Abstract:
A compact bearing system is capable of variable load carrying capability and precision motion on a guideway. The compact bearing system includes a bearing body, a fluid medium, a sealing bearing member and a pressurizing action means. The bearing body includes an internal pocket with an open side facing a surface of said guideway. The fluid medium is contained inside said internal pocket and is contacted with said surface of said guideway on said open side. The sealing bearing member is contacted with said surface of said guideway and seals a clearance between said bearing body and said surface of said guideway to limit leakage of said fluid medium. The pressurizing actuation means pressurizes the confined fluid medium through a small passage by the principle of connected vessels, and the pressurized fluid medium carrying main portion of load carried on said compact bearing system.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 62/192,722, filed Jul. 15, 2015, which is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Technical Field 
         [0003]    The present disclosure relates to a compact bearing system and a machine stage system equipping the same. More particularly, the present disclosure relates to a compact bearing system featured new “sealed pad hydrostatic bearing” with a simple, compact and low cost design in order to reduce or eliminate the disadvantage of requiring complex oil supply and handling system on existing hydrostatic bearing systems. 
         [0004]    Description of Related Art 
         [0005]    Existing hydrostatic baring system has the following disadvantages: (1) the construction is complicated; (2) the system cost is high; (3) oil temperature tends to increase at high traveling speeds. The complicated construction requires a complex oil supply system, including pumps, motors, pressure vessels, oil tanks, various oil filters, valves and controllers, pipes, flow regulators and oil temperature control systems, in order to maintain the pressure and temperature of the circulating oil. These factors result in the high system cost. In addition, the bearing gap for oil passage is very small. When the stage travels at high speed, the shear stress in the thin oil film becomes very high (viscous fluid shear stress τ=F/A=U/h), which induces heat by friction. The viscosity of oil is generally very sensitive to temperature change. The viscosity variation can cause changes of flow resistance in the land area, thereby resulting in deviations of pocket oil pressure and temperature. As a result, stages with hydrostatic bearings usually travel at speed not higher than 2 m/sec (Ref. Slocum, A. H., Precision Machine Design, Prentice Hall, New Jersey, 1992, Chapter 9). 
         [0006]    In modern arts of bearing technologies, sliding contact bearings have been used together with hydraulic or pneumatic bearings as backup bearings. And in machine stages using sliding contact bearings as primary load carrying bearings, air or oil assist has been used to relieve load and reduce drag in order to make high-speed positioning (i.e. rapid traverse). This entails providing pockets in the bearings: one pocket is required for pressurized fluid to take the weight of the machine and preload off the bearing, and often a second, smaller pocket is required on the opposing pad to relieve the opposing preload force (Ref. Slocum, A. H., Ibid, Chapter 8, Section 8.2.1, sub-section “Preload”). Details of such arrangement were not described, but the air or oil assisting bearings available in the arts are likely to be either the hydrostatic (including pneumatic) bearings described earlier, which require constant pumping of fluid through finite bearing gaps, or simple closed pad hydrostatic bearing. An example of the use of simple closed pad hydrostatic bearing is described in Trinh and Sullivan (U.S. Pat. No. 7,621,346) for a rotary head of an oil drilling machine. In journal bearings or plane bearings, it is quite common that slots or circular recesses are made on the contacting surfaces in order to assist sliding bearings or hydrodynamic bearings. However, these simple closed pad hydrostatic bearings only provide the function of some load relief and friction reduction. They themselves generally do not provide precision movement or very significant load carrying. Nor can they adjust pressure in response to varying loading conditions actively. 
       SUMMARY 
       [0007]    According to one aspect of the present disclosure, a compact bearing is provided. The compact bearing system is capable of variable load carrying capability and precision motion on a guideway. The compact bearing system includes a bearing body, a fluid medium, a sealing bearing member and a pressurizing action means. The bearing body includes an internal pocket with an open side facing a surface of said guideway. The fluid medium is contained inside said internal pocket and is in contact with said surface of said guideway on said open side. The sealing bearing member is in contact with said surface of said guideway and seals clearance between said bearing body and said surface of said guideway to confine said fluid medium and limit leakage out of said internal pocket at a predetermined minimal level. The pressurizing actuation means pressurizes the confined fluid medium through a small passage by the principle of connected vessels, and the pressurized fluid medium carrying main portion of load carried on said compact bearing system. 
