Abstract:
The invention concerns a water-hydraulic machine with at least two mutually movable parts, of which one has a surface of a plastic material with friction-reducing properties. In such a machine it is desired to enable the use of demineralised water. For this purpose, it is ensured that a layer of a carbon-containing material is arranged between the plastics material and the other part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in German Patent Application No. 102 23 844.8 filed on May 28, 2002.  
         FIELD OF THE INVENTION  
         [0002]    The invention concerns a water-hydraulic machine with at least two mutually movable parts, of which one has a surface of a plastic material with friction-reducing properties.  
         BACKGROUND OF THE INVENTION  
         [0003]    A water-hydraulic machine of this kind is known from the “Nessie” project of Danfoss A/S, Nordborg, Denmark. An example of a publication of such a machine exists in DE 43 01 124 A1.  
           [0004]    In a water-hydraulic machine, water is used as a hydraulic medium. Compared with the normally used hydraulic oils, water has the advantage that leakages causes practically no pollution of the environment.  
           [0005]    However, water has the disadvantage that, contrary to oil, it has no lubricating properties. In a water-hydraulic machine, this usually leads to problems, as mutually movable parts cannot be lubricated and cooled to a sufficient extent.  
           [0006]    Therefore, in the Nessie project mentioned above, when working with pairings of two mutually movable parts, one part has been provided with a layer or an insert of a plastics material, which is made of a friction-reducing plastics material. A preferred plastic material for this purpose comes from the group of high-performance thermoplastic plastics materials on the basis of polyarylether ketones, in particular polyetherether ketones (PEEK). Basically, the use of PEEK has turned out to be successful. Water-hydraulic machines provided with such a plastics material on the contact surface of mutually movable parts could also be operated reliably with water over long periods.  
           [0007]    However, in one specific application, problems sometimes occur, when demineralised water is used as hydraulic fluid. Such applications comprise, for example, systems working according to the principle of reverse osmosis. As soon as “pure” water is used, wear phenomenon occur on mutually moving parts, particularly, when these parts are provided with PEEK.  
           [0008]    The invention is based on the task of enabling operation of a water-hydraulic machine also with demineralised water.  
         SUMMARY OF THE INVENTION  
         [0009]    In a water-hydraulic as mentioned in the introduction, this task is solved in that a layer of a carbon-containing material is arranged between the plastics material and the other part.  
           [0010]    Thus, the plastics material, for example PEEK, is not replaced by another plastics material. On the contrary, an additional layer of a carbon-containing material is used, which is arranged between the plastics material and the other part. In connection with demineralised water, the use of such carbon-containing layers leads to astonishing results. The wear of the plastics material layer and the layer sliding upon it, for example a steel layer, is drastically reduced. The life of the machine is substantially extended. At the same time, the working characteristics of a water-hydraulic machine with this equipment does practically not change compared with the machines already known from the Nessie project. The reason for the improvement during operation with demineralised water, when using a layer of a carbon-containing material, is not yet fully disclosed. It is assumed that, further to an improved “lubrication ability” caused by the layer, also an improved corrosion protection occurs. This is particularly important in connection with the use of demineralised water as hydraulic fluid.  
           [0011]    Preferably, the layer is arranged on the other part. In other words, two materials with friction-reducing properties are made to work together, namely, firstly, the plastics material, which already has friction-reducing properties, and secondly the layer of the carbon-containing material, which is arranged on the other part. This layer of carbon-containing material will protect the other part. At the same time, a mutual movement of the two parts will cause a minor wear away of the layer with a consequent settling on the plastics material.  
           [0012]    Preferably, the layer is made on the basis of diamond-like carbon. Such a layer of “DLC” (diamond-like carbon) has outstanding frictional properties on the corresponding counter-surface, that is, it keeps the wear small. This particularly applies, when the DLC layer cooperates with the plastics material. An improvement can hardly be anticipated, as the plastics material, particularly PEEK, has already outstanding frictional coefficients. With demineralised water, however, the DLC layer even further improves these.  
           [0013]    Preferably, the layer is applied during a plasma-activated steam-phase deposit. Thus, relatively thin layers can be achieved. At the same time occurs a very close connection of the diamond-like carbon with the base, so that even with heavy loads the risk of a detachment of the DLC-layer from the base is extremely small. The use of a steam-phase deposit, particularly a plasma-activated steam-phase deposit, permits the application of the diamond-like carbon practically independently of the shape of the base. This gives a relatively high degree of freedom when designing the mutually movable parts.  
           [0014]    Preferably, the layer has a thickness in the range from 0.5 to 10 μm. The exact thickness depends on the desired load. The use of a very thin layer has the advantage that the base, that is, the surface of the other part, is practically identically reproduced. When designing the mutually movable parts, the layer of diamond-like carbon requires practically no attention at all. Still, however, the frictional properties and the wear properties are reduced so much that even with demineralised water a long life is ensured.  
