Backaround to the invention Typically guide rails are machined to a high accuracy and usually provide two outwardly facing parallel faces to serve as a linear guide for a worktable or tool carriage to slide on and to be guided along a linear path. The bearing normally comprises a metal housing which straddles the guide rail and presents opposed pads containing hydrostatic pocket to the two outwardly facing parallel machined faces of the guide rail.

By providing an appropriate small dimensional clearance between the pads and the parallel faces of the guide rail, the latter can be clamped between the two pads by the supply of high pressure oil to the pockets in the two pads, via restrictors, in manner known per se.

Whilst the resulting bearing assembly can be considered to be stiff in a direction perpendicular to the parallel machined faces of the guide rail, the film of oil between the pads and the guide rail ensures that the housing can slide along the guide rail with minimal friction.

The stiffness and other properties of the bearing assembly are governed to a large extent by the size of the gaps between the pads and the opposed parallel faces of the guide rail. Any variation in the gap size results in variable bearing characteristics and consequently such guide rails have been manufactured using precise dimensional control to ensure a high degree cf parallelism of the faces. Whilst any slight non parallelism cr local low or high points on either surface can usually be accommodated by the gaps between the pads and the opposed faces of the guide rail, the presence of any such imperfections will normally appear as a deviation from the linear and straight line path which the carriage should follow as it slides from one end to the other of the rail, since the oil pressure and servo feedback associated with the hydrostatic bearing will tend to manoeuvre the carriage so as to follow a mean path, so that the gaps on opposite sides of the guide rail are always equal. Thus if one face happens to be true and the other face not, the imperfection in the other face will govern the accuracy of the linear motion of the carriage as it tries ço accommodate the varying distance between the two opposed faces of the guide rail.

The machining of a single linear surface to serve as a guide presents far fewer problems than when two such surfaces are to be machined, particularly when they are on opposite faces of a relatively long length of metal and the two surfaces not only have to be flat but also parallel. Where a carriage is to be hung from a rail, a single flat true surface can be arranged to face upwardly and hydrostatic bearing pads can be provided in a carriage assembly which is located on the rail so that the pads face downwardly to co-act with the upwardly facing guide rail surface. The supply of oil under pressure to the bearing pads generates a thin film of oil in manner known per se on which the carriage can slide. An unopposed pad arrangement such as this will normally use the mass of the hanging, carriage to generate the pre-load on the pads for stiffness, but this type of arrangement is only applicable to vertical loading applications. Where the hydrostati bearing arrangement is to provide lateral control of an otherwise suspended assembly, the pre-load has to be provided by an opposed pair of pads and it is in such situations that the need for parallel accurately machined opposed outwardly facing surfaces has been so essential.

It is an object of the present invention to provide an- alcernativc hydrostatic bearing arrangement for use with opposed parallel vertical faces of a guide rail in which only one of the faces has to be machined to ultra high accuracy so as to define the linear path which the carriage or other machine component is to follow.

Summarv of the invention According to one aspect of the present invention in a carriage which straddles and is guided by a guide rail which presents two outwardly facing parallel faces to cooperate with a pair of inwardly directed spaced apart hydrostatic pads such that a working clearance exists between the pads and the parallel faces of the guide rail, and wherein one of the said parallel faces is machined to a high level of precision alignment and flatness, and the hydrostatic pad cooperating therewith (the fixed pad) is adapted to exert gap control via a servo system so as to control the motion of the carriage along the rail, the second pad which cooperates with the other face is adapted to apply force only thereto without gap control, se that the motion of the assembly along the rail is dictated by the flatness and alignment of the precision machined surface of the rail.

According to another aspect of the invention, the second pad is adapted as aforesaid to exert force only on the said other face of the guide rail, by mounting the second pad so as to be freely movable relative to the carriage, and hydrostatic fluid is supplied not only to the one or more pockets in the face of the pad to form a hydrostatic bearing, but also to the rear of the floating pad at a pressure sufficient to generate a force on the floating pad between it and the carriage to balance the force exerted on the opposite face of the guide rail exerted by the hydrostatic fluid in the one of more pockets in the fixed pad on the other side of the guide rail.

The hydrostatic fluid applied to the rear of the floating pad- may be derived from a different source of hydrostatic fluid from that which is supplied to the pocket or pockets in the pads.

Alternatively the same source of hydrostatic fluid may be applied to the pockets in the pads and to the rear of the floating pad, in which event the area of the floating pad presented to the hydrostatic fluid must be selected so that the product of (pressure x area) on the rear of the floating pad, is the same as the product of (pressure x area) of the pockets.

Where the area of the floating pad which is to be presented to the hydrostatIc fluid is less than the overall area of the pad, flexible sealing means may be provided to define the area acted on by the hydrostatic fluid. The seal must be capable of accommodating movement of the pad relative to the housing without the leakage of hydrostatic fluid.

Sealing means for achieving this may comprise a rolling diaphragm, an O-ring, or telescoping annular rings with sealing therebetween.

