Abstract:
A displaceable carriage ( 10 ) and a linear guide for guiding the carriage, which includes a first guide path slideway ( 13, 31 ) and a second guide path slideway ( 32 ). To form the first slideway ( 13, 31 ), the carriage ( 10 ) includes at least one pocket ( 13 ), which slides along the first guide path part ( 31 ) and can hold lubricant under pressure to create a relieving force on the carriage ( 10 ). The second guide path part ( 32 ) includes an island-shaped surface to create a frictional force on the carriage ( 10 ). A pressure medium held in the at least one pocket ( 13, 14, 15 ) creates the relieving force, whereby the first slideway ( 13, 31 ) relieves the load on the second slideway ( 11   a   , 12   a   , 12   b   , 32 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device comprising a displaceable carriage and a linear guide. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are known linear guides which operate hydrostatically. Pockets that can hold a lubricant under pressure are provided for this purpose. It has been found that such guides are susceptible to interfering influences, so it becomes difficult to hold the carriage in a certain position. For example, if the carriage is used for machining a workpiece, even minor ground vibrations may make it difficult to hold the carriage in a certain position, which thereby impedes an accurate machining. Furthermore, the achievable path accuracy is restricted in the case of long guides. 
         [0003]    DE 31 31 566 A1 describes a linear guide comprising a hydrodynamic guide rail and a hydrostatic guide rail which is arranged opposite the hydrodynamic guide rail and thus applies a load to it. The design of the guide rails is not explained in detail. 
       SUMMARY OF THE INVENTION 
       [0004]    One object of the present invention is to create a device of the type mentioned in the introduction in which the carriage is precisely movable and positionable to an improved extent. This object is achieved by a device comprising a displaceable carriage with at least one pocket, in which lubricant can be held under pressure, and an island-shaped surface to produce a frictional force acting on the carriage. This makes it possible to provide a hydrodynamic slideway, for which the friction is adjustable through the hydrostatic relief. The carriage is precisely movable and positionable with this type of guide. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in greater detail below on the basis of an exemplary embodiment with reference to the figures, in which 
           [0006]      FIG. 1  shows a perspective view of a device according to the invention having two feed axes X and Z; 
           [0007]      FIG. 2  shows a carriage of the device from  FIG. 1  in a view from underneath; 
           [0008]      FIG. 3  shows a detailed view from  FIG. 2 ; 
           [0009]      FIG. 4  shows a part of the linear guide from  FIG. 1  in a perspective view, with the carriage being shown in a partial sectional view; 
           [0010]      FIG. 5  shows the part of the linear guide from  FIG. 4  in a sectional side view; and 
           [0011]      FIG. 6  shows a schematic diagram of the device according to  FIG. 1  in a sectional side view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]      FIG. 1  shows a device having a machine bed  1 , on which a carriage  10  is arranged. The carriage can travel in one axis, namely the Z axis here, its path of movement being defined by a linear guide  20 . In the present example, the linear guide  20  is embodied as a guide freely supported, i.e., it is free of wrap-arounds, for example, in the form of a V-flat guide, as also shown in  FIG. 2 . Other guides are also conceivable, e.g., only a V guide or only a flat guide. 
         [0013]    The linear guide  20  comprises two guide profiles  21  and  22 , which extend in the direction of the axis Z of movement. The respective guide profile  21 ,  22  is embodied, for example, so that it is integrated into the machine bed or is attached to the machine bed in the form of a separate rail. A carriage drive  5 ,  6  for moving the carriage  10  is arranged between the guide profiles  21 ,  22 . In the present example, the carriage drive  5 ,  6  is embodied in the form of a linear motor, which contains a primary part  5  mounted on the underside of the carriage  10  and containing electric windings, as well as a secondary part  6  extending between the guide profiles  21 ,  22  in the direction of the axis of travel Z and being provided with permanent magnets. A traveling field can be generated by supplying the windings in the primary part  5  with a suitable current, exerting a thrust from the primary part  5  on the secondary part  6  and thus inducing movement of the carriage  10 . To effectively dissipate the heat generated by the linear motor  5 ,  6  during its operation, a cooling system with a circulating cooling fluid, which may be brought into contact with the primary part  5  and/or the secondary part  6 , may be used. 
         [0014]    Instead of a linear motor  5 ,  6 , other drives are also conceivable for moving the carriage  10 , for example, a rotary drive, e.g., one with a ball screw spindle. 
