WO1999053207A1

WO1999053207A1 - Self-compensating hydrostatic bearing

Info

Publication number: WO1999053207A1
Application number: PCT/US1999/008097
Authority: WO
Inventors: Gregory S. Lyon
Original assignee: Thomson Industries, Inc.
Priority date: 1998-04-13
Filing date: 1999-04-13
Publication date: 1999-10-21
Also published as: AU3745899A

Abstract

In accordance with the present disclosure, a self-compensating bearing assembly is provided. The bearing assembly includes a bearing carriage (14) having a plurality of spaced bearing races configured to be slideably supported on a support rail (12). A gap (72) is defined between each bearing race and the surface of the rail (12). Each bearing race includes a supply groove formed thereon. A manifold (16) having a plurality of distribution channels (30, 32, 34, 36) and a valve assembly (26) associated therewith is secured to one end of the carriage (14). Each distribution channel communicates with one of the supply grooves. The valve assembly (26) includes at least one valve (74, 76) having an inlet (77) and a pair of outlets. Each outlet communicates with a second end of one of the distribution channels (30, 32, 34, 36). The valve assembly also includes an actuator (80) which is connected to an electrical circuit which senses a change in the size of the gap (72) between the rail (12) and any one of the bearing surfaces. Upon sensing a change in gap size, the actuator (80) operates to regulate the flow of fluid from the valve outlets to the distribution channels to compensate for loading of the bearing carriage.

Description

SELF-COMPENSATING HYDROSTATIC BEARING

BACKGROUND

1. Technical Field

The present invention relates to a hydrostatic bearing assembly and, more particularly, to a self-compensating hydrostatic bearing assembly having an automatically controlled valve assembly associated therewith which regulates ϋie flow of hydrostatic fluid flow to bearing races of the bearing assembly to compensate for loading of the bearing assembly.

2. Background of Related Art Hydrostatic bearings are well known and have been used for many years for the near frictionless movement of masses. Hydrostatic bearings are characterized, as having excellent low friction, accuracy, and repeatability characteristics, with a theoretically infinite life. Hydrostatic bearings also have excellent damping characteristics which result from hydrostatic fluid acting as a shock absorber between the apparatus to which the bearing is associated and an applied load.

Typically, hydrostatic bearings maintain the distance between a bearing race and a support rail by providing a thin pressurized film of fluid between the bearing race and the support rail. One type of hydrostatic bearing is a self-compensating hydrostatic bearing. Self-compensating hydrostatic bearings respond automatically to a change in bearing gap, i.e., the gap between a bearing race and a suppoπ rail, by changing the flow of fluid to pockets positioned along the bearing race.

U.S. Patent No. 5,104,237 ("Slocum") discloses a self-compensating hydrostatic bearing for supporting a bearing carriage along a bearing rail. Slocum's hydrostatic bearing includes geometrically opposed pockets formed in the bearing carriage surfaces facing the bearing rail and compensating units in fluid communication with each of the pockets. Each compensating unit includes an annulus and a hole positioned on the carriage surface. The hole is connected to one of the pockets positioned on the opposite side of the bearing carriage by a channel formed in the bearing carriage. A constant pressure source of fluid is connected to each annulus to supply fluid to the annulus, where it flows across the bearing gap into the hole and to the opposite side pocket. As a load is applied to the bearing carriage, the resistance of fluid exiting the bearing pocket on the load side increases while the resistance of fluid exiting the bearing pocket on the opposite side decreases. The pressure increases in the pocket on the load side until the differential pressure generated between the two pockets balances. Thus, self-compensation is provided. Although Slocum's hydrostatic bearing provides some degree of self- compensation, because of the complexity of the fluid circuit connecting the pockets formed in the bearing carriage surfaces and the compensating units, the stiffness of the bearing is highly non-linear. The bearing also requires high flow rates and has a low load capacity.

Accordingly, a need exists for an improved self-compensating hydrostatic bearing having a higher load capacity and uniform stiffness. Moreover, a need exists for a self-compensating hydrostatic bearing that is able to compensate for errors and/or irregularities in a rail system.