Since the actual movement of the pad relative to the housing should be very small since it will only be occasioned by imperfections in machining of the rail, an O-ring seal has been found to be sufficient to accommodate all the relative movement involved. Where a greater movement is to be accommodated for any reason, the next preferred seal is a rolling diaphragm seal.

Each pad, both fixed and floating, may comprise a plurality of pockets in its surface which cooperate with a face of the guide rail. Preferably four such pockets, of equal size and symmetrically arranged in the face of the pad are provided.

In known manner, flow restrictors are provided in the hydrostatic fluid feeds to the pockets in the fixed and floating pads.

Hydrostati fluid may be conveyed via flexible conduit means to the floating pad and via passages therein thereby to the pocket(s) in the face thereof. Alternatively where fluid for the pads is derived from the supply of fluid to the rear of the floating pad, fluid passage means may be provided through the floating pad to convey hydrostatic fluid to the pccket(s) in the front face thereof. In either event fluid flow restriction means is incorporated in the fluid passage means supplying hydrostatic fluid to the pockets in the floating pad, and where the fluid for the pads is derived from the supply thereof to the rear sf the floating pad, the restrictor means must be located in the passage means In the pad.

Although an arrangement as described may only exhibit less stiffness than that of a conventional opposed pad system in which both pads are fixed, the lack of mechanical stiffness within the machine structure of normal systems greatly exceeds the potential gains in film stiffness in an opposed design.

The advantage of the present invention is that the provision of the floating pad obviates the need for ultra-precision parallelism between the two surfaces of the guide rail and ultra-precision flatness of the guide rail surface cooperating with the floating pad.

According to another aspect of the present invention a method of guiding a housing relative to a fixed structure means of a rail having parallel opposed faces thereon, comprises forming a first hydrostatic bearing comprising a pad with at least one pocket therein, which pad is fixed in a first part of the housing and faces and cooperates with one of the said parallel faces, and forming a second hydrostatic bearing comprising a second pad and pocket means in the housing so as to face and cooperate with the opposite face of the guide rail to form a second hydrostatic bearing, the second pad being movable relative to the housing towards and away from the guide rail,- and applying hydrostatic fluid under pressure to the rear of the movable (floating) pad so as to force the latter in a direction towards the guide rail, whilst providing hydrostatic fluid under pressure in manner known per se through fluid flow restrictor means to the pockets in the fixed and floating pads.

Preferably the area of the floating pad over which the hydrostatic fluid acts and/or the pressure of the hydrostatic fluid acting thereon is/are selected or adjusted, so as to just compensate for the hydrostatic fluid force acting between the fixed pad and the other face of the guide rail.

In the aforementioned method, a common source of hydrostatic fluid may be used to supply fluid to the pockets of the hydrostatic bearing pads and to the rear of the floating pad, in which event fluid flow restriction means must be inserted in passage means feeding the hydrostatic fluid to the pocket(s) in the floating pad and in passage means supplying hydrostatic fluid to the pocket(s) in the fixed pad.

The invention also lies in a machine tool having a movable carriage guided by a guide rail wherein the carriage includes a hydrostaric bearing assembly constructed in accordance with the present invention, for guiding the carriage along the rail.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows part of a machine tool in which a carriage assembly is guided laterally by a guide rail, Figure 2 is a cross-section through a hydrostatic bearing assembly straddling a guide rail to provide lateral support for a component or carriage attached to the assembly, and Figure 3 shows diagrammatically the construction of a floating pad assembly for use in the hydrostatic bearing assembly of Figure 2.

Detailed descrittion of drawings Figure 1 illustrates part of a machine tool comprising a machine base 10 and a carriage such as a worktable 12 which is shown partly in cross-section and which is slidable relative to the machine base on two linear hydrostatic bearing assemblies 14 and 16. Whilst the hydrostatic bearings 14 and 16 provide vertical support and guidance, lateral guidance is provided by a central guide rail 18 which is straddled by part of the carriage housing generally designated 20, which presents opposed,hydrostatic bearing pads to opposite parallel faces of the guide rail 18. These will be described in more detail in relation to Figure 2. Clearance is provided at 22 so that the only faces cf the guide rail 18 which provide the guidance for the carriage 12 are the two side faces designated 24 and 26.

Figure 2 shows to a greater scale and in cross-section the rail 18 and the housing which interacts therewith. The same reference numerals have been used throughout and reference is made to the description of Figure 1 for items 10 to 26 inclusive.

The invention concerns the replacement of one of the hydrostatic pads with a floating pad assembly as will hereinafter be described. The more conventional fixed pad is retained on the right hand side of Figure 2 and this is formed by machining the inwardly directed face of the housing leg 28 so as to leave an annular land designated 30 which surrounds a shallow depression 32 in the centre of the face to which hydrostatic oil is supplied via a passage 34 which contains a flow restrictor 36.

