions of the housings 23' and 25' sandwiched beLween the metallic 'annular members 41 and 46. the tightening force is circumferenttally uniformly applied to the housings 23' and 25' through the annular members 41 and 46. thus making It possible to tighten the joining bolts 27 with higher torque. Thereby, the rigidity of the bearing portions can be further Improved.

The arrangements of the second and third embodiments described above are not limited to an eternally pressurized air bearing spindle of the type in which each 21 bearing portion Is constructed by Integrally fittIng the housing and the bearing sleeve together, but are also applicable to an eternally pressurized air bearing spindle of the type In which each bearing portion Is of sIngle cylindrical construction, as In the first embodimment, In which case the same effects can be achieved.

In addition, the present Invention Is not limited to an eternally pressurized air bearing spindle of the air turbine drive type, but Is also applicable to an eternally pressurized air bearing spindle of the type driven by an electric motor, such as an Induction motor or any of various servomotors.

22

Claims

WHAT IS CLAIMED IS:

I. An eternally presstiri7ed air bearing spindle, having a housing directly provided with a bearing surface opposed to an outer surface of a main shaft with a slight bearing clearance defined therebetween; a plurality of nozzles disposed on the same circumference and opened to said bearing surface; and a continuous polygonal air feed passage putting said plurarity of nozzles In circumferential communication with each other.

wherein compressed air Is ailowed to finw Into said bearing clearance throurh said plurIty of nozzles to support said main shaft in a state of non-contact with respect to said bearing surface of said housing.
2. An eternally pressuri2ed air bearing spindle as set forth In Claim 1. wherein said polygonal air feed Passage Is composed of a plurality of straight passage portions formed from an outer surface of said housing in predetermined directions.
3. An eternally pressurized air bearing spindle as set forth In Claim I or 2. wherein said housing Is made of a material of low specific gravity, Stich as ceramic materlal.synthette resin, graphite or aluminum alloy.
4. An eternally pressurized air bearing spindle as set forth In. Claim 3. wherein said material of low specific gravity is a material liftving a self-lubricating 23 property.
S. An eternally pressurized air bearing spindle as set forth In Claim 1. 2 or 3, wherein said bearing surface of said housing Is formed wIth a lubrIcating film or wear resistant film.
6. An externally pressurized air bearing spindle,com.p rising a first bearing portlon having a journal bearing surface opposed to an outer surface of a main shaft with a slight clearance defined therebetween, and a thrust bearing surface opposed to one epd surface of a thrust plate provided on said main shaft with a slight clearance defIned therebetween; a second bearing portion having a thrust hearing surface opposed to the other end surface of said thrust plate with a slight clearance therebetween; said first bearing portion and said second bearing portion being put togetherouter walls of said bearing portions.and being coaxIally joined to each other with Joining bolts Inserted Into said fixing portion of one of said bearing portions and threadedly engaged with Internal threaded portions formed on Rald fixing portion of tile other, wherein at least said member constituting said outer wall of said first bearing portion is made of a matertal of low specific grayfry, such as ceramic material. synthetic resin. or graphite while said Internally 24 threaded portions are made of a metallic material.
7. An eternallY pressurized air bearlng spindle as set forth In Claim 6. wherein said other fixing portIon Is Integrally forined on the outer surface of said other end with a flange having a mating surface. and has a metallic annular member with said flange disposed therehetween, said annular member havIng said Internal threads.
a. An eternally pressurized air bearing spindle as set forth In Claim a or 7, wherein a metallic annular or circular member having a seat surface for the head nt said joining bolt to be seated thereon.
9. An eternally pressurized air bearing spindle substantially as hereinbefore described with reference to the accompanying drawings.