SUMMARY

In accordance with the present disclosure, a self-compensating fluid bearing assembly is provided. The bearing assembly includes a bearing carriage having a plurality of spaced bearing races configured to be slidably supported on a rail. A gap is defined between each bearing race and the adjacent rail surface. Each bearing race also includes a supply groove formed thereon. A manifold having a plurality of distribution channels and a valve assembly associated therewith is secured to one end of the bearing carriage. Each distribution channel has a first end that communicates with one of the bearing race supply grooves. The valve assembly has an inlet in fluid communication with a source of hydraulic fluid and a plurality of outlets. Each outlet communicates with a second end of one of the distribution channels. The valve assembly includes an actuator which is connected to an electrical circuit which senses a change in the gap between the rail surface and any one of the bearing races. Upon sensing a change in gap size, the actuator operates to regulate

-2- the flow of fluid to the bearing races via the distribution channels and supply grooves to compensate for loading of the carriage.

In a preferred embodiment, the bearing carriage includes a pair of upper and a pair of lower bearing races. Each of the bearing races includes a race sensor secured thereto by an electrically insulative bonding media. A pair of electric contacts are positioned on the manifold in contact with the race sensors and a voltage source is provided to create an electric potential across the gap between the race sensors and the rail. The potential across this gap will behave as a capacitor whose value will be a function of the distance between the race sensors and the rail surface. When the size of the gap between the race sensors and the rail changes, and thus the capacitance changes, the voltage supplied to the valve actuators changes to regulate the valve assembly and compensate for loading of the carriage.

BRIEF DESCRIPTION OF THE DRAWINGS

Various preferred embodiments are described herein with reference to the drawings, wherein:

FIG. 1 is a perspective view of one embodiment of the presently disclosed self-compensating bearing assembly positioned on a support rail;

FIG. 2 is a top perspective view with parts separated of the bearing assembly shown in FIG. 1; FIG. 3 is a bottom perspective view with parts separated of the bearing assembly shown in FIG. 1 ;

FIG. 4 is a partial cross-sectional view taken along section line 4-4 of FIG. 3;

FIG. 5 is a cross-sectional view taken along section line 5-5 of FIG. 1 ; FIG. 6 is a cross-sectional view taken along section line 6-6 of FIG. 1;

- 3 FIG. 7 is a schematic diagram of an electrical circuit associated with the bearing assembly shown in FIG. 1 ;

FIG. 8 is a bottom perspective view with parts separated of an alternate embodiment of the presently disclosed bearing assembly; FIG. 9 is a partial cross-sectional view of the manifold and rail of the bearing assembly shown in FIG. 8;

FIG. 10 is a perspective view of another alternate embodiment of the presently disclosed bearing assembly;

FIG. 11 is a perspective view with parts separated of the bearing assembly shown in FIG. 10;

FIG. 12 is a top view of a race sensor of the bearing assembly shown in FIG. 10;

FIG. 13 is a cross-sectional view taken along section line 13-13 of FIG. 12; FIG. 14 is a cross-sectional view taken along section line 14-14 of FIG.

10; and

FIG. 15 is a cross-sectional view taken along section line 15-15 of FIG.

10.

DETAT ED DESCRIPTION OF PREFERRED EMBODIMENTS Preferred embodiments of the presently disclosed self-compensating hydrostatic bearing assembly will now be described in detail with reference to the drawings, in which like reference numerals designate identical or corresponding elements in each of the several views.

FIGS. 1 and 2 illustrate one embodiment of the presently disclosed self- compensating hydrostatic bearing assembly, shown generally as 10. The bearing assembly

4 - 10 is slidably supported on rail 12. Briefly, bearing assembly 10 includes bearing carriage 14, bearing manifold 16 and fluid supply hose 22. Supply hose 22 has a first end adapted to be connected to a source of hydrostatic fluid (not shown). Bearing manifold 16 is secured to one end of bearing carriage 14, preferably by brazing, although other attachment devices may be used, e.g. , screws, interlocking structure, etc. Although not shown, a gasket may be positioned between manifold 16 and carriage 14 to provide more effective sealing therebetween.

Manifold 16 has four fluid distribution channels 30, 32, 34 and 36 formed on an interior face thereof and a valve assembly 26 supported within a manifold recess 18. Valve assembly 26 has an inlet fitting 28 adapted to be connected to supply hose 22 and is electrically connected to electrical contacts 38 which are positioned on the interior face of manifold 16 and will be discussed in detail below. Operation of valve assembly 26 will also be described in detail below.

Referring to FIGS. 3 and 4, bearing carriage 14 includes a pair of upper bearing races 46 and 48 and a pair of lower bearing races 50 and 52. Upper bearing races 46 and 48 are angled downwardly at an angle of about 30° to about 60° towards the centerline of carriage 14 and lower bearing races 50 and 52 are angled upwardly at an angle of about 30° to about 60° towards the centerline of carriage 14. Preferably, upper and lower bearing races define an angle of about 45 ° with respect to a vertical plane extending through the longitudinal centerline of the carriage, although other bearing carriage configurations are envisioned. A truncated central portion 54 of carriage 14 is positioned between each respective upper and lower bearing race.

Bearing carriage 14 is constructed of multiple components including carriage body 56, race sensors 58a-d, and a high strength electrically insulative bonding media 62, such as high performance epoxy. Race sensors 58a-d are constructed of an electrically conductive material. Bonding media 62 is used to attach race sensors 58a-d

5 - within grooves 63 formed along each of the bearing races of carriage body 56. Supply grooves 66, 68 and 70 are formed in race sensors 58b-d along the upper and lower bearing races and extend substantially but not completely the full length of carriage body 56. A supply groove (not shown) is also formed in race sensor 58a. One end of each supply groove communicates with a respective one of distribution channels 30, 32, 34 and 36 via a supply channel 65 formed through each sensor. Hydrostatic fluid delivered to channels 30, 32, 34 and 36 flows through channels 65 into the supply grooves and into the gaps 72 defined between each bearing race and the surface of rail 12 (See FIGS. 5 and 6).

Carriage body 56 can be manufactured from steel by cold drawing, cold rolling, casting, and machining, e.g., grinding. Bonding media 62 is applied to carriage body 56 and race sensors 58a-d are subsequently positioned and fixed in place. Then, bonding media 62 is allowed to cure. Preferably, each of the upper and lower bearing races is finish ground, and the manifold is attached to one end of carriage 14 using, for example, a brazing procedure. When manifold 16 is attached to carriage 14, each of the electrical contacts 38 engages a respective race sensor.

Referring to FIGS. 5 and 6, valve assembly 26 houses first and second double acting valves 74 and 76. Each double acting valve is in fluid communication with a valve inlet port 77 which communicates with inlet fitting 28 and with two distribution channels formed in manifold 16 which deliver fluid to diametrically opposed bearing races on bearing carriage 14. For example, double acting valve 74 controls the flow of fluid into distribution channels 30 and 36, which deliver fluid to supply grooves 68 and 66, respectively, formed on bearing races 50 and 48, respectively. Likewise, double acting valve 76 controls the flow of fluid into distribution channels 34 and 32, which deliver fluid to the supply groove formed in race sensor 58a and to supply groove 70, respectively, formed on bearing races 46 and 52, respectively. A cross-over block 78 facilitates communication of each valve with the appropriate distribution channels. Each double acting valve operates to

- 6 regulate the flow of fluid to each distribution channel at a rate inversely proportional to the flow of fluid to the other distribution channel communicating with the valve, i.e. , if valve 74 is actuated to decrease the flow of fluid delivered to distribution channel 30, valve 74 also operates to increase the flow of fluid delivered to distribution channel 36 a proportionate amount. A pair of actuators 80 and 82 are associated with valve assembly 26 and operate in response to a change in the gap between the bearing races and the rail surface to alter the flow distribution flowing through double acting valves 74 and 76. The valve actuators may be piezoelectric devices, although other types of actuators may be used. Alternately, the two double acting valves can be replaced by four single acting valves. FIG. 7 illustrates one example of a control circuit, shown generally as

84, which can be used to automatically control operation of the valve assembly. Control circuit 84 includes a voltage source 86 which is connected to a resistor Rl and valve actuators 80 and 82. Race sensors 58a-d and the adjacent rail surfaces will behave as a capacitor and are connected in parallel with actuators 80 and 82. In operation, voltage source 86 creates an electrical potential across gaps 72 (FIG. 6) defmed between race sensors 58a-d and the adjacent surfaces of the rail 12 by supplying a voltage to electrical contacts 38 which are in contact with race sensors 58a-d. The potential across gaps 72 will behave as a capacitor, whose value will be a function of the width of the gap. As the width of the gaps change due to a change in the load applied to the bearing carriage 14, the voltage supplied to the valve actuators will change, effecting a change in the distribution of fluid by valves 74 and 76. Thus, the relative distance between the race sensors 58a-d and rail 12 directly influences the amount of fluid delivered to each of the supply grooves formed along the bearing races.

Referring again to FIGS. 5 and 6, when a load "L" is applied to carriage 14, the gap between race sensors 58a-b and rail 12 increases while the gap between race sensors 58c-d and rail 12 decreases. As discussed above, the change in gap size

7 -

SUBSTΓΓUTE SHEET (RULE 26) changes the potential across each gap, and thus changes the voltage supplied to the valve actuators 80 and 82. Since the control circuitry 84 is designed to compensate for loading of the bearing carriage 14, valve actuator 82 actuates valve 74 to increase flow into distribution channel 30 and supply groove 68 while simultaneously decreasing flow into distribution channel 36 and supply groove 66, and valve actuator 80 actuates valve 76 to increase flow into distribution channel 32 and supply groove 70 and to decrease flow into distribution channel 34 and the supply groove formed on race sensor 58a. It is noted that the change in flow distribution between the bearing races will also result in a change in gap size. Therefore, when a load is applied to the bearing carriage, a continuous readjustment of the flow rate of fluid to the bearing races will occur until the loading is compensated for and the gap sizes between each bearing race and rail 12 are uniform.

An alternate embodiment of the presently disclosed hydrostatic bearing assembly shown generally as 100 will now be described with reference to FIGS. 8 and 9. Referring to FIG. 8, bearing assembly 100 includes bearing carriage 114, bearing manifold 116, and sensors 158a and 158b. Bearing manifold 116 is secured to one end of bearing carriage 114, preferably by brazing. Alternately, other methods of attachment may be used, e.g., screws, interlocking structure, etc. Although not illustrated, a sealing gasket may be positioned between manifold 116 and carriage 114. Bearing carriage 114 is substantially similar in construction to bearing carriage 14 discussed above and includes a pair of upper bearing races 146 and 148 and a pair of lower bearing races 150 and 152. A supply groove 164, 166, 168 and 170 is formed in each bearing race. A truncated central portion 154 of carriage 114 is positioned between each respective upper and lower bearing race.

Sensors 158a and 158b are constructed from a conductive material having an electrically insulative backing, such as a polymer in conjunction with an adhesive, e.g., an acrylic adhesive. The sensors are positioned to extend within carriage 114 in a direction parallel to the longitudinal axis of carriage 114. Sensors 158a and 158b may be

- 8 -

SUBSTΓΓUTE SHEET (RULE 26) attached to carriage 114 using an adhesive as discussed above. Alternately, other methods of attachment may be used. Sensor 158a is secured to one of the truncated central portions 154 of carriage 114 with its transverse axis vertically oriented. Sensor 158b is secured to the inner top wall 155 of carriage 114 and is positioned with its transverse axis horizontally oriented. Each of sensors 158a and 158b includes an oπhogonal end portion 159a and 159b, respectively, which is positioned between carriage 114 and manifold 116.

Referring also to FIG. 9, manifold 116 has four distribution channels 130, 132, 134 and 136 formed on an interior face thereof. A recess 118 formed in the manifold is configured to receive valve assembly 126. Valve assembly 126 includes four valves 173-176. Each valve includes an inlet (not shown) in communication with inlet port 177 of the valve assembly and an outlet in communication with one of the manifold distribution channels 130, 132, 134 or 136. R-,ch valve includes an actuator 180 which is connected to a pair of contacts 138 positioned on the interior face of manifold 116 for engagement with one of orthogonal end portions 159a and 159b of sensors 158a and 158b. Preferably, actuator 180 is a piezoelectric actuator, although other types of actuators may be used.

A control circuit (not shown) such as the one shown in FIG. 7 is used to automatically control operation of the valve assembly. Sensors 158a and 158b and adjacent surfaces of rail 112 behave as a capacitor. In operation, when a voltage is applied ^'to the control circuit, an electric potential is created across the gaps defined between sensors 158a and 158b and rail 12. The potential across these gaps will be a function of the width of the gaps. When a load is applied to bearing assembly 100 and the width of the gaps change, the voltage supplied to valve actuators 180 will change, effecting a change in the distribution of fluid by valves 173-176 to bearing races 146, 148, 150, and 152 via distribution channels 130, 132, 134 and 136 and supply grooves 164, 166, 168 and 170. Sensor 158a will function to correct horizontal misalignment of carriage 114 while sensor 158b will function to correct vertical misalignment of carriage 114.

