GB2248892A - Fluid bearing - Google Patents

Abstract

A fluid bearing comprises a plenum chamber 28 having a source of pressurised fluid supplied thereto, a reaction member 42 adapted to co-operate with a deflectable bearing face member 24, and duct means 30 to allow passage of fluid from the plenum chamber to the interface between the bearing face member and a co-operating sliding surface 32. A vent 38 allows the space 40 between members 24, 42 to be at ambient pressure. The reaction member 42 may be provided on a part which is movable relative to the housing. The fluid may be either a gas or a liquid. Such bearings may provide linear bearings for a tool slide, or be incorporated in thrust bearing assemblies. <IMAGE>

Description

Bearings The present invention relates to linear bearings operating on a fluid film, the fluid being particularly, though not exclusively, a gas.

Low friction linear bearings are necessary in some applications such as in machine tool slides to locate a rapidly oscillating cutting tool, for example. High stiffness is required of the bearing for tool position accuracy and low friction to avoid high power losses and sluggish response in driving the tool.

As a general consideration low friction and high stiffness are usually always advantageous in bearing designs. In the present case such an application may be a thrust bearing, for example.

In applications such as the machine tool slide, lowest friction is most easily achieved by a gas bearing where fluid viscosity and hence viscous drag on the relatively moving components is reduced to a minimum.

In the case of a thrust bearing, minimum friction may not be the primary consideration and the bearing may have to be able to work efficiently with a liquid film.

According to the present invention a bearing for supporting load via a fluid film comprises a plenum chamber having a source of pressurised fluid supplied thereto , a reaction member adapted to co-operate with a deflectable bearing face member and duct means to allow passage of fluid from the plenum chamber to the interface between the bearing face member and a co-operating sliding surface.

In B preferred embodiment the deflectable bearing face member may be substantially circular.

In one embodiment of the present invention the reaction member itself may be initially moveable to allow "setting up" or "zeroing" of the bearing against the co-operating sliding surface which may, for example, be a slideway; the reaction member then being fixed in the desired position by, for example, adhesive means.

In another embodiment the reaction member may itself be continuously axially moveable to provide a "fully floating" bearing in order to be able to accommodate surface and/or machining imperfections greater than the fluid bearing film thickness.

In operation the bearing face member deflects according to the applied load to increase or decrease the convergence in the fluid film as the load increases or decreases respectively.

For a given supply pressure of fluid to the plenum chamber the resulting fluid film thickness may be controlled by the diameter of the deflecting bearing face member whilst maintaining a thickness of not less than about 0.5mm of the deflecting face member. The reason for this is that at thicknesses below about 0.5mm the face member becomes difficult to machine accurately and reproduceably. Where, for example, the fluid is air having a supply pressure of about 600 kPa, a circular bearing face member having a diameter of about l9mm may sustain a film thickness in the range from about 2 to 5 micrometres. A face member of about lOOmm diameter may, under the same operating conditions, sustain a film thickness of up to 100 micrometres.The latter bearing, therefore, is suitable where it is required to travel relative to an uneven or rough surface; an example of this may be in a bearing or bearings following the surface of a continuously moving strip, such as rolled steel strip, for example. The earing or bearings may carry a transducer for the purpose of measuring the thickness of the strip and still maintain a high film stiffness thus allowing the bearings to follow the larger waviness of the moving strip for measuring purposes.

A further advantage of bearings according to the present invention, and operating on a gas film, is that they are able to utilise and operate at high gas pressures which have hitherto been considered impractical. High pressures in known gas bearings have caused bearing instability due to the gas flow velocity within the main body of the supporting film exceeding supersonic speed. It has been found that control of the thickness of the deflecting face member allows the use of gas pressures up to and exceeding lOMPa. The only proviso to the use of such high pressures is that exit film thicknesses are maintained to about 1 to 2 micrometres . The advantage of bearings operating under these conditions is that very high loads may be sustained on very stiff films in bearings of compact size.