Although not shown, an additional annular protrusion forming an inner land (not shown) may be formed within the land 30 so as to define an annular trough within 30, the inner land being- machined so that the inwardly directed face thereof is a little below the plane containing the inwardly directed machined face of the land 30, so that the inner land serves as a weir between the central region of the pad and the annular trough.

Additional drillings and passageways (not shown) serve to convey hydrostatic oil away from the annular trough (not shown) all in manner known per se.

Although the land 30 and the central depression 32 have been described as annular, it is of course not necessary for them to be circularly shaped as viewed from the guide rail 18, and the shape of the land 30 may be ovaloid or square or rectangular.

It is also to be understood that although the pad is described as having a single land 30 defining a single depression 32, a number of such depressions and bounding lands may be formed in the inwardly directed face of the housing part 28 with drillings and passageways feeding and retrieving hydrostatic oil to and from each of the areas defined by the separate lands. Where a number of such depressions and bounding lands are provided, they are typically of the same size and are arranged symmetrically relative to the centre of the inwardly directed face of the housing part containing them.

Each of the lands 30 and bounded recesses 32, with or without an inboard annular weir, is conventionally referred to as a pocket.

In a conventional design, the face of the housing 20 which is to co-act with the opposite face 2 of the guide rail 18 would be formed in a similar way to that described in relation to housing part 28. However in accordance with the invention, the hydrostatic pad and pocket assembly for cooperating with the face 24 is formed as a floating member generally designated 38 which is retained in a recess 0 within the left hand housing section 42. The rear wall of the recess 40, item 44, is spaced from the face 24 by a distance such that when the inwardly directed faces of the fixed pad and floating pad engage the guide rail 13, there is a small clearance 44, (of the order of lmm or so) between the rear face 46 of the floating pad 38 and the face 48.

An O-ring seal 50 is located in one, or two cooperating grooves, in the opposed faces 46 and 48.

Centrally cf the face 48 is an opening communicating with a passage 52 through which oil can be supplied to the rear of the floating pad 38. This hydrostatic oil serves two purposes.

1. the oil pressure exerts a force on the floating pad dictated by the pressure and the area bounded by the O-ring 50), in a direction towards the guide rail 18, 2. passages through the floating pad 38 such as 54 and 56 (and internally communicating passages 58 and 60 shown in dotted outline) serve to communicate the oil to pockets in the inwardly directed face of the floating pad 38. Each pocket may include a weir as previously described and additional passages and drilling (not shown) may be provided in the floating pad 38 to convey oil away from the pockets.

Flow restrictors are provided in the passages in the floating pad 38 as at 62 and 64.

The hydrostatic oil supplied to the pockets on the inboard face of the floating pad 38 serves to create a hydrostatic bearing between it and the face 24 of the guide rail 18 in the same way as the hydrostatic oil forms a hydrostatic bearing between the fixed pad on the other side of the housing and the face 26 of the guide rail 18. However by virtue of the floating nature of the pad 38, its ability te move rearwardly into the gap 44 (albeit thereby having to compress the o-ring 50) and by virtue of the constant pressure supplied to the pockets in both of the- two pads, a constant will can be maintained by the servo effect between the pads and the surfaces 24 and 26, and any variation in spacing between the two surfaces 24 and 25 due to faulty machining or lack of alignment or parallelism will be accommodated by the movement of the floating pad 38.

Since a constant gap is now maintained between the fixed pad in the housing section 28 and the face of the guide rail 18, if the latter is straight and true and machined perfectly flat, there is little reason for the carriage 20 to follow anything other than its straight line path dictated by the surface 26.

The surface 26 is therefore the major factor determining the linear movement of the carriage 20 and the purpose of the floating pad 38 is simply to maintain the hydrostatic pressure needed to form the hydrostatic bearing.

It will be appreciated that in this sense the floating pad 38 can be thought of as a piston acting against the face 24 so as to maintain an appropriate gap between the opposite face 26 and the fixed pad hydrostatic bearing. The reason for incorporating a hydrostatic bearing into the floating pad/piston is to reduce the friction which would otherwise exist between it and the face 24.

A preferred arrangement for the pockets in the floating piston 38 is shown in Figure 3. Here four pockets 68, 70, 72 and 74 are shown in the end face of the floating pad 38, each supplied via a drilling 76, 78, 80 and 82 respectively.

Since it may be necessary to adjust the size of the flow restrictor to the pockets 68, 70 etc, a restrictor shim 84 may be employed instead of separate restrictors in each of the passages, the shim containing a plurality of apertures which align with the other ends of the drillings 76, 78 etc, and permit the flow of oil from a common gallery or common drilling such as shown at 86, which is supplied with hydrostatic oil- from drillings such as 5, 5.

To facilitate the removal of the shim, the floating pad 38 may be constructed as shown in Figure 2 with a removable plug 88, held in place by means of a screw 90, and the shim 8z is located between the end face of the plug 88 and the internal face of the recess in the rear face of the pad.

Removal of the shim to permit a shim with differently sized restrictors is thus a relatively simple exercise.