         [0008]    According to another aspect of the present disclosure, a machine stage system is provided. The machine stage system is capable of variable load carrying capability and precision motion and includes a guideway and a carriage having a set of compact bearing systems riding on said guideway. Each of the compact bearing systems includes a bearing body, a fluid medium, a sealing bearing member and a pressurizing actuation means. The bearing body includes an internal pocket with an open side facing a surface of said guideway. The fluid medium is contained inside said internal pocket and is in contact with said surface of said guideway on said open side. The sealing bearing member is in contact with said surface of said guideway and seals clearance between said bearing body and said surface of said guideway to confine said fluid medium and limit leakage out of said pocket at a predetermined minimal level, an the sealing bearing member includes a coupling means that allows relative displacement with respect to said bearing body. The pressurizing actuation means pressurizes the confined fluid medium through a small passage by the principle of connected vessels, the pressurized fluid medium carrying main portion of load carried on each of said compact bearing systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
           [0010]      FIG. 1  illustrates a basic concept of a compact bearing system of the present disclosure. 
           [0011]      FIG. 2  illustrates a first preferred embodiment of the compact bearing system according to the present disclosure. 
           [0012]      FIG. 3  illustrates a second preferred embodiment of the compact bearing system according to the present disclosure. 
           [0013]      FIG. 4  illustrates a third preferred embodiment of the compact bearing system according to the present disclosure, featuring multiple layers of fluid pockets. 
           [0014]      FIG. 5  illustrates a simplified view of a machine stage system with multiple compact bearing systems of the present disclosure. 
           [0015]      FIG. 6  illustrates a perspective view of a machine stage system with multiple compact bearing systems of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    It is therefore the purpose of the present disclosure to provide a new bearing system featured new “sealed pad hydrostatic bearing” with a simple, compact and low cost design in order to reduce or eliminate the disadvantage of requiring complex oil supply and handling system on existing hydrostatic bearing systems. 
         [0017]    It is also the purpose of the present disclosure to provide a new bearing system with a simple and compact design that allows active adjustment of bearing pressure and load carrying capacity and at the same time also allows for precision movement. 
         [0018]    Furthermore, it is the purpose of the present disclosure to provide a machine stage system that uses the new bearings system together an active bearing pressure adjustment system that enables precise, micro-scale adjustments of the displacement or the orientation of a machine carriage on a linear or rotary guideway in order for compensating loading variations and maintaining positioning precision. 
         [0019]    The basic concept of the Sealed Pad Hydrostatic Bearing (SPHB), as depicted in  FIG. 1  in half-cross-sectional view, uses a pressurized fluid medium  11  confined in a fluid pocket  3  between a bearing body  2  and a guideway  1  to carry load, as in the case of a conventional hydrostatic bearing, but it doses the gap (clearance)  19  between the hydrostatic bearing block and the guideway (i.e. the land) with a sealing bearing member  4  and pressurizes the enclosed fluid by a small pressurizing actuator  7  through a small diameter duct  71  to generate required loading capacity. By the principle of connected vessels, a small force from the small pressurizing actuator  7  pressurizing oil through the small sliding contact bearing can generate a large pressure that can results in a large force in the pocket  3  with a large load carrying area. The contact between the sealing bearing member  4  and the guideway  1  is a solid to solid contact similar to the case of a sliding contact bearing. However, the difference is that the pressurized oil inside the pocket  3  takes up most of the load, with the sealing bearing member  4  under only a fraction of the total load, which can be much lower than those in cases of stages using only general sliding contact bearings. The pressure between the sealing bearing member  4  and the guideway  1  only needs to be large enough to seal the pressurized fluid and can be much lower than the pressure inside the pocket  3 . As a result, the friction of the contact is also much lower than those in cases of stages using only general sliding contact bearings. Thus, the effect of the SPHB is similar to the combination of a conventional hydrostatic bearing with a lightly-loaded sliding contact bearing, but without the complex fluid circulation system. Compared to a conventional hydrostatic bearing, despite of including the sliding contact sealing bearing member  4 , the SPHB can still have high precision. For example, in modern arts of bearing technologies, lightly preloaded sliding contact bearings can still reach sub-micrometer precision and are applied in wafer steppers and instruments (Ref. Slocum, A. H., Precision Machine Design, Prentice Hall, New Jersey, 1992, Chapter 8, which is incorporated herein by reference). 
         [0020]    In addition, a pressure regulation means  8  can be used to maintain pressure of the confined fluid medium  11  within a predetermined range, thereby maintaining load carrying range during system operation. 
         [0021]      FIG. 2  illustrates a first preferred implementation of the SPHB concept in cross-sectional view. The bearing body  2  includes a pocket  3  filled with oil (or any suitable hydraulic fluid). The pocket  3  has one side open toward the guideway  1 . A sealing bearing member  4  is assembled to the bearing body  2  from the open side. A seal, such as an O-ring  6  seals the clearance  19  between the external side surface of the sealing bearing member  4  and the internal wall of the pocket  3 . The lower rim of the sealing bearing member  4  is in contact with the guideway  1 , forming a seal, which is also a contact sliding bearing. To pressurize the oil, an actuator  73  pushes a piston  72  in the duct  71 . By the principle of connected vessels, the load carrying capacity FL of the SPHB and the force Fp required to push the piston  72  are as follows respectively, FL=P At; and Fp=P Ap, wherein P is the pressure in the pocket  3  and the duct  71 , At is the total acting area of hydraulic pressure that can generate upward load carrying force, and Ap the cross sectional area of the pressurizing duct  71 . For example, assuming Ap=1 mm 2 , the actuator  73  needs only to exert a force of 0.31 kgw (3N), a bearing pocket pressure of 30 bar can be generated, which is equivalent to bearing hydraulic pressure on conventional hydrostatic bearings. Under this pocket pressure, a bearing pad area of 10 mm×10 mm can carry 32 kgw (313 N). 