In one application employing bearings of the present invention there is provided a machine tool slide wherein the tool holder is supported by gas bearings according to the invention, the bearings being arrayed about a rectangular section frame work to locate the tool holder slide in two mutually perpendicular directions, the tool holder axial position being controlled by, for example, a linear motor.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a cross section through a bearing according to the present invention in the unloaded condition; Figure 1(a) shows part of the bearing of Figure 1 in the loaded condition; Figure 2 shows a cross section through a first modification of the bearing of Figure 1; Figure 3 shows a cross section through a second modification of the bearing of Figure 1; Figure 4 shows a cross section through a third modification of the bearing of Figure 1; Figure 5 is a graph showing the bearing face member profile and the fluid pressure distribution of the bearing of Figures 1 and l(a), when under load;; Figure 6 shows a schematic perspective view of a arrangement for a machine tool slide employing bearing as shown in Figures 2 and 3 or 4; Figure 7 shows a cross section of the tool slide of Figure 6; Figure 8 shows schematically a cross section through a rotary thrust bearing employing bearings of the type shown in Figures 2 and 3 or 4; and Figure 9 which shows a second embodiment of a thrust bearing.

Referring now to Figure 1 where a fluid bearing, which in this instance is operating on air, is shown at 10. The bearing comprises a fixed housing 12 having a co-operating slideable bearing member 14 contained therein. The bearing member 14 has a generally cylindrical body 16 with an annular recess 18 to accept an '0' ring seal 20 which co-operates with and seals against a bore 22 in the housing 12. A bearing face member in the form of a thin, and relatively flexible flange 24 is provided to form a deflectable bearing face member which forms part of the bearing sliding surface . A pressurised supply of gas (not shown) is provided through a conduit 26 in the housing 12, the conduit 26 opening into a plenum chamber 28 formed between the housing and the member 14. A duct 30 in the member 14 supplies gas to the interface between face member 24 and the co-operating sliding surface 32. The outer periphery 34 of the face member 24 contacts a rebated surface 36 of the housing, which surface constituting a reaction member. A vent 38 is formed so that the space 40 formed between the inner face 42 of the bearing face member 24 and the housing below the seal 20 is at ambient pressure.

The bearing in Figure 1 is shown in the unloaded position.

As the load on the bearing increases (Figure 1(a)) the face member 24 deflects to increase the convergence of the air film in the space 44 (the clearances and changes in convergence are shown greatly exaggerated). The bearing member body 16 moves in the bore 22 under the influence of the change in the pressure distribution shown in Figure 5.

Curve 'A' shows the pressure distribution of air within the bearing face which has a radius of about 9mm, whilst curve 'B' shows the curvature of the flange 24 which has a thickness of about 0.5 mm, is made from a steel material and where the air supply pressure is approximately 600 kPa. The line 'C' in Figure 2 corresponds approximately to the outer extent of the body 16. In practice the flanged bearing face 24 is freqently made so that a slight convergence exists in the unloaded condition (ie in the free condition the member 24 adopts a form as shown greatly exaggerated in Figure l(a).

Figure 2 shows a construction of an embodiment for use where the tolerances between the lower face 50 of the housing 52 and the co-operating sliding surface 54 either cannot be predicted or perhaps, alternatively , cannot be machined with sufficient accuracy or is dependent upon some other variable. The bearing described has the ability to co-operate with variations in surface topography which are beyond the capability of the deflecting face member alone. A bearing member 56 is provided with a body 58 which slides in a bore 60 of a reaction member 62. In this case the bore 60 is lined with a low friction PTFE based material 64 which also acts as a seal with the body 58.

The bearing member has a deflectable bearing face member 66; the periphery 68 of which co-operates with a downwardly and outwardly directed surface 70 form a space 72 which is maintained at ambient pressure by a vent duct 74. The reaction member 62 itself has a cylindrical sliding surface 76 which slides in a bore 78 formed in the housing 52. The outer surface 76 of the reaction member has an annular groove 80 to receive an '0' ring seal 82 which co-operates with the bore 78 in the housing 52. A plenum chamber 82 is formed in the space between the housing 52, the reaction member 62 and the bearing member 56. Conduits 84, 86 are provided to conduct pressurised gas from a supply (not shown) to the interface 88 between the sliding face 90 of the bearing member and the co-operating sliding surface 54.In operation the bearing member 56 operates in substantially the same manner as does the bearing of Figures 1 and 1 (a). However, the ability of the reaction member 62 to slide in the bore 76 to take up any large clearance or other surface variations which may exist allows the bearing to be used in applications where extremely small machining tolerances are not necessary.