FIGS. 10-15 disclose another alternate embodiment of the presently disclosed self-compensating bearing assembly shown generally as 200. Referring to FIGS. 10 and 11 , bearing assembly 200 includes a pillow block 210 having a throughbore 211 , a bearing element 215 positioned within throughbore 211 , a bearing manifold 216, a valve assembly 226 and a supply hose 222. Bearing element 215 includes a throughbore 213 dimensioned to slidably receive support rail 212.

Referring also to FIGS. 12 and 13, throughbore 213 of bearing element 215 defines a cylindrical bearing race having four longitudinally extending grooves 263 spaced evenly about the throughbore. Each groove is dimensioned to receive an elongated electrically conductive race sensor 258. As discussed above with respect to bearing 10, a high strength insulative bonding media, such as high performance epoxy, is used to attach race sensors 258 within grooves 263. Each race sensor 258 includes a supply groove 264 and a supply channel 265. Supply channels 265 are positioned to interconnect one end of a respective distribution channel formed in manifold 216 with a respective supply groove.

Referring again to FIG. 11 and also to FIGS. 14 and 15, manifold 216 includes four distribution channels 230, 232, 234 and 236 formed on interior face thereof. Manifold 16 also includes a recess configured to receive valve assembly 226. Valve assembly 226 which includes first and second double acting valves 274 and 276, is substantially identical to valve assembly 26 of bearing assembly 10 discussed above and will not be discussed in further detail herein. Alternately, valve assembly 226 may include four separate valves such as disclosed with respect to valve assembly 126 discussed above. Each valve 274 and 276 has an inlet in fluid communication with valve assembly inlet port 277 which communicates with supply hose 222, and an outlet (not shown) which communicates with two distribution channels which deliver fluid to diametrically opposed locations of

- 10 - bearing race 213. Manifold 216 also includes four contacts 238. Each contact 238 engages a race sensor 250 when manifold 216 is secured to bearing element 215.

Bearing assembly 200 also includes a control circuit such as the one shown in FIG. 7. Bearing assembly 200 operates in substantially the same manner as bearing assembly 10 discussed above. Thus, the operation of the device need not be disclosed in further detail.

A bearing assembly incorporating the same principals discussed above may be utilized to greatly enhance the performance of a machine of which it is a component. This bearing assembly may be used to compensate for errors in a rail system by making small vertical or horizontal positional compensation. Thus, near perfect linear travel may be realized. Similarly, the stiffness of the bearing system may be programmed to correspond with an operation of a particular machine which requires a particular stiffness such as an aggressive clip making operation or a stamping operation.

It will be understood that various modifications may be made to the embodiments disclosed herein. For example, the valve assembly need not be supported within the manifold but rather may be positioned externally thereof and connected thereto by hoses. Further, the configuration of the bearing carriage and support rail need not be as illustrated, rather other configurations are envisioned. Moreover, other devices or circuitry can be used to automatically operate the valve assembly. For example, an optical device or assembly positioned to react to any change in gap size may also be used to control operation of the valve assembly. Linear variable displacement transducers, lasers and interferometry and sonar devices may also be used. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.

11 -

Claims

WHAT IS CLAIMED IS;

1. A self-compensating bearing assembly comprising: a support rail; a carriage having at least two bearing races configured to be slidably supported on the support rail, each of the at least two bearing races having a supply groove formed thereon and being positioned adjacent the rail to define a gap therebetween; a manifold attached to the carriage, the manifold having at least two distribution channels, each of the distribution channels communicating with one of the supply grooves; a sensor positioned on the carriage to sense a change in the gap between the support rail and any of the bearing races; and a valve assembly communicating with the at least two distribution channels and being adapted to communicate with a source of fluid, the valve assembly being operable to regulate fluid flow between the at least two distribution channels in response to the sensor sensing a change in the gap to compensate for loading of the carriage.

2. A self-compensating bearing assembly according to claim 1, wherein the valve assembly is supported within the manifold.

3. A self-compensating bearing assembly according to claim 1, wherein the sensor includes control circuitry operatively connected to the valve assembly, the control circuitry being operable to actuate the valve assembly in response to a change in the size of the gap between the support rail and one of the bearing races.

- 12 -

4. A self-compensating bearing assembly according to claim 3, wherein the control circuitry includes a race sensor insulatively attached to one of the bearing races, and a voltage supply connected to the race sensor and to the valve assembly, wherein the voltage supply creates an electrical potential across the gap between the support rail and the bearing race causing the rail and the race sensor to behave as a capacitor.

5. A self-compensating bearing assembly according to claim 4, wherein the carriage includes a pair of upper and a pair of lower bearing races, each bearing race having a supply groove formed thereon, and wherein a race sensor is insulatively attached to each of the bearing races.

6. A self-compensating bearing assembly according to claim 5, wherein each race sensor forms an integral part of one of the bearing races, and wherein each bearing race provides the dual function of providing a bearing surface and functioning as part of the control circuitry.

7. A self-compensating bearing assembly according to claim 5, wherein the at least two distribution channels includes four distribution channels, each of the distribution channels communicating with a respective supply groove.

8. A self-compensating bearing assembly according to claim 5, wherein the valve assembly includes two double acting valves, each double acting valve having an inlet and a pair of outlets, wherein the outlets of each pair of outlets communicate with the supply grooves formed on opposed bearing races.

- 13

9. A self- compensating bearing assembly according to claim 8, wherein each of the double acting valves operates to regulate the flow of fluid to one outlet of the pair of outlets at a rate inversely proportional to the other of the pair of outlets.

10. A self-compensating bearing assembly according to claim 5, wherein the valve assembly includes four valves.

11. A self-compensating bearing assembly comprising: a bearing carriage configured to be slidably received on a support rail, the bearing carriage including opposed bearing races, each bearing race being positioned adjacent a rail surface to define a gap therebetween; and a valve assembly including a valve actuator, the valve assembly being operable to direct fluid to the bearing races, the valve actuator being operable in response to a change in the size of the gap to regulate the flow of fluid to the bearing races to compensate for loading of the carriage.

12. A self-compensating bearing assembly according to claim 11, wherein the carriage includes a pair of upper and a pair of lower bearing races, and each bearing race includes a supply groove formed thereon.

13. A self-compensating bearing assembly according to claim 12, wherein the valve assembly includes two double acting valves, each double acting valve haying an inlet and a pair of outlets, wherein the outlets of each pair of outlets communicate with opposed bearing races.

- 14

14. A self-compensating bearing assembly according to claim 13, wherein each of the double acting valves operates to regulate the flow of fluid to each outlet of each pair of outlets at a rate which is inversely propo╧Çional to the other outlet of the pair of outlets.

15. A self-compensating bearing assembly according to claim 13, wherein each of the double acting valves is a piezoelectrically actuated valve.

16. A self-compensating bearing assembly according to claim 11, further including a sensor supported on the bearing carriage and being positioned to sense a change in the gap between the support rail and the bearing races, the sensor being operably associated with the valve actuator to cause the valve assembly to compensate for loading of the bearing assembly.

17. A self-compensating bearing assembly according to claim 16, wherein the sensor is selected from the group consisting of optical sensors, laser sensors, capacitance sensors, linear variable displacement transducers, and interferometry and sonar devices.

18. A self-compensating bearing assembly according to claim 16, wherein the sensor is integrally formed with at least one of the bearing races.

19. A self-compensating bearing assembly according to claim 18, wherein the sensor includes a plurality of electrically conductive members, each conductive member being bonded to one of the bearing races by an insulative bonding media, wherein a control circuit including a voltage source is connected to each of the electrically conductive

- 15 -

SUBST╬áTJTE SHEET (RULE 26) members and to the valve actuator such that the electrically conductive members and the support rail behave as a capacitor and the voltage supplied to the valve actuator changes as the width of the gap changes.

20. A self-compensating bearing assembly according to claim 19, wherein the insulative bonding media is high performance epoxy.

21. A self-compensating bearing assembly comprising: a support rail; a carriage having at least two bearing races configured to be slidably supported on the support rail, each of the at least two bearing races being positioned adjacent the rail to define a gap therebetween; a sensor positioned on the carriage to sense a change in the gap between the support rail and any of the bearing races; and an adjustment assembly being operable to regulate position of the bearing races in relation to the support rail in response to the sensor sensing a change in the gap to compensate for loading of the carriage.

22. A self-compensating bearing assembly according to claim 21, . wherein the adjustment assembly includes a valve assembly including a valve actuator, the valve assembly being operable to direct hydrostatic fluid to the bearing races in response to a change in the size of the gap to compensate for loading of the carriage.

-16-