         [0015]      FIG. 2  shows the underside of the carriage  10  without the primary part  5  of the carriage drive. The carriage  10  has two guide profiles  11  and  12 , which protrude out of the underside of the carriage  10  and slide along the guide profiles  21  and  22  when traveling. The first guide profile  11  has a slide plane  11   a  in which recesses  13 ,  16  have been created (cf. also the detailed view in  FIG. 3 ). The first recess  13  extends in the axis of travel Z and serves as a pocket to form a hydrostatically relieving slideway. The second recess  16  runs between the pocket  13  and the inside edge of the guide profile  11  in a zigzag pattern along the slide plane  11   a . The second recess  16  serves as a lubrication groove to form a hydrodynamic slideway. 
         [0016]    The second guide profile  12  has two slide planes  12   a  and  12   b , which are arranged inclined at an angle according to the V shape of the guide profile  22  and are each provided with recesses  14 ,  15 ,  17  like the slide plane  11   a . The recesses  14  and  15  formed in the slide plane  12   a ,  12   b  extend in the axis Z of travel, such that they are arranged in line, and serve to form hydrostatic pockets like the pocket  13  on the first guide profile  11 . The recess  17  formed in the slide plane  12   a ,  12   b  has a zigzag-shaped course along the slide plane  12   a ,  12   b  and serves as a lubrication groove like the lubrication groove  16  on the first guide profile  11 . 
         [0017]      FIG. 4  shows a detailed view of the carriage  10  which rests on the guide profile  21  arranged on the stand  25 . The profile  21  has a guide path running substantially horizontally and comprises a first guide path part  31 , a second guide path part  32  and a third guide path part  33 . 
         [0018]    The guide path parts  31 - 33  are designed to be substantially flat and have a sliding path surface, which is structured in different ways. 
         [0019]    The first guide path part  31 , which is designed to be closed and extends in an uninterrupted fashion along the edge of the stand  25 , serves as the closure of the pocket  13  on the carriage  10 , as also shown in  FIG. 5 . Lubricant can be directed into the pocket  13  through a feed line and suitable bores in carriage  10  and can be put under pressure there. 
         [0020]    The second guide path part  32  is designed to be interrupted by the provision of islands. The islands  32  may also have a shape different than the nubs shown here. For example, it is conceivable that individual islands or all of the islands  32  may be connected to the first guide path part  31  and/or to the third guide path part  32 . The islands  32  are surrounded by a recess  32   a  for receiving lubricant (cf.  FIG. 5 ). The second guide path part  32  serves as a closure to the part of the slide plane  11   a  in which the lubrication groove  16  runs. 
         [0021]    The third guide path part  33  is designed to be closed and extends in an uninterrupted manner along the edge of the stand  25 . The two guide path parts  31  and  33  form a lateral closure so that lubricant cannot escape at the side. 
         [0022]    The V-shaped guide profile  22  is designed according to the guide profile  21  shown in  FIG. 4  in that the two flanks of the guide profile  22  each have two guide path parts in the manner of the guide path parts  31  and  33  and, arranged in between, a structured guide path part in the manner of the guide path part  32 . 
         [0023]    The guide path  31 - 33  is manufactured from a wear-resistant material such as plastic. The recesses  13 - 17  can be created at the time of manufacture of the carriage  10 , for example, by milling and/or molding. 
         [0024]    The functioning of the linear guide  20  described above will now be explained below on the basis of  FIG. 5  and the schematic diagram in  FIG. 6 . Since the two guides  11 ,  21  and  12 ,  22  function in the same manner, for the sake of simplicity only the functioning of the one guide  11 ,  21  will be explained here. 
         [0025]    A normal force Fn directed against the guide  11 ,  21  acts on the carriage  10 . The force Fn is obtained from the weight force originating from the mass of the carriage  10  and from the load Q additionally acting on the carriage as well as from an additional preload. In the present example, this additional preload is created by the attractive force of the carriage drive  5 ,  6  designed as a linear motor. An additional preload may also be supplied by other means, for example, by providing additional hydrostatic pockets which act in the direction of the weight force. 
         [0026]    During operation of the carriage  10  the pocket  13  is filled with a fluid lubricant, for example, a lubricating oil, which is put under pressure. This results in a relieving force Fe, which acts against the normal force Fn and relieves the load on the carriage  10 . The surface pressure is thereby minimized. Pocket  13  and guide path part  31  thus form a hydrostatically relieved slideway. In this area of the guide, friction is created only in the lubricant when displacing the carriage  10 . The corresponding frictional force Fr′ is negligible, i.e., Fr′≈0. 
         [0027]    Fluid lubricant is also added between the two guide path parts  31  and  33 . The carriage  10  is in contact with the second guide path part  32  via the slide plane  11   a . The distribution of the lubricant in the recess  32   a  of the guide path part  32  is supported by the lubrication groove  16  when displacing the carriage  10 . On the whole, the slide plane  11   a  and the guide path part  32  form a hydrodynamic slideway. In this area of the guide, friction is created primarily between the surfaces  11   a  and  32 , which are rigid. The corresponding frictional force Fr is given by the difference between the normal force Fn and the relieving force Fe: Fr=μ·(Fn−Fe), where μ is the coefficient of friction. 