Figure 3 shows a bearing which may be used in circumstances where there is a relatively large, but relatively constant clearance which must first be accommodated before the bearing may operate. The bearing has a similar construction to that shown in Figure 2 but the groove and '0' ring seal 80, 82 are replaced by a layer of an adhesive 96, such as for example, an epoxy resin, immediately prior to assembly and first operation of the bearing. On initial operation the member 62 slides relative to the housing 52 to take up any excess clearance which may exist between the face 90 and the surface 54.

The bearing is maintained in this position until the adhesive 96 cures. The use of epoxy resin as the adhesive is, of course, exemplary and any other suitable adhesive may be used.

Figure 4 shows an alternative embodiment of bearing having the same objective as the bearing of Figure 3, to remove a relatively constant, but excess clearance. The bearing assembly comprises a housing 100 having a cylindrical bore 102 therein to receive a bearing reaction member 104 which has a cylindrical body 106, which co-operates with the bore 102, and a body portion 108 which receives a bearing member 110. The body 106 has an annular groove 112 and '0' ring seal 114 to seal against and co-operate with the bore 102.The body portion 108 has a bore 116 to receive the body portion 118 of the bearing member 110 which functions substantially as described with reference to Figure 1 and l(a). The reaction member 104 has a circular, central recess 120 formed in the top face 122 thereof and a slot 124 also formed in the top face which allows gas access to the duct 126 from a plenum chamber 128. The top surface 130 of the bore 102 has a counterbore 132 which allows an easy sliding fit to a steel ball 134; a conduit 136 allows access of pressurised gas to the counter bore 132 and a second conduit 138 allows access of pressurised gas to the plenum chamber 128. In operation the bearing is supplied with pressurised gas via the conduits 136, 138 which moves the reaction member 104 towards the sliding surface 140 and keeps the ball 134 in contact with the central recess 120.Adhesive 142 is injected to fix the ball 134 in a final position, the assembly being maintained in its stationary position until the adhesive cures. The fixed ball 134 acts a a pivot for the reaction member 104 via the recess 120 and allows a small degree of compliance of the bearing member 110 with the sliding surface 140.

The bearing of Figure 4 may be modified by removal of the ball 134, counterbore 132, conduit 136, recess 120 and slot 124 to provide a more compliant bearing, such as that described with reference to Figure 2.

Figures 6 and 7 show schematically a tool slide which employs bearings as exemplified in Figures 1,2 and 3 or 4.

The slide in Figure 6 is shown, for the sake of clarity, without the internal sliding member which carries cutting tool bits in this instance. The slide frame 150 comprises two main slide frame members 152 of generally 'U' shaped section made fron Sintox (trade mark) material. Tool bit positions are shown at 154, 156 and which are carried on a sliding member 158. The sliding member 158 has an accurately ground rectangular section with flat faces 160, 162, 164. 166 which co-operate with gas bearings as shown in Figures 1, 2 and 3 or 4. Only the positions of the bearings are shown for the sake of clarity. Bearings of the type shown in Figure 1 may be installed in positions 168, bearings of the type shown in Figure 2 are installed in positions 170, 172, 174 and bearings of the type shown in Figures 3 or 4 are fitted in positions 176 , 178. The tool slide 158 may be moved, and its axial position controlled by a linear motor, part of the drive magnets of which are shown at 180, 182. The position of the tool slide 158 is measured by a transducer, the position of which is shown at 184 and which feeds signals to a known closed loop control system (not shown).