         [0022]    Regarding the sealing bearing member  4 , the force pushing it toward the guideway  1  must be larger than the force pushing it away from the guideway  1  in order for it to seal the contact surface and prevent leakage of the pressurized oil in the pocket. That is, Fs+PAi≧0.5 PAo. 
         [0023]    Wherein Fs is the force of the preloading spring  5 ; Ai is the acting area of hydraulic pressure pushing the sealing bearing member  4  toward the guideway  1 ; Ao is the acting area of hydraulic pressure pushing the sealing bearing member  4  away from the guideway  1 ; and 0.5 P is averaged oil pressure in the contact area assuming a linearly decreasing pressure profile from P. From the above relation, we see that when Ai=0.5 Ao, the system is theoretically self-sealing; with the preloading spring  5  providing extra sealing force Fs. When Ai&gt;0.5 Ao the system will have “over balanced” sealing force. This principle of “seal hydraulic balance” is basically the same principle applied in the art of mechanical seal (or mechanical face seal), which is widely used on rotating shafts in high pressure liquid pumps (Ref. Handbook of Fluid Sealing, ed. by Brink, R. V., McGraw Hill, New York, 1993, Chapter 6, which is incorporated herein by reference). If the hydrostatic load carrying force FL takes all the load from a machine (or a stage carriage  20 ), then the force exerted by the sealing bearing member  4  on the guideway  1  surface, i.e. solid to solid contacting force, only comes from the spring force Fs of the preload spring  5  and the hydraulic force due to area difference (Ai−0.5 Ao). Under the condition of keeping the oil sealed, adjusting the spring force Fs and the area difference (Ai−0.5 Ao) can adjust the solid to solid contacting force so that the difference between the static friction and the dynamic friction can be minimized, thereby minimizing the stick-slip phenomenon. 
         [0024]    The sealing bearing member  4  can be the type similar to a mechanical face seal or can be the type of molder packing. Mechanical face seal materials include carbon, graphite or resin or metal matrix composites. Molded packing type of seal materials include leather, rubber, fiber reinforced rubber, or polymer materials such as NBR or PTFE. These are some common materials used in the art of fluid sealing. On the other hand, materials commonly used for sliding contact bearings on machine tools include PTFE film (such as Turcite and Glacier DU) and metal pairs (such as brass to steel). Other engineering materials such as Acetron and Derlin can also be used. 
         [0025]    The actuator  73  can apply stacked piezoelectric actuators or a servo motor with force amplification mechanism such as worm gears, in order to generate required force. Pressurization can even be done manually. 
         [0026]    The pressure regulator  8  can be used to adjust the stiffness and damping characteristics of the system. Its construction is similar to the pressurizing unit, except that a spring  83  and a displacement clearance adjusting mechanism replace the actuator to adjust and set the reaction force and the allowed displacement of the piston  82 . The pressure regulator  8  can also be a passive unit or can be an active unit applying powered actuator to enhance its capability. A passive air-chamber type pressure compensator commonly used for stabilizing flow pressure can also be used. 
         [0027]    The preloading spring  5  is preferred for handling operational variations but is not necessary. If the load is large and stable, the bearing body  2  and the sealing bearing member  4  can be an integral part and the pressure regulator  8  can handle operational variations. 
         [0028]    There can be other constructions of the SPHB based on the same principle described above. For example,  FIG. 3  illustrates a second preferred example embodiment. A membrane spring (diaphragm)  5 A replaces the spring  5  in  FIG. 1 . It also has the function of sealing, replacing O-ring  6 . In addition, in the pressurizing system  7 A, a membrane spring  5 A also replaces a molded packing seal  74 . A pushing pin  75  now pushes on the membrane to cause pressurization. 