         [0028]    On the whole, a total frictional force F of F=Fr′+Fr≈μ(Fn−Fe) acts on the carriage  10  because of the hydrostatic and hydrodynamically supported slideway  11   a ,  13 ,  31 ,  33 . 
         [0029]    To ensure accurate ventilation of the pockets  13 - 15 , it is conceivable to provide nozzles and/or throttles on the carriage  10  to permit a continuous flow of lubricant through the pockets  13 - 15 . Therefore, air which is in the pockets  13 - 15  due to a longer standing time, for example, can be drawn off. The nozzles and/or throttles are to be selected, so that the flow is as small as possible and so that a sufficiently great lubricant pressure can be built up in the pockets  13 - 15 . 
         [0030]      FIG. 6  shows schematically the pocket  13 , which is connected fluidically to a feed line  45  on the one hand and to a drain  46  on the other hand. The feed line  45  is connected to external means for generating the lubricant pressure. 
         [0031]    In summary it can be concluded that the linear guide  20  described here functions according to the principle of hydrodynamic guidance with hydrostatic relief. Due to the pressure acting on the relief pockets  13 - 15 , the normal force Fn and thus the frictional force Fr between the carriage  10  and the guide path part  32  are reduced. 
         [0032]    The linear guide  20  can be designed as a static system so that the relief remains constant once it has been set. The size of the relieving force Fe is designed according to the minimal load Q which is provided so that the guide qualities and a desired measure of stiffness are preserved. If the guide  20  is designed as a guide freely supported as in the present example, then it is necessary to ensure, among other things, that the carriage  10  still rests securely on the guide profiles  21 ,  22 , even at the minimal load Q. 
         [0033]    The measure of relief can be defined by various parameters such as the number, length and width of the pockets  13 - 15  and their position on the carriage  10  as well as the lubricant pressure in the pockets  13 - 15 . Regions may be relieved in different ways depending on the choice of these parameters. This is necessary, for example, when the load profile Q is asymmetrical which is in contrast with the case shown in  FIG. 6  where the arrows Q indicate a constant load profile. 
         [0034]    Providing the linear guide  20  described here has the advantage that, among other things, a carriage  10  can be moved precisely to a certain position and held there. It has been ascertained that the frictional force Fr generated on the basis of the linear guide  20  has a damping effect on interfering factors. These may be caused internally, e.g., by a change in the load on the carriage  10  over time, or externally, e.g., due to ground vibrations. Because of the damping, these interfering factors have no effect on a change in position of the carriage  10 . It is therefore simpler to regulate the device in such a way that the carriage  10  is held in a certain position. 
         [0035]    Providing a linear guide as a combination of a hydrodynamic guide and a hydrostatic guide also has the advantage that the linear guide has an improved stiffness and an improved path precision, in particular when the guide length is great. 
         [0036]    If a linear guide  5 ,  6  is provided as a carriage drive, then a precise distance between the two parts  5  and  6  is ensured by the linear guide  20  and a precise positioning of the carriage  10  is possible. In addition high speeds and accelerations can be generated by means of the linear motor  5 ,  6  and thus rapid movements of the carriage  10  are possible. 
         [0037]    The linear guide  20  can be used in a variety of ways, in particular on machines which are designed for machining workpieces, e.g., by grinding. In the example according to  FIG. 1  a workpiece holder which has a workpiece headstock and a tailstock, for example, can be mounted on the carriage  10 , for example.  FIG. 1  shows another carriage  10 ′, which can travel in the X axis, wherein the guide  20 ′ is designed like the linear guide  20 . For example, a linear motor  5 ′,  6 ′, which is designed like the linear motor  5 ,  6  is used for the drive. A rotatable grinding disk can be mounted on the second carriage  10 ′, for example. The device according to  FIG. 1  is thus suitable for use on a grinding machine, in particular a cylindrical grinding machine, in order to be able to machine workpieces accurately. 
         [0038]    Numerous modifications are accessible to those skilled in the art from the preceding description without going beyond the scope of the invention, which is defined by the claims. 
         [0039]    For example, the definite geometric embodiment of the linear guide  20 ,  20 ′ can be adjusted. For example, instead of the V-flat guide shown here, the guide may also be embodied as a pure V guide, a pure flat guide or also as a guide having a different geometric shape. 
         [0040]    In addition it is conceivable to provide the island-shaped guide path part  32  on the carriage  10  and to design the area between the two guide path parts  31  and  33  in the form of the slide plane  11   a . A lubrication groove in the form of the lubrication groove  16  can be provided in this slide plane. 
         [0041]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.