In operation the slide 158 is forced up against the bearings in positions 168, 170 and 172 which are effectively fixed and their positions and operating film thicknesses accurately known. The self adjusting nature of the bearings in positions 174, 176 and 178 take up any unwanted clearances to leave the slide fully supported by the bearings at the optimum film thicknesses. Cutting forces may be in either direction as the preload applied to the bearings by the gas pressure will always be in excess of the cutting forces.

The tool slide as shown will operate at about 500 kPa in practice.

Figure 8 shows an arrangement for a thrust bearing assembly 190 where a rotatable shaft 192 supported on journal bearings (not shown) has a thrust flange 194 having two thrust faces 196, 198. Bearings 200 of the type shown in either Figures 1 or 3 or 4 are arranged on face 196 of the flange 194, whilst bearings 202 of the type shown in Figure 2 are arranged on face 198. Only the bearing face members and positions are shown in the drawing for the sake of clarity. Bearings may be provided every 600 about the flange, or in any desired quantity or spacing to support the anticipated load.

The annular duct 230 may be made small enough to restrict the gas supply to the interface 224 so as to cause a significant pressure drop in the gas from the plenum chamber 222. This feature may be used to tailor the characteristics of the bearing in response to a variable supply pressure. Alternatively, small holes or orifices may be used for the same purpose in other geometrics where the bore 214 does not exist e.g where the shaft 210 is on the opposite face of the flange 212 and does not pass through the bearing member 216.

Figure 9 shows a thrust bearing assembly where a shaft 210 supported on journal bearings (not shown) has a thrust flange 212. The shaft 210 passes through a bore 214 of a bearing member 216 which is received in sealed, sliding engagement with a bore 218 in a housing 220 (shown in part only). Pressurised gas is supplied to a plenum chamber 222, which is represented by the space to the right of the bearing member 216. The gas is conducted to the interface 224, between the bearing face member 226 and the thrust face 228 of the flange 212, via an annular duct 230 constituted by the clearance between the shaft 210 and the bore 214 of the bearing member 216. The bearing face member 226 reacts in substantially the same manner as described with reference to Figures 1 and 1(a) in response to load indicated by the arrow 232.

Although the bearings have generally been described with reference to being supported by a gas film, they may be operated with a liquid. The corresponding clearances and film thicknesses are somewhat greater due largely to the increased difficulty of filtering liquids efficiently.

Claims (13)

1. A bearing for supporting load via a fluid film, the bearing comprising a plenum chamber for receiving pressurised fluid, a reaction member adapted to co-operate with a deflectable bearing face member and duct means to allow passage of fluid from the plenum chamber to the interface between the bearing face member and a co-operating sliding surface.

2. A bearing according to claim 1 wherein the fluid is a gas.

3. A bearing according to claim 1 wherein the fluid is a liquid.

4. A bearing according to any one preceding claim wherein the deflectable bearing face member is substantially circular.

5. A bearing according to any one preceding claim wherein the reaction member is initially moveable within a housing and is subsequently fixable in a desired position.

6. A bearing according to claim 5 wherein the reaction member includes a cylindrical body portion which is slideable in a bore in a housing, the interface between the reaction member cylindrical body portion and housing bore being supplied with adhesive means for sealably fixing the position of the reaction member.

7. A bearing according to claim 5 wherein the reaction member is slideable within a housing and has positionable stop means to maintain the reaction member in a desired position.

8. A bearing according to any one of preceding claims 1 to 4 wherein the reaction member is continuously, axially moveable.

9. A bearing according to claim 8 wherein the reaction member has a cylindrical body part which is slideable in sealed engagement within a bore of a housing.

10. A tool slide assembly having a bearing according to any one of preceding claims 1 to 9.

11. A thrust bearing assembly having a bearing according to any one of preceding claims 1 to 9.

12. A thrust bearing according to any one of claims 1 to 9 wherein the co-operating sliding surface rotates about an axis which is substantially coincident with the axis of the bearing face member.

13. A bearing substantially as hereinbefore described with reference to the accompanying specification and Figures 1 and l(a), 2, 3, 4, 6 and 7, 8 or 9 of the drawings.