Abstract:
A self-compensating hydrostatic (pressurized fluid film) linear bearing that maintains a fluid gap between a carriage and a rail when relative forces are applied. The geometric shape of the rail and mating carriage enable the bearing to have very high stiffness and load capacity without exessive detrimental carriage deformation. The carriages contain bearing grooves and lands which control and use fluid pressure to provide a very high degree of restoring force in response to changes in the fluid gap. The fluid emanating from the bearing gap is prevented from immediately leaking from the bearing carriage, and is instead routed back to the source from which it is pumped, thereby sealing the bearing carriage and simplifying the handling of the lubricating fluid. The hydrostatic bearing is particularly designed to be compact and to be bolt-for-bolt compatible with conventional linear bearings.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/617,390, filed Jul. 11, 2003, and claims priority from U.S. Provisional Patent Application No. 60/406,933, filed on Aug. 30, 2002, the entire contents of both of which are hereby incorporated into this application by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to mechanical bearings and, more particularly, to hydrostatic bearings for linear motion guidance.  
         [0004]     2. Description of Related Art  
         [0005]     A linear bearing typically includes a carriage and a rail slideably mounted on the carriage. A component, such as a moveable portion of a machine tool, is typically removably mounted on the carriage for sliding movement along the rail with the carriage. A conventional linear bearing uses rolling elements or polymer linings to reduce friction between the carriage and rail.  
         [0006]     In a hydrostatic linear bearing, lubricating fluid is pumped into the carriage and rail at high pressures so that a thin film of lubricant is maintained between the carriage and rail as the carriage slides along the rail, even when large loads are applied to the carriage and rail. The lubricating fluid flows into shallow cavities and channels provided in the carriage and rail. These cavities in the carriage and rail are sometimes referred to as bearing pockets.  
         [0007]     In order to maintain the thin fluid film between the carriage and the rail, some fluid flow resistance or compensation must be provided in the bearing. Typically, capillary tubes, orifices, and control valves are used to provide the required resistance or compensation. A hydrostatic bearing may also be of the self-compensating type, in which resistive lands in the bearing pockets (i.e., planar areas over which fluid flow is restricted), or other bearing pocket features, are used to provide the required flow resistance or compensation.  
         [0008]     Hydrostatic bearings a very desirable in a number of applications because they generally have very high stiffness, high load capacity, low friction, no wear, high damping, and resistance to contamination. All of these advantages make hydrostatic bearings particularly desirable in machine tool applications, where linear bearings with high rigidity and damping capabilities are needed to enable very precise motion that is free of excessive vibration.  
         [0009]     Despite their advantages, hydrostatic bearings have not been widely used in the machine tool industry due to a number of practical problems with their installation and use. For example, the typical compensating devices, orifices, and control valves are often too difficult to install properly in machine tools, and may also be delicate, expensive, or too prone to contamination to provide a reasonable useable lifetime. Additionally, the fluid used for lubrication is easily contaminated by chips and coolant used in the machining process. For these reasons, linear bearings based on rolling elements have been used predominantly in the machine tool industry.  
       SUMMARY OF THE INVENTION  
       [0010]     One aspect of the invention relates to a self-compensating hydrostatic bearing. The self-compensating hydrostatic bearing includes a bearing rail and a bearing carriage constructed and arranged to be mounted for hydrostatically supported movement on the bearing rail. The bearing carriage includes a plurality of self-compensating bearing pads provided on surfaces that oppose the bearing rail. The bearing pads are constructed and arranged to be in fluid communication with one another and with a pressurized fluid source.  
         [0011]     End sealing structures are provided on end portions of the bearing carriage. At least one edge of the end sealing structures engages the bearing rail to prevent hydrostatic fluid from leaking from between the bearing carriage and the bearing rail. Side sealing structures are provided on side portions of the bearing carriage and extend at least a portion of the length of the bearing carriage. At least one edge of the side sealing structure engages the bearing rail to prevent hydrostatic fluid from leaking from between the bearing carriage and the bearing rail.  
         [0012]     The bearing also includes a fluid return system provided within portions of the bearing carriage that are sealed by the end and side sealing structures. The fluid return system is constructed and arranged to route fluid towards the pressurized fluid source.  
         [0013]     Another aspect of the invention relates to a self-compensating hydrostatic bearing. The bearing includes a bearing rail having at least one substantially contiguous support surface constructed and arranged to support the hydrostatic bearing and a bearing carriage constructed and arranged to be mounted for hydrostatically supported movement on the bearing rail.  
         [0014]     The bearing carriage includes a plurality of self-compensating bearing pads provided on surfaces that oppose the bearing rail. The bearing pads are constructed and arranged to be in fluid communication with one another and with a pressurized fluid source. Sealing structure is provided on portions of the bearing carriage. At least one edge of the sealing structure engages the bearing rail to prevent hydrostatic fluid from leaking from between the bearing carriage and the bearing rail. The bearing carriage also includes a fluid return system provided within portions of the bearing carriage that are sealed by the sealing structure. The fluid return system is constructed and arranged to route fluid towards the pressurized fluid source.  
         [0015]     A further aspect of the invention relates to a bearing carriage that comprises one or more bearing pads and a fluid recovery system. The bearing pads are constructed and arranged to receive fluid from a pressurized fluid source and to cause that fluid to flow selectively over a collection of bearing grooves and resistive lands so as to create a supporting fluid layer between the bearing carriage and a structure on which the bearing carriage is mounted for movement.  
         [0016]     The fluid recovery system is constructed and arranged to prevent fluid from flowing out of the space between the bearing carriage and the structure on which the bearing carriage is mounted for movement and to route the fluid back towards the pressurized fluid source. The fluid recovery system includes sealing structure having contiguous end and side portions. The end portions are constructed and arranged to seal ends of the bearing carriage and the side portions are constructed and arranged to extend along at least a portion of sides of the bearing carriage to seal the sides. The end portions include a double-lipped seal. A first lip of the double-lipped seal engages the structure on which the bearing carriage is mounted for movement and the second lip of the double-lipped seal prevents debris from entering the bearing carriage. The fluid recovery system also includes reservoir structure defined by portions of the bearing carriage and sealed by the sealing structure and drain grooves constructed and adapted to conduct pressurized fluid from the bearing pads to the reservoir structures.  
         [0017]     Further aspects of the invention relate to machine tools or portions thereof mounted on hydrostatic bearings.  
         [0018]     Yet another aspect of the invention relates to a bearing carriage. The bearing carriage comprises one or more bearing pads constructed and arranged to receive fluid from a pressurized fluid source and to cause that fluid to flow selectively over a collection of bearing grooves and resistive lands so as to create a supporting fluid layer between the bearing carriage and a structure on which the carriage is mounted for movement.  
         [0019]     The bearing carriage also includes a fluid recovery system constructed and arranged to prevent fluid from flowing out of the space between the bearing and the structure on which the bearing carriage is mounted for movement and to route the fluid back towards the pressurized fluid source. The fluid recovery system includes a sealing structure having contiguous end and side portions. The end portions are constructed and arranged to seal ends of the bearing carriage. The side portions are constructed and arranged to extend along at least a portion of the sides of the bearing carriage to seal the sides. The end portions include a double-lipped seal. A first lip of the double lipped seal engages the structure on which the bearing carriage for movement, and a second lip of the double-lipped seal prevents debris from entering the bearing carriage.  
         [0020]     The bearing carriage also includes reservoir structures defined by portions of the bearing carriage and sealed by the sealing structure and drain grooves constructed and arranged to conduct pressurized fluid from the bearing pads to the reservoir structures.  
         [0021]     Another further aspect of the invention relates to a hydrostatic bearing. The hydrostatic bearing comprises a bearing rail and a bearing carriage constructed and arranged to be mounted for hydrostatically supported movement on the bearing rail. The bearing carriage includes one or more bearing pads provided on surfaces that oppose the bearing rail. The bearing pads are constructed and arranged to be in fluid communication with a pressurized fluid source.  
         [0022]     The bearing carriage also includes seal receiving grooves and a sealing structure having contiguous end and side portions. At least a portion of the sealing structure is adapted to be received in the seal receiving grooves. End portions of the sealing structure include double-lipped seals.  
         [0023]     A fluid return system is also included in the bearing carriage. The fluid return system includes a plurality of drain grooves in fluid communication with the bearing pads. At least some of the plurality of drain grooves are positioned between the bearing pads and the side portions of the sealing structure.  
         [0024]     Yet another further aspect of the invention relates to a method of sealing a hydrostatic bearing carriage. The method comprises causing or allowing hydrostatic fluid to flow from hydrostatic bearing pads provided in the bearing into drain grooves provided along the sides of the bearing carriage. The method also involves preventing leakage from the drain grooves by positioning sealing structures along the sides of the bearing carriage so as to capture hydrostatic fluid flowing out from the drain grooves, collecting the hydrostatic fluid in a reservoir provided as a portion of the hydrostatic bearing carriage, preventing the hydrostatic reservoir from leaving the reservoir except through designated outlets using a first portion of an end sealing structure, and preventing debris from entering the bearing carriage using a second portion of the end sealing structure.  
         [0025]     An additional aspect of the invention relates to a hydrostatic bearing pad. The hydrostatic bearing pad comprises a compensating groove, an adjacent pocket groove enclosing a first planar area therein, and a second planar area interposed between the compensating groove and the adjacent pocket groove. The first and second planar areas are constructed and arranged to resist the flow of pressurized fluid when the bearing pad is in a load supporting position relative to another surface. The bearing pad does not include grooves between the compensating groove and the pocket groove.  
         [0026]     Other additional aspects of the invention relate to self-compensating hydrostatic bearings having bearing pads as described in the preceding paragraph.  
         [0027]     These and other aspects, features and advantages of the invention will be described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The invention will be described with reference to the following drawing figures, in which like numerals represent like features throughout the figures, and in which:  
         [0029]      FIG. 1  is a perspective view of a hydrostatic bearing in accordance with the invention without end caps or seals installed;  
         [0030]      FIG. 2  is a side elevational view of the carriage of  FIG. 1 ;  
         [0031]      FIG. 3  is a schematic diagram of the vertical bearing pads in the carriage of  FIG. 1 ;  
         [0032]      FIG. 4  is a fluid circuit diagram showing the resistances of the bearing pads of  FIG. 3 ;  
         [0033]      FIG. 5  is a schematic diagram of the horizontal bearing pads of the carriage of  FIG. 1 ;  
         [0034]      FIG. 6  is a fluid circuit diagram showing the resistances of the bearing pad of  FIG. 5 ;  
         [0035]      FIG. 7  is another perspective view of the hydrostatic bearing of  FIG. 1 , with end caps and seals installed;  
         [0036]      FIG. 8  is a sectional view through Line  8 - 8  of  FIG. 7  showing the reservoir end caps and end seals of the hydrostatic bearing;  
         [0037]      FIG. 9  is a close-up sectional view of a portion of the structure shown in  FIG. 8 , showing the end caps and seals in more detail;  
         [0038]      FIG. 10  is a sectional elevational view of the carriage of  FIG. 1  illustrating the side seals;  
         [0039]      FIG. 11  is a close-up sectional view of a portion of the structure shown in  FIG. 10  in more detail;  
         [0040]      FIG. 12  is a side elevational view showing a machine tool table supported on several hydrostatic bearings of the type shown in  FIG. 1 ;  
         [0041]      FIG. 13 a  perspective view showing the underside of the bearing carriage of  FIG. 1 ;  
         [0042]      FIGS. 14 and 15  are perspective views of the keeper portions of the bearing carriage of  FIG. 1 ;  
         [0043]      FIG. 16  is a perspective view of the side and end seals of the bearing carriage of  FIG. 1  in isolation without the bearing carriage itself;  
         [0044]      FIG. 17  is a close-up perspective view of a portion of the side and end seals shown in  FIG. 16 , illustrating the engagement of the side and end seals; and  
         [0045]      FIG. 18  is a schematic perspective view of several hydrostatic bearings according to the invention connected to a hydraulic power unit. 
     
    
     DETAILED DESCRIPTION  
       [0046]      FIG. 1  is a perspective view of a hydrostatic linear bearing, generally indicated at  10 , according to the present invention. The bearing  10  is comprised of a carriage  12  that is mounted for sliding, hydrostatically supported movement along a rail  14 . The direction of movement is shown by arrow M in  FIG. 1 .  
         [0047]     In the embodiment shown in  FIG. 1 , the rail  14  has a “T shaped” cross section. The carriage  12  has a central portion  16  and two keepers,  18 A,  18 B that are clamped or bolted to the central portion  16  of the carriage  12 . Alternatively, the carriage  12  may be fabricated as a single-piece structure; however, the use of the two separable keepers  18 A,  18 B makes the carriage  12  easier to fabricate, and, in particular, easier to finish grind. If the carriage  12  is fabricated as a single-piece structure, special finish grinding equipment may need to be used.  
         [0048]     The carriage  12  also includes a number of drain grooves  106 ,  108 A,  108 B,  110 A,  110 B,  112 A, and  112 B extending substantially the entirety of the length of the carriage. The drain grooves  106 ,  108 A,  108 B,  110 A,  110 B,  112 A, and  112 B will be described in more detail below.  
         [0049]     The carriage  12  and rail  14  have rectilinear cross-sections in this embodiment of the invention. (The term “rectilinear,” as used herein, refers to any shape comprised of line segments without substantial curvature between adjacent segments.) Although rectilinear cross sectional shapes are generally preferred because they are easier to machine, the carriage and rail of a hydrostatic bearing according to the invention may have any desired cross sectional shape. More generally, the carriage  12  may be shaped to engage a rail of substantially any cross-sectional shape.  
         [0050]     As shown in  FIG. 1 , the rail  14  includes drilled and counterbored holes  20  that are used to secure it to a machine tool bed or other rigid structure. The carriage  12  includes drilled and tapped holes  22  such that raised surfaces  24 A,  24 B,  24 C may be clamped rigidly to the mating surface of a machine tool table or other structure that requires linear motion guidance. (The use of the hydrostatic bearing  10  will be described in more detail below.)  
         [0051]     In general, the overall size and shape of the carriage  12  and rail  14 , and the locations of the holes  20 ,  22  in the rail and carriage may be selected so as to be “bolt-for-bolt” compatible with and of the same size as standard rolling element linear bearings. It is advantageous if this type of compatible configuration is used, because a hydrostatic bearing  10  according to the invention may then be directly substituted for a rolling element-type linear bearing in an existing machine tool or tool design.  
         [0052]      FIG. 2  is a side elevational view of the carriage  12 . The carriage  12  is hydrostatically supported by a number of bearing pads provided in interior surfaces of the carriage  12 . The locations of the vertical bearing pads  26 A,  26 B,  28 A,  28 B and the horizontal bearing pads  30 A,  30 B are also shown in the perspective views of  FIGS. 13-15  and will be described in more detail below with respect to those figures. (The terms “vertical” and “horizontal,” as used with respect to the bearing pads, refer to the direction of the applied loads that the respective bearing pads resist.) Fluid pressure exerted through the bearing pads  26 A,  26 B,  28 A,  28 B maintains the bearing carriage  12  at a small distance from the bearing rail  14 . Typically, the clearance between the bearing pads  26 A,  26 B,  28 A,  28 B,  30 A,  30 B and the rail  14  would be on the order of about 0.001 inches to about 0.005 inches.  
         [0053]     In this description, the terms “fluid” and “hydrostatic fluid” are used interchangeably to refer to any fluid that may be used in a bearing  10  according to the present invention. Many such fluids are known in the art, including hydrocarbon-based oils, silicone-based oils, water, water-based compositions, and air or another suitable gas. In machine tool applications, hydrocarbon-based oils may be preferred for some applications. These oils tend to reduce or eliminate corrosion problems, and may also have relatively high viscosities, which help to reduce the bearing flow rate and associated pumping power needed to pressurize the bearing  10 .  
         [0054]     Water-based hydrostatic fluids also have certain advantages and may also serve in hydrostatic bearings  10  according to the invention. One advantage of water-based hydrostatic fluid is that if machining coolant (typically a water-based composition) leaks into or mixes with the hydrostatic fluid, it may not present a serious contamination problem. Water-based hydrostatic fluids may also be used in bearings  10  that are produced for the food industry, because of the reduced risk of contaminating the consumable product. Additionally, water-based fluids generally have high thermal conductivities, which enables the heat generated by the pumping process to be removed much more easily.  
         [0055]      FIG. 3  is a schematic diagram of the vertical pads  26 A and  28 A, showing their basic geometry and illustrating the route fluid takes through the bearing pads  26 A,  28 A. Vertical bearing pad  26 B is similar in design to pad  26 A and is therefore not shown. Vertical bearing pad  28 B is identical in design to  28 A and is therefore not shown. In the following description, it is assumed that the fluid path is the same in the non-illustrated bearing pads  26 B,  28 B. However, as those of ordinary skill in the art will realize, the design of the various vertical bearing pads  26 A,  26 B,  28 A,  28 B need not be identical.  
         [0056]     A lubricating fluid is pressurized and supplied by pump  32  to the upper and lower bearing pads  26 A and  28 A. (The details of the hydraulic supply of bearings  10  according to the invention will be described below with respect to  FIG. 18 .) The fluid enters the lower pad  28 A at supply groove  34  which has a depth sufficient to allow free flow of fluid within it. Some fluid crosses leakage lands  36 A and  36 B, which are at a tight gap distance from rail  14 , and exits bearing pad  28 A. Some fluid crosses land  38  and enters pocket groove  40 . Some fluid also crosses compensating land  42  which is at a small distance from the rail  14 ; this tight gap creates a pressure drop as the fluid enters compensator groove  44 . Some fluid leaks from compensator groove  44  across lands  46 A and  46 B and exits bearing pad  26 A. Some fluid is routed from compensator groove  44  to pocket groove  48  of bearing pad  26 A. Some fluid leaks out of pocket groove  48  across lands  50 A,  50 B, and  50 C where it exits bearing pad  26 A. Fluid is free to flow in the tight gap region between rail  14 , and central bearing pad  52 , at a pressure that is equal to the fluid pressure in pocket groove  48 . Fluid is also supplied at supply pressure from pump  32  to the supply grooves  54 A and  54 B of pad  26 A. Some fluid leaks across lands  56 A,  56 B,  56 C, and  56 D and exits the bearing pad  26 A. Some fluid crosses from supply grooves  54 A and  54 B across lands  58 A and  58 B to pocket groove  48 . Some fluid crosses from supply grooves  54 A and  54 B across compensator lands  60 A and  60 B to compensator groove  62 . Some fluid leaks from compensator groove  62 , crosses land  64  and exits bearing pad  26 A. Some fluid is routed from compensator groove  62  to bearing pad  28 A where it enters pocket groove  40 . Some fluid then flows from pocket groove  40  across lands  66 A,  66 B, and  66 C where it exits bearing pad  28 A. Fluid can flow between compensator groove  44  and pocket groove  40  but is largely restricted from doing so by land  68 . Fluid can flow between compensator groove  62  and pocket groove  48  but is largely restricted from doing so by land  70 .  
         [0057]     Grooves  54 A,  54 B,  62 ,  48 ,  34 ,  44 , and  40  all should have a depth that is at least about three times larger than the clearance between the pads  28 A and  26 A and the rail  14  to ensure uniform pressure within each of these grooves. In the case of grooves  48  and  40 , uniform pressure is desired to spread the load-supporting pressure over the entire pocket area. In the case of grooves  54 A,  54 B,  62 ,  34 , and  44 , uniform pressure is desired in order to yield the proper hydraulic resistance on the adjacent lands so that the pressure in the respective bearing areas can be adequately controlled.  
         [0058]     Pad  26 A should be fabricated such that lands  52 ,  50 A,  50 B,  50 C,  60 A,  60 B,  56 A,  56 B,  56 C,  56 D,  64 ,  58 A,  58 B, and  70  are preferably all on the same plane and at the same tight gap distance to rail  14 . Pad  28 A should be fabricated such that lands  66 A,  66 B,  66 C,  42 ,  46 A,  46 B,  36 A,  36 B,  38 , and  68  are preferably all on the same plane and at the same tight gap distance to rail  14 .  
         [0059]      FIG. 4  is a fluid circuit diagram of the vertical bearing pads  26 A and  28 A (which are identical to the counterpart vertical bearing pads  26 B and  28 B). The various lands described above with respect to  FIG. 3  are shown in  FIG. 4  as circuit resistors. The values of the land resistances, which can be calculated by those skilled in the art of fluid dynamics, is dependent upon the fluid viscosity, the length and width of the lands, and the clearance between each land and the rail  14 . The fluid circuit shown in  FIG. 4  can be solved by those skilled in the art of circuit analysis to compute the pressure in each of the bearing grooves. These pressures may then be multiplied by the corresponding bearing areas to yield the overall vertical force developed by the bearing.  
         [0060]     In order to evaluate how the bearing force changes in response to a change in vertical position of the carriage  12  with respect to the rail  14 , the fluid gap between the carriage  12  and the rail  14  that was used to calculate the land resistances would be changed and the analysis described above would be repeated with the new fluid gap data. A computer program could be used to carry out this repetitive analysis. Although the bearing pad geometries may be chosen to suit particular applications of the hydrostatic bearing  10 , it is preferable if the the bearing groove and land geometry are optimized to provide very high bearing stiffness and load capacity in the vertical direction with the minimum possible flow rate of fluid through the bearing  10  because high fluid flow rates typically require great amounts of pumping power.  
         [0061]     Grooves  54 A,  54 B,  62 ,  48 ,  34 ,  44 , and  40  are shown in  FIG. 3  with rounded corners; however, they may be fabricated with sharp square corners or another geometric profile without considerable effect on bearing operation, since the hydraulic resistances of the adjacent lands will change by a very small percentage of their overall resistance values.  
         [0062]     As shown in  FIG. 3  and described above, fluid is routed between pad  28 A and pad  26 A in two places, from compensator groove  44  to pocket groove  48 , and from compensator groove  62  to pocket groove  40 . These fluid transfers may be accomplished by the use of drilled holes in carriage  12  and keeper  18 A, or they may be accomplished with the use of rigid tubing external to carriage  12 . Similarly, fluid may be routed at supply pressure from pump  32  to supply grooves  34 ,  54 A, and  54 B with the use of external tubing followed by holes drilled in carriage  12  and keeper  18 A.  
         [0063]      FIG. 5  is a schematic view of the horizontal bearing pads  30 A and  30 B, showing their basic geometry and illustrating the route that fluid takes through the bearing pads  30 A,  30 B. A lubricating fluid is pressurized and supplied by pump  32  to the upper and lower bearing pads  30 A and  30 B. (The same pump  32  may be used to supply the horizontal bearing pads  30 A,  30 B and the vertical bearing pads  26 A,  26 B,  28 A,  28 B, or two different pumps  32  may be used.) The fluid enters pad  30 A at supply groove  72 A which is at a depth sufficient to allow free flow of fluid within it. Some fluid leaks from supply groove  72 A across leakage lands  74 A and  76 A, which are at a tight gap distance from rail  14 , and exits bearing pad  30 A. Some fluid flows from supply groove  72 A across lands  78 AA and  80 AA to pocket groove  82 AA and some flows across lands  78 AB and  80 AB to pocket groove  82 AB. Some fluid flows from supply groove  72 A across compensator lands  84 AA,  86 AA,  88 AA to compensator groove  90 AA. Some of the fluid which enters compensator groove  90 AA leaks to or from pocket groove  82 AA across land  100 AA. The remainder of the fluid which enters compensator groove  90 AA is routed to bearing pad  30 B where it enters pocket groove  82 BA and provides uniform pressure to pocket groove  82 BA before leaking across lands  92 BA,  94 BA, and  96 BA and exiting bearing pad  30 B. The fluid in the tight clearance of bearing pad  98 BA will be at a pressure equal to the fluid pressure in pocket groove  82 BA because pocket groove  82 BA completely surrounds bearing pad  98 BA. Fluid is also supplied at supply pressure from pump  32  to supply groove  72 B of bearing pad  30 B. Some of the fluid which enters supply groove  72 B leaks across lands  74 B and  76 B and exits bearing pad  30 B. Some of the fluid which enters supply groove  72 B leaks across lands  78 BA and  80 BA to pocket groove  82 BA, and some leaks across lands  78 BB and  80 BB to pocket groove  82 BB. Some of the fluid which enters supply groove  72 B leaks across compensator lands  84 BA,  86 BA, and  88 BA to compensator groove  90 BA. Some fluid may across land  100 BA between compensator groove  90 BA and pocket groove  82 BA. The remainder of fluid entering compensator groove  90 BA is routed to pad  30 A where it enters pocket grooves  82 AA and leaks across lands  92 AA,  94 AA, and  96 AA and exits bearing pad  30 A. The fluid in the tight gap clearance of bearing pad  98 AA will be at a pressure equal to the fluid pressure in pocket groove  82 AA because pocket groove  82 AA completely surrounds bearing pad  98 AA. Some of the fluid which enters supply groove  72 A leaks across compensator lands  84 AB,  86 AB, and  88 AB to compensator groove  90 AB. Some fluid may across land  100 AB between compensator groove  90 AB and pocket groove  82 AB. The remainder of fluid entering compensator groove  90 AB is routed to pad  30 B where it enters pocket groove  82 BB and leaks across lands  92 BB,  94 BB, and  96 BB and exits bearing pad  30 B. The fluid in the tight gap clearance of bearing pad  98 BB will be at a pressure equal to the fluid pressure in pocket groove  82 BB because pocket groove  82 BB completely surrounds bearing pad  98 BB. Some of the fluid which enters supply groove  72 B leaks across compensator lands  84 BB,  86 BB, and  88 BB to compensator groove  90 BB. Some fluid may across land  100 BB between compensator groove  90 BB and pocket groove  82 BB. The remainder of fluid entering compensator groove  90 BB is routed to pad  30 A where it enters pocket groove  82 AB and leaks across lands  92 AB,  94 AB, and  96 AB and exits bearing pad  30 A. The fluid in the tight gap clearance of bearing pad  98 AB will be at a pressure equal to the fluid pressure in pocket groove  82 AB because pocket groove  82 AB completely surrounds bearing pad  98 AB.  
         [0064]     Grooves  82 AA,  82 AB,  82 BA,  82 BB,  90 AA,  90 AB,  90 BA,  90 BB,  72 A, and  72 B all should have a depth that is at least three times larger than the clearance between the pads  30 A and  30 B and the rail  14  to ensure uniform pressure within each of these grooves. In the case of grooves  82 AA,  82 AB,  82 BA, and  82 BB, uniform pressure is desired in order to spread the load-supporting pressure over the entire pocket area. In the case of grooves  90 AA,  90 AB,  90 BA,  90 BB,  72 A, and  72 B, uniform pressure is desired in order to yield the proper hydraulic resistance on the adjacent lands so that the pressure in the respective bearing areas can be adequately controlled.  
         [0065]     Pad  30 A is fabricated such that lands  98 AA,  98 AB,  84 AA,  84 AB,  86 AA,  86 AB,  88 AA,  88 AB,  92 AA,  92 AB,  94 AA,  94 AB,  96 AA,  96 AB,  78 AA,  78 AB,  80 AA,  80 AB,  100 AA,  100 AB,  74 AA,  74 AB,  76 AA,  76 AB are preferably all on the same plane and at the same tight gap distance to rail  14 . Pad  30 B should be fabricated such that lands  98 BA,  98 BB,  84 BA,  84 BB,  86 BA,  86 BB,  88 BA,  88 BB,  92 BA,  92 BB,  94 BA,  94 BB,  96 BA,  96 BB,  78 BA,  78 BB,  80 BA,  80 BB,  100 BA,  100 BB,  74 BA,  74 BB,  76 BA,  76 BB are preferably all on the same plane and at the same tight gap distance to rail  14 .  
         [0066]      FIG. 6  is a schematic diagram showing the fluid resistances of the horizontal bearing pad  30 A. Each of the resistances shown in  FIG. 6  represents one of the lands of the horizontal bearing pad  5 A. The values of the resistances of the horizontal bearing pad  30 A may be calculated as was described above with respect to the vertical bearing pads  26 A,  26 B,  28 A,  28 B.  
         [0067]     Grooves  82 AA,  82 AB,  82 BA,  82 BB,  90 AA,  90 AB,  90 BA,  90 BB,  72 A, and  72 B are shown in  FIG. 5  with rounded corners; however, they may be fabricated with sharp square corners or another geometric profile without considerable effect on bearing operation since the hydraulic resistances of the adjacent lands will change by a very small percentage of their overall resistance values.  
         [0068]     As shown in  FIG. 5 , fluid is routed between pad  30 A and pad  30 B in four places: from compensator groove  90 AB to pocket groove  82 BB, from compensator groove  90 AA to pocket groove  82 BA, from compensator groove  90 BA to pocket groove  82 AA, and from compensator groove  90 BB to pocket groove  82 AB. As with the fluid transfers in the vertical bearing pads  26 A,  26 B,  28 A,  28 B, these fluid transfers may be accomplished by the use of drilled holes in carriage  12 , or they may be accomplished with the use of rigid tubing external to carriage  12 . Similarly, fluid may be routed at supply pressure from pump  32  to supply grooves  72 A and  72 B with the use of external tubing followed by holes drilled in carriage  12 .  
         [0069]     In the vertical and horizontal bearing pads shown in  FIGS. 3 and 5  and described above, lands  58 A,  58 B,  70 ,  38 ,  68 ,  78 AA,  78 AB,  78 BA,  78 BB,  80 AA,  80 AB,  80 BA,  80 BB,  100 AA,  100 AB,  100 BA, and  100 BB allow leakage paths between adjacent compensators, pockets, and supply grooves. These leakage paths tend to reduce the pressure response of the bearing and therefore reduce its stiffness and load-carrying capability. However, a greater factor that overcomes the effect of these fluid leakage paths is the ability to arrange pocket grooves  48 ,  40 ,  82 AA,  82 AB,  82 BA, and  82 BB such that they are closer to the compensating grooves, and, therefore, spread the load-supporting pocket pressures over a larger area. By better utilizing the available pad area, the bearing pad configurations of the hydrostatic bearing  10  provide higher stiffness and load capacity.  
         [0070]      FIG. 7  is another perspective view of the hydrostatic bearing of  FIG. 1 , with its seals and endcaps installed.  FIG. 8  is a sectional view through Line  8 - 8  of  FIG. 7 , and  FIG. 9  is a close-up view of portion A (enclosed in dotted line) of  FIG. 8 .  FIGS. 7-9  show the hydrostatic bearing of  FIG. 1  with end caps  102 A and  102 B attached to carriage  12  and keepers  18 A and  18 B. End caps  102 A and  102 B contain reservoirs  104 A and  104 B (visible in the views of  FIGS. 8 and 9 ) to which the fluid flows into from bearing pads  26 A,  26 B,  28 A,  28 B,  30 A, and  30 B as well as from drain grooves  106 ,  108 A,  108 B,  110 A,  110 B,  112 A, and  112 B. (As was described above, the drain grooves are provided at the corners of the carriage  12  and are visible in the views of  FIGS. 1 and 2 .) Double-lipped end seals  114 A and  114 B are attached to end caps  102 A and  102 B. The double-lipped end seals  114 A,  114 B are attached to rigid plates  113 A,  113 B in order to provide them with additional stiffness. Lips  116  of end seals  114 A and  114 B are in sliding engagement with rail  14  and serve to trap the fluid into reservoirs  104 A and  104 B and largely prevent fluid from leaking directly out of the hydrostatic bearing  10 . The fluid flows out of reservoirs  104 A or  104 B through at least one drain outlet  118 A and/or  118 B. One or more of the drain outlets  118 A,  118 B may be plugged, but at least one drain outlet  118 A,  118 B is used to route the fluid to a hose or tubing assembly, where the fluid is returned to the hydraulic supply source.  
         [0071]      FIG. 10  is a sectional side elevational view of the hydrostatic bearing  10 , illustrating side seals  120 A and  120 B that are received by acceptor grooves  122 A and  122 B within keeper portions  18 A and  18 B of the bearing carriage  12 .  FIG. 11  is an enlarged sectional view of portion B of  FIG. 10 , illustrating the side seals  120 A,  120 B in more detail. The side seals  120 A,  120 B slidingly engage the bearing rail  14 , serve to trap fluid, and allow the trapped fluid to be routed through drain grooves  112 A and  112 B into reservoirs  104 A and  104 B to prevent fluid from leaking directly out of the hydrostatic bearing  10 . As shown in  FIG. 11 , the side seals  120 A,  120 B have a generally u-shaped portion  121  that opens upwardly, towards the top of the drain groove  112 A,  112 B. The side seals  120 A,  120 B are positioned in the acceptor groove  122 A,  122 B such that one wall of the u-shaped portion  121  of the side seal  120 A,  120 B is in contact with the keeper  18 A,  18 B and the other wall of the u-shaped portion  121  is in contact with the bearing rail  14 .  
         [0072]      FIG. 13  is a perspective view of the underside of the central portion  16  of the carriage  12  without the keepers  18 A,  18 B installed.  FIG. 13  shows the relative locations and extents of the vertical bearing pads  26 A,  26 B and the horizontal bearing pads  30 A,  30 B.  FIGS. 14 and 15  are perspective views of the keepers  18 A and  18 B, showing the locations and extents of vertical bearing pads  28 A and  28 B on the keepers  18 A and  18 B. The positions of the drain grooves  106 ,  108 A,  108 B,  110 A,  10 B,  112 A, and  112 B and seal acceptor grooves  122 A,  122 B are also shown.  
         [0073]     Each side of the central portion  16  of the bearing carriage  12  has a set of threaded holes  222  provided in respective connecting surfaces  220 A and  220 B. A set of complimentary, counterbored through holes  226  are provided in the keepers  18 A and  18 B. When the keepers  18 A and  18 B and central portion  16  of the carriage  12  are assembled, bolts are inserted through the holes  226  in the keepers  18 A,  18 B and into the threaded holes  222  of the central portion  16  of the carriage  12  such that the engaging surfaces  220 A,  220 B of the central portion  16  and the engaging surfaces  224 A,  224 B of the keepers  18 A,  18 B are adjacent, as shown in  FIG. 10 .  
         [0074]     The bearing pad grooves and other surface features shown in  FIGS. 13-15  may be formed by milling, electrical discharge machining, or other known techniques.  
         [0075]      FIG. 16  is a perspective view showing the end seals  114 A,  114 B and side seals  120 A,  120 B in isolation. As was described above, the end seals  114 A,  114 B are constructed of a rubber material molded so as to attach to rigid plates  113 A,  113 B, for example, steel or aluminum plates, to provide them with greater rigidity. In alternative embodiments, the end seals  114 A,  114 B may not be attached to rigid plates  113 A,  113 B  
         [0076]     As is shown best in  FIG. 17 , a close-up perspective view of portion “C” of  FIG. 16 , the side seals  120 A,  120 B are inserted into receptacles  115  formed in the end seals  114 A,  114 B such that they have an interference fit with the receptacles  115 . In one embodiment, the side seals  120 A,  120 B may be made slightly longer than required, such that they can be maintained in compression during operation. In alternative embodiments of the invention, the side seals  120 A,  120 B and the end seals  114 A,  114 B may be molded or cast as a single structure, bonded together, or otherwise caused to adhere to one another to form a unitary structure.  
         [0077]     The bearing pads  26 A,  26 B,  28 A,  28 B,  30 A,  30 B described above are designed for a self-compensating hydrostatic bearing. However, those of ordinary skill in the art will realize that the other features of the carriage  12  and rail  14 , including the sealing structures (i.e., the end seals  114 A,  114 B and side seals  120 A,  120 B) and the drain grooves  106 ,  108 A,  108 B,  110 A,  10 B,  112 A, and  112 B may be used without the particular bearing pads  26 A,  26 B,  28 A,  28 B,  30 A,  30 B described above. For example, in alternative embodiments of the invention, a carriage having end seals, side seals and a drain groove arrangement similar to that described above could be used with bearing pads that are not self-compensating. Bearing pads that are not self-compensating could use capillary tubes or valves for compensation purposes, as one of ordinary skill in the art will readily be able to appreciate.  
         [0078]     Conversely, the self-compensating bearing pads  26 A,  26 B,  28 A,  28 B,  30 A,  30 B described above may be used on other types of hydrostatically supported devices and in other types of fluidstatic bearings without the other features described herein.  
         [0079]      FIG. 18  is a schematic perspective view illustrating four bearing carriages  12  riding on two carriage rails  14 . In general, several bearing carriages  12  may be provided on the same carriage rail  14 , particularly if those bearing carriages  12  are fixed in position with respect to one another (e.g., by being bolted to the bed of a machine tool, as will be described below). Alternatively, several shorter segments of bearing rail  14  could be provided, one segment for each bearing carriage  12 .  
         [0080]      FIG. 18  also illustrates the details of the hydraulic fluid connections for the bearings  10  according to the present invention. A hydraulic power unit  230  delivers hydraulic fluid under high pressure through a conduit  232 . The hydraulic power unit  230  includes all of the components necessary to deliver temperature controlled fluid that is relatively free of contaminant particles at high pressure with minimal pressure pulsations. For example, the hydraulic power unit  230  may include a reservoir, a pump, an electric motor, a filter, a pressure regulating valve, a pressure gauge, and a heat rejection system, such as an air-to-oil heat exchanger.  
         [0081]     The conduit  232  from the hydraulic power unit  230  branches such that one branch connects with each bearing carriage  12 . The branches of the conduit  232  are received by a fluid inlets  119  in the end seals  114 A,  114 B of the bearing carriages  12 . (Depending on the configuration of the bearings  10 , the conduit  232  may connect to a fluid inlet  119  on either end seal  114 A,  114 B. The unused fluid inlet  119  may be plugged or omitted.) The connection between the conduit  232  branch and the fluid inlet  119  of the end seal may be any appropriate type of conventional hydraulic connection. From the fluid inlet  119 , the pressurized fluid is distributed to the supply grooves  34 ,  54 A,  54 B by an internal network of passageways. Once used, the fluid is collected in the reservoirs  104 A,  104 B and returned via return conduits  238 , which connect to the drain outlets  118 A,  118 B and the return portions of the hydraulic power unit  232 .  
         [0082]      FIG. 12  is a side elevational view of a machine tool  200 , illustrating a typical application for a hydrostatic bearing  10  according to the present invention. A machine tool table  66  is supported by four bearing assemblies  10  which ride on two rails  14 . Although only two bearing assemblies  10  are shown, at least four are typically used to provide adequate pitch and yaw stability to table  202 . The rails  14  of the hydrostatic bearings  10  are horizontally clamped to a machine bed  204  using wedges  206 A and  206 B. The rails  14  are clamped vertically to machine bed  204  using a plurality of bolts  208  threadedly secured within machine bed  204  through counterbored holes  20  provided in the rail  14 . Two of the hydrostatic bearings  10  are clamped horizontally to the table  202  using wedges  210  (one wedge  210  is shown in the view of  FIG. 12 ). The other two hydrostatic bearings  10  are floated into alignment by pressurizing them with lubricating fluid, thus allowing hydrostatic bearings  10  to float horizontally into a self-aligning position. Once the two wedge-secured hydrostatic bearings  10  are in alignment, the bolts that secure them to the table  202  are tightened. Although  FIG. 12  illustrates the use of wedges  206 A,  206 B, and  210 , many other mechanisms to clamp the rails  14  and the hydrostatic bearings  10  are possible and are within the scope of the invention.  
         [0083]     A hydrostatic bearing  10  may be used in a number of different types of machine tools, and in any other application in which linear motion guidance is required. However, hydrostatic bearings  10  according to the invention may be particularly beneficial when used in lathes. For example, hydrostatic bearings  10  may be used in the QUEST® turning machines manufactured by HARDINGE, Inc. (Elmira, N.Y., United States). Hydrostatic bearings  10  may also be useful in grinding machines, milling machines, boring machines, and other machine tools in which a combination of high stiffness and damping are beneficial.  
         [0084]     A hydrostatic bearing  10  according to the present invention may have certain advantageous performance characteristics. For example, a hydrostatic bearing  10  according to the invention would typically have high static and dynamic stiffnesses. A hydrostatic bearing  10  may also operate with very low friction, because the seals described above with respect to  FIGS. 7-11  would generally be the only components creating friction. Because the carriage  12  rides on a layer of fluid, and for other reasons, the hydrostatic bearing  10  may have up to ten times the force damping capabilities of a conventional rolling element linear bearing. Additional advantages may include an essentially unlimited translational (feed) rate, an essentially unlimited fatigue life (with substantially no component wear because the carriage  12  and rail  14  are not in contact), substantially no change in positioning accuracy of a machine tool mounted on hydrostatic bearings  10  over time, substantially no damage to the hydrostatic bearing  10  under heavy “crash” loads (i.e., when the bearing  10  stops suddenly at the ends of its travel range). Moreover, the hydrostatic bearing  10  is self cleaning if fluid flow is maintained between the carriage  12  and rail continuously  14 .  
         [0085]     When installed in a machine tool and used to produce parts, the features of the hydrostatic bearing  10  may also lead to certain other advantages. For example, the hydrostatic bearing  10  may improve tool life. Additionally, parts may be produced with better surface finishes and better roundnesses for round parts. A machine tool mounted on hydrostatic bearings  10  may also have improved hard turning capability, improved interrupted cutting capability, and improved positioning accuracy. Some of the advantages and benefits described above will become apparent from the following example.  
       EXAMPLE 1  
       [0086]     A hydrostatic bearing  10  according to the invention is installed so as to support operational movement in a QUEST® 51 turning machine (Hardinge, Inc., Elmira, N.Y., United States) using the installation procedure described above. Four hydrostatic bearings  10  according to the present invention are installed to guide motion in the X-axis and four are installed to guide motion in the Z-axis. No adaptations to the turning machine are required in order to accommodate the hydrostatic bearings  10 ; however, hydraulic hoses are provided for each hydrostatic bearing  10 . A two-inch round A2 tool steel blank was prepared with four slots milled around its circumference for interrupted cutting. It was then hardened to 60-62 Rc. The part was then roughed with a 5/16 inch diameter round cubic boron nitride (CBN) insert at 450 SFM/0.002 ipr/0.030 doc with five passes. Subsequently, the part was finished with a 55 degree CBN insert at 550 SFM/0.003 ipr/0.005 doc with one pass, and then threaded with a CBN triangular insert. The surface finish of the part was consistently in the 5 to 6 microinch range, an improvement of approximately a factor of two when compared with an identical part machined on a comparable QUEST® 51 turning machine without a hydrostatic bearing. Additionally, the tool life of the interrupted turning insert was increased by a factor of three when compared to the life of an insert used on the turning machine without the hydrostatic bearing.  
         [0087]     Although the invention has been described with respect to certain embodiments, those embodiments are intended to be illustrative, rather than limiting. Modifications and variations to the invention are possible, within the scope of the appended claims.