           [0015]    Preferably, the layer has a temper of 25 Gpa. Such a temper permits a relatively high loadability. Thus, also with higher pressure and the use of demineralised water, the wear is kept low.  
           [0016]    Preferably, the plastics material is chosen from a group of the high-performance thermoplastic plastics materials on the basis of polyarylether ketones, in particular polyetherether ketones, polyamides, poly-acetalenes, polyarylethers, polyethyleneterephthalates, polyphenylene sulphides, polysulphones, polyether-sulphones, polyetherimides, polyamidimides, polyacrylates, phenol resins, such as novolak resins, or similar substances; glass, graphite, polytetra-fluoroethylene or carbon, particularly in fibre form, can be used as fillers. With this selection of material, already now the use of water as hydraulic fluid provides excellent working characteristics. When using demineralised water or pure water, this working characteristics will be maintained, on condition that a layer of a carbon-containing material, particularly a diamond-like carbon, is used on the other part.  
           [0017]    Preferably, one of the two parts is a cylinder with a sleeve of the plastics material and the other of the two parts is a piston, provided with the layer, at least on its friction surface. The pairing cylinder-piston is one of the heaviest loaded elements, for example in an axial or radial piston machine. Here, the combination of the friction-reducing plastics material and the DLC-layer is particularly effective to keep the wear small when using demineralised water.  
           [0018]    Alternatively, or additionally, one of the two parts is a holddown plate, which is supported on a cylinder drum via a ball joint, one sliding surface of the ball joint being provided with the plastics material, the other sliding surface having the layer. Also in the area of the ball joint, with which the holddown plate is supported on the cylinder drum, substantial loads occur. These loads can then be absorbed without problems, when one sliding surface is provided with the plastics material, for example PEEK, and the other sliding surface with a DLC-layer.  
           [0019]    It is also preferred that one of the two parts is a sliding shoe, which bears on a swashplate and carries the plastics material, the swashplate, which is the other of the two parts, being provided with the layer. Of course, the embodiment can also be vice versa, that is, the swashplate is provided with the plastics material, and the sliding shoe carries the layer. Here, the material pairing is also particularly important, as the sliding shoes are pressed against the swashplate with relatively high pressures.  
           [0020]    Finally, it is preferred that one of the two parts is a pressure plate, which is arranged between a control plate, being the other of the two parts, and the cylinder drum, and which turns during operation together with the cylinder drum in relation to the control plate, the control plate being provided with the plastics material and the pressure plate having the layer. Also in this area substantial loads occur, which can be absorbed by the DLC-layer, when demineralised water is used as hydraulic fluid, without causing any deterioration of the working characteristics of the machine.  
           [0021]    Preferably, the sliding shoe is connected with the piston via a ball joint, and, at least in the area of the ball joint, provided with the plastics material, the counter-surface of the ball joint having the layer. The load of the ball joint on the sliding shoe is somewhat smaller than the load of the ball joint, with which the holddown plate is supported on the cylinder drum. Anyway, the result when using demineralised water is an advantageous material pairing of the plastics material with friction-reducing properties and the DLC layer.  
           [0022]    Preferably, the cylinder drum is supported on a housing via a radial bearing surface, the cylinder drum having the layer and the bearing surface carrying the plastics material. Applying the layer on the cylinder drum is somewhat simpler than applying the layer on an inner surface of the housing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    In the following, the invention is explained in detail on the basis of preferred embodiments in connection with the drawings, showing:  
         [0024]    [0024]FIG. 1 a first embodiment of a water-hydraulic machine  
         [0025]    [0025]FIG. 2 a second embodiment of a water-hydraulic machine  
         [0026]    [0026]FIG. 3 a third embodiment of a water-hydraulic machine 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    [0027]FIG. 1 shows a water-hydraulic machine  1  with a housing  2 , in which a cylinder drum  3  is arranged to be rotatable.  
         [0028]    In the cylinder drum  3  is arranged at least one cylinder  4 , which is surfaced with a sleeve  5 . The sleeve  5  is made of a plastics material from the group of high-performance thermoplastic plastics materials on the basis of polyarylether ketones, in the present case polyetherether ketones (PEEK). PEEK cooperates in a low-friction manner with the material of a piston  6 , which is in the present case made of stainless steel.  
         [0029]    The piston  6  is movable in the cylinder drum in the direction of a double arrow  7 . The control of the piston  6  movements in the cylinder  4  occurs by means of a sliding shoe  8 , which is held against a swashplate  10  by the effect of a holddown plate  9 .  
         [0030]    The holddown plate  9  is supported on the cylinder drum  3  via a ball joint with one ball  11 . The ball  11  is also made of stainless steel. In the contact area with the ball  11 , the holddown plate  9  has an insert  12  made of PEEK.  
         [0031]    The sliding shoe  8  is encased by a moulded element  13  made of PEEK, that is, the moulded element  13  forms both the bearing surface of the sliding shoe  8  on the swashplate  10  and the bearing surface of the sliding shoe  8  on the holddown plate  9 . Finally, the moulded element  13  has an extension, which permits it to encase a ball  14  at the front end of the piston  6 , this ball  14  forming a part of a ball joint.  
         [0032]    The cylinder drum  3  is supported in the housing  2  on a bearing surface  15  made of PEEK, that is, the bearing surface  15  forms a radial bearing.  
         [0033]    At the end facing away from the swashplate  10  is provided a pressure plate  16 , into which sleeves  17  are inserted, which form a connection between the pressure plate  16  and the cylinders  4 . The pressure plate  16  bears on a control plate  18 , which is provided with a cover  19  made of PEEK. The control plate  18  is arranged to be fixed in the housing  2 . Here, a bolt  20  retains it. The pressure plate  16  rotates together with the cylinder drum  3  in relation to the control plate  18 , so that the control plate  18 , can position the inlet and outlet of hydraulic fluid for the cylinder  4  correctly.  
         [0034]    The pressure plate  16  is pressed against the control plate  18  by the force of a spring  21 . The sleeves  17  permit a slight axial movement of the cylinder drum  3  in relation to the pressure plate  16 . At the same time, the spring  21  provides a certain pressure, with which the holddown plate  9  presses the sliding shoe  8  against the swashplate  10 .  
         [0035]    In principle, such a machine is known. It can both be used as a motor, when the cylinder  4  is supplied with pressurised hydraulic fluid, and as a pump, when the movement of the piston  6  in the cylinder  4  produces a certain pressure in the hydraulic fluid. Due to the use of PEEK in the areas, where two parts of the machine  1  are mutually moving, it is even possible to operate the machine with water as hydraulic fluid. The plastics material PEEK reduces the friction between mutually movable parts to such a degree that a serious wear no longer exists.  
         [0036]    When, however, this machine is operated with demineralised water, additional measures must be taken, which are explained in the following.  
         [0037]    The surfaces bearing on the PEEK surfaces are namely provided with a layer of a diamond-like carbon. Such a DLC layer, the abbreviation “DLC” meaning “diamond-like carbon”, has an extremely high temper, which is in the range of 3,000 HV or can be measured in the range of 2,000 to 5,000 kg/mm 2 . In another notation, the temper is at least 25 Gpa. Such a DLC layer can, for example, be applied on the corresponding parts during a plasma-activated steam-phase deposit. Such a layer is extremely thin. Its thickness is in the range from 0.5 to 10 μm. Thus, basically the surface geometry of the part carrying the DLC layer is not changed. The roughness of the layer is maintained. Anyway, this DLC layer, in cooperation with the PEEK plastics material, provides a relatively small friction between the mutually movable parts, also when demineralised water is used as hydraulic fluid. This friction ensures a reliable protection against the wear of the mutually movable parts. Also when using demineralised water as hydraulic fluid, a satisfactory life is ensured.  
         [0038]    On the friction surface cooperating with the sleeve  5 , the piston  6  has a layer  22  of diamond-like carbon. In the area of the ball  14 , the piston has an additional DLC layer  23 . Of course, the two layers  22 ,  23  can also extend into one another.  
         [0039]    On the swashplate  10  is arranged a DLC layer  24  in the contact area with the sliding shoes  8 . The ball  11  of the ball joint between the holddown plate  9  and the cylinder drum has a layer  25 , which cooperates with the insert  12  of PEEK. The cylinder drum  3  has a layer  26 , with which it bears on the bearing surface  15 .  
         [0040]    Also at the other end of the cylinder drum  3  corresponding layers of diamond-like carbon are provided. This particularly concerns the pressure plate  16 , which has a layer  27 , which bears on the PEEK cover  19  of the control plate  18 .  
         [0041]    [0041]FIG. 2 shows a similarly designed hydraulic machine  1 ′, in which mainly the support of the holddown plate  9  on the cylinder drum  3  has changed. Same parts have the same reference numbers. Corresponding parts have marked reference numbers.  
         [0042]    Here, the ball  11 ′ is arranged in the holddown plate  9 . The ball  11 ′ bears on an insert  12 ′, which is supported in the cylinder drum  3  under the effect of a spring  21 . The insert  12 ′ is made of a plastics material, particularly PEEK. The ball  11 ′ carries the layer  25 ′ of diamond-like carbon.  
         [0043]    [0043]FIG. 3 shows a further embodiment of a hydraulic machine  1 ″, in which same parts as in FIG. 1 have the same reference numbers and corresponding parts have double marked reference numbers. Here, the cylinder drum  3 ″ no longer bears on the housing  2 . On the contrary, two ends of a shaft  28  are supported in the housing  2 . Further, here the piston  6 , the swashplate  10 , the pressure plate  16  and the ball  11  are provided with a DLC layer in a manner as shown in FIGS.  1  und  2 .