         [0029]    Still another variation of the SPHB concept is to use multi-layered pockets  3 E,  3 F, as shown in  FIG. 4 . That is, the fluid pocket  3 E,  3 F in the bearing body  2  can have a structure with concentric walls separating the pocket  3 E,  3 F into concentric regions, with each region using a separate sealing bearing member  4 E,  4 F. That is, the multiple concentric structures of ring shaped pockets  3 E,  3 F and sealing bearing members  4 E,  4 F form cascaded separated oil pockets  3 E,  3 F. The oil pocket  3 E,  3 F pressures can also be made cascaded. For example, in the center pocket  3 E the pressure is P. In the ring pockets  3 F next to the center one the pressure is P 1 , slightly lower than P. In the next adjacent ring pocket  3 F, the pressure is P 2 , and so on. In this way, pressure difference between adjacent pockets  3 F can be smaller than in the case of using a single pocket. This allows the sealing bearing member  4 E,  4 F to have better sealing ability. This is illustrated in  FIG. 4 . 
         [0030]    In case of fluid leakage, as this may be inevitable in some types of mechanical seals, the leakage rate is usually several orders of magnitude smaller than the fluid circulation rate of conventional hydrostatic bearing. This is because the effective bearing gap is now very small, leading to very small flow rate. In this case, a small pump can be applied to pressurize as well as to replenish the oil. Because the flow rate is so low, the pump can be small enough to be placed on the stage and its effect on the machine precision can also be minimized even if it sits on the stage. Further, because the bearing still has a solid to solid contact, flow resistance is of less concern here. The slow rate oil replenishing and circulating system can be much simpler than conventional hydrostatic bearings. Such a fluid recycling pumping system is illustrated in  FIG. 4 ; a small circulating pump system  10  is used. 
         [0031]    When the sealing bearing member  4  includes a coupling means to the bearing body, such as the springs  5 ,  5 A and O-ring  6  mechanism in  FIGS. 2 and 3 , it can have a small relative displacement with respect to said bearing body mainly in the direction perpendicular to the guideway  1  surface. If incompressible fluid, such as oil or water, is used, because the fluid confined inside the pocket  3  is of a fixed volume, a change of position of the piston  72  or the pushing pin  75  will cause expansion or contraction of the fluid volume. This allows minute adjustment of the clearance  19  between the bearing body  2  and the guideway  1  surface, that is, the vertical position of a carriage  20  on the guideway  1 . 
         [0032]    In order to apply the SPHB to a linear stage or a rotary table, a bearing system of multiple pads with opposed pads arranged in proper directions at selected locations can be used, similar to the cases of using conventional hydrostatic bearing. The multiple pads can be inter-connected or can be independent. In the case of independent pads, the pressure in each pad can be independently controlled. By using piezo or small servo motors as actuators on each pad, the pressure in each pad can be actively and independently controlled. This allows minute adjustment, as allowed by the coupling mechanism between the bearing body  2  and the sealing bearing member  4 , of both the displacement and the orientation of a machine carriage  20  on a linear or rotary guideway, which can be very useful for compensating loading variations and maintaining positioning precision. And all these controls can be done by “wire”, that is, by electrical signals. 
         [0033]      FIG. 5  illustrates a simplified view of a machine stage with multiple bearings of the present disclosure. With independently adjusted bearing, even if the guideway surface  1 A is not completely flat, a control system  500  is able to “pose” the carriage  20  to maintain desired orientation and elevation by driving the SPHB sealing bearing members  401 ,  402  via electric signal on wires  501  to push the sealing bearing members  401 ,  402  to different heights. 
         [0034]      FIG. 6  illustrates a perspective view of a machine stage with multiple bearing systems of the present disclosure. For example, in  FIG. 6, 403-408  six independently controlled SPHB bearing systems  403 - 408  are used. The orientation of the carriage  20  on the guideway  1 B can be adjusted in microscale. 
         [0035]    To ensure good sealing, the flatness and the surface finish of the contacting surfaces on both the guideway  1 B and the sealing bearing member  4  need to be high. Polishing a long guideway  1 B to high flatness can involve very high cost. A solution to the issue to use polished or glazed porcelain tiles as guideway surface  1 A. These tiles are usually used for construction or interior decoration purposes. We have discovered that they provide acceptable surface finish and local flatness for the purpose of the present disclosure. 
         [0036]    The fluid medium  11  described in the present disclosure should be understood as any suitable medium that can change shape and can take pressure, including oil, water air and other suitable fluids and fluids with any additives and multi-phase members. 
         [0037]    In sum, the present disclosure provides a new bearing system with a simple, compact and low cost design in order to reduce or eliminate the disadvantage of requiring complex oil supply and handling system on existing hydrostatic bearing systems. Moreover, the compact design of the new bearing system allows active adjustment of bearing pressure and load carrying capacity and at the same time also allows for precision movement. 
         [0038]    Furthermore, the present disclosure also provides a machine stage system that uses the new bearings system together an active bearing pressure adjustment system that enables precise, micro-scale adjustments of the displacement or the orientation of a machine carriage on a linear or rotary guideway in order for compensating loading variations and maintaining positioning precision. 
         [0039]    Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
         [0040]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims.