Abstract:
The invention concerns a hydraulic machine ( 1 ) with an externally toothed gear wheel ( 2 ) and an internally toothed ring ( 3 ), which is made of a sintered material or of a ceramic material. It is endeavored to provide good operation behaviour at low costs. For this purpose, the toothed ring ( 3 ) is provided with a sealing element ( 7 ) in the area of its outer circumference.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Applicant hereby claims foreign priority benefits under U.S.C. § 119 from German Patent Application No. 10 2007 015 626.1 filed on Mar. 27, 2007, the contents of which are incorporated by reference herein. 
       FIELD OF THE INVENTION 
       [0002]    The invention concerns a hydraulic machine with an externally toothed gear wheel and an internally toothed ring. 
       BACKGROUND OF THE INVENTION 
       [0003]    Such a hydraulic machine is, for example, known from U.S. Pat. No. 3,460,481. Also U.S. Pat. No. 3,506,383 shows a toothed ring of such a hydraulic machine, which is made of a sintered material. 
         [0004]    Using a sintered material or a ceramic material has the advantage that with a relatively small effort a high accuracy can be achieved during manufacturing. In fact, it is merely required to put a powder-shaped material into a mould and, if required, heat it up under pressure to eventually receive the sintered or ceramic material. The accuracy requirements on the mould are severe. As, however, the mould can be used for a plurality of toothed rings, a higher accuracy when manufacturing the mould will not cause a significant increase in the costs of the toothed rings. 
         [0005]    However, sintered material has the disadvantage that it is not completely fluid-tight. Particularly, when higher pressures occur, there is a risk of loosing hydraulic fluid. Therefore, for the toothed ring shown in U.S. Pat. No. 3,460,481, a plastic-impregnated, sintered iron is used to keep the toothed ring tight. Alternatively, the inner side of the toothed ring can be lined with a thin layer of plastic or copper, the layer being anticipated to smooth the surface to keep the frictional losses between the toothed gear wheel and the toothed ring small. Further, this layer is supposed to maintain a hydrodynamic fluid film between the toothed ring and the toothed gear wheel. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention is based on the task of providing good operation behaviour at low costs. 
         [0007]    With a hydraulic machine as mentioned in the introduction, this task is solved in that in the area of its outer circumference the toothed ring is provided with a sealing element. 
         [0008]    Thus, it is accepted that the hydraulic fluid can penetrate the toothed ring. Still, the sintered material or the ceramic material does not have to be tight towards the hydraulic fluid. Also other materials, which are not “tight”, can now be used, for example plastic. The fluid is now retained by the sealing element, so that it cannot penetrate to the outside. Further hydraulic fluid penetrating into the sintered material, the ceramic material or another material of the toothed ring then causes an increase in the static pressure, until this pressure balances with the working pressure in the space between the toothed gear wheel and the toothed ring. Therefore, extensive working of the inner circumference of the toothed ring can be avoided. Still, the machine has good operation behaviour, as the desired accuracy can already be set when shaping the material of the toothed ring. 
         [0009]    Preferably, the sealing element surrounds the toothed ring. This has the advantage that the toothed ring and the sealing element can be manufactured separately. The sealing element is not mounted until the toothed ring has been finished. The mounting is simple, as the circumference of the toothed ring is free to access. 
         [0010]    Preferably, the sealing element has a higher tensile strength than the toothed ring. Thus, further to the sealing, the sealing element gets an additional task. It provides the toothed ring with a larger load capacity, so that the hydraulic machine can also be operated with higher hydraulic pressures. Without countermeasures hydraulic pressures acting inside the toothed ring, that is, between the toothed ring and the toothed gear wheel, would cause an expansion of the toothed ring. In the extreme case, this expansion could cause a damaging of the toothed ring. Also when otherwise the toothed ring remains intact, this expansion may cause that the tightness between the contact points between the toothed ring and the toothed gear wheel does no longer exist to the desired extent. When, now, a sealing element with the higher tensile strength than the toothed ring ensures that this expansion or damaging does not appear, or that the expansion can be kept smaller, the operation behaviour is improved. 
         [0011]    Preferably, the sealing element is, at least partly, made of steel. On the one hand, steel cannot be penetrated by the hydraulic fluid and on the other hand has a relatively high tensile strength, so that with the use of steel the desired properties can be maintained at low costs. A large amount of steel is not required. 
         [0012]    Preferably, the sealing element comprises carbon fibres, glass fibres or plastic fibres. Particularly, these fibres can be enclosed in a plastic matrix. These fibres primarily serve the purpose of increasing the stability of the sealing element against forces acting from the inside of the toothed ring, for example forces, which are caused by high hydraulic pressures. 
         [0013]    Preferably, the sealing element is provided with several layers, which follow each other in the radial direction of the toothed ring. Then each layer can, for example, be made somewhat weaker, so that the total load capacity of the sealing element with regard to stability and/or tightness only occurs by means of the combination of all layers. 
         [0014]    It is particularly preferred that at least two layers have different properties. Then, for example, one layer can be made with a good tightness and another layer can be made with a good stability. Thus, the combination of two different layers can provide the sealing element with different properties. 
         [0015]    Preferably, the sealing element is made as a closed ring. On the one hand, a closed ring has the advantage of being tight on its whole circumference. On the other hand it has the advantage of being able to adopt tension forces in a good manner. 
         [0016]    In an additional or alternative embodiment the sealing element may have a winding. The winding can be made so that it forms a closed ring, which is formed by a plurality of helically arranged layers. Also this provides both a good tightness and a high stability. 
         [0017]    Preferably, the toothed ring has several sections bearing on each other in the axial direction. When a desired displacement cannot be realised with one section, it is possible to achieve the desired displacement by adding several sections. 
         [0018]    It is preferred that the sealing element overlaps at least one abutting joint between two sections. The sealing element is then additionally used to seal the abutting joint. Further, the sealing element can then be used to align the individual sections in relation to each other. As all the sections must have the same shape, it is relatively easy to surround them tightly in common by the sealing element, so that it must merely be ensured that the angle alignment is correct. The individual sections can then be joined by means of an adhesive or the like. It is also possible to press the individual sections together under pressure, which is particularly advantageous, if a sintered material is used for the toothed ring. 
         [0019]    Preferably, the sections have the same axial length. Thus, within certain limits it is possible to select the displacement of hydraulic machines, simply in that several sections are joined in a module-like manner. 
         [0020]    Preferably, the sections are clamped together axially by at least one bolt. Such a screw bolt can easily provide the axial tension force. 
         [0021]    Preferably, the sections are clamped together by several bolts, at least one bolt being guided through a through opening, whose circumferential wall is only partly in contact with the bolt. In other words, parts of the through opening are larger than the cross-section of the bolt. Thus, the through opening can not only be used to guide the bolt through the sections. The through opening can at the same time be used as a channel, through which hydraulic fluid is led, for example to perform certain control tasks. As the toothed ring is manufactured by sintering or ceramic forming, it is possible without problems to provide the through opening with almost any arbitrary contour. 
         [0022]    Preferably, several bolts are provided, the part of the circumference at n bolts being 360°/n. Thus, a coincidence of the positioning between the bolts and the sections in the circumferential direction is avoided. It is also possible to distribute the part of circumference being in contact with a bolt on two or more sections, whereby usually two sections are preferred. 
         [0023]    Preferably, the toothed ring comprises at least one control channel. As the toothed ring is made as a sintered element, or a ceramic element, or of another shapeable material, substantial freedom is, as already mentioned, given for the shaping without influencing the accuracy during manufacturing. A control channel can be used for many purposes, for example, for guiding a hydraulic pressure from one front side of the toothed ring to the other. 
         [0024]    It is also advantageous that a pressure connection ends between the toothed ring and the sealing element. Thus, it can be ensured that in the radial direction the same pressure rules outside and inside the toothed ring. Also this counteracts an expansion of the toothed ring. The sealing element ensures that the hydraulic fluid cannot escape to the outside. 
         [0025]    Preferably, the toothed ring is made as a sintered element or a ceramic element. Alternatively, the toothed ring can be made of a light metal, for example aluminium, or a plastic material, for example by way of moulding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    In the following, the invention is described on the basis of preferred embodiments in connection with the drawings, showing: 
           [0027]      FIG. 1  is a schematic, perspective view of a toothed set of a hydraulic machine. 
           [0028]      FIG. 2  is a perspective view of a toothed ring of a modified embodiment of a hydraulic machine. 
           [0029]      FIG. 3  is a perspective view of a sealing element. 
           [0030]      FIG. 4  is a perspective view of a unit of a toothed ring and a sealing element according to  FIGS. 2 and 3 . 
           [0031]      FIG. 5  is a section of a toothed ring of a modified embodiment with fixing bolt. 
           [0032]      FIG. 6  is a further modified section of a toothed ring with a fixing bolt. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  shows essential parts of a hydraulic machine  1  with an externally toothed gear wheel  2  and an internally toothed ring  3 . The gear wheel  2  is provided with a torque application surface  4  in the form of a multi-spline involute profile. Thus, the gear wheel  2  can be driven, if the hydraulic machine is used as a pump. If the hydraulic machine  1  is used as a motor, the gearwheel  2  can transfer a torque to a connected shaft via this torque application surface  4 . During operation the gear wheel  2  rotates and orbits in relation to the toothed ring  3 . Usually, the toothed ring  3  has a number of teeth  5  that exceeds the number of teeth  6  of the gear wheel  2  by one. 
         [0034]    For reasons of clarity, further elements of the hydraulic machine  1 , which are required for the operation, for example a control valve and the like, are not shown. 
         [0035]    In a hydraulic steering the hydraulic machine  1  can be used as a measuring motor. 
         [0036]    The toothed ring  3  is made of a sintered material or a ceramic material. In both cases, the material of the toothed ring  3 , which is available as a powder before the manufacturing of the toothed ring  3 , is inserted in a press mould. The press mould can be made with a relatively high accuracy, so that after the sintering or the thermal hardening of the ceramic material the toothed ring  3  has the desired, equally high accuracy. The pressed powder is thermally treated. Different temperatures may be required for the sintering or the manufacturing of the ceramic material. For the sintering a pressing power may also be required. Then, the finished toothed ring  3  is removed from the mould and is practically finished. In many cases an additional working is not required. 
         [0037]    Alternatively, the toothed ring  3  can be made of aluminium or of a plastic material, for example by way of moulding. 
         [0038]    However, sintered materials and also some ceramic materials as well as many plastic materials are not completely tight to the hydraulic fluid. If a high hydraulic pressure builds up in the pressure pockets, which are formed between the toothed ring  3  and the gear wheel  2 , there is a risk that the hydraulic fluid can reach the outside through the toothed ring  3 , which is undesirable. 
         [0039]    For this reason, a sealing element  7  is used, which surrounds the whole circumference of the toothed ring  3 . Here, the sealing element  7  is made as a closed ring, which is, for example, made of steel. Steel is tight towards hydraulic fluid and additionally has the advantage that it has a larger tensile strength than the ceramic material or the sintered material, of which the toothed ring  3  is made. Then, further to the sealing, the sealing element  7  assumes the task of improving the stability and thus also the load capacity of the toothed ring  3 . 
         [0040]    Alternatively or additionally, the sealing element can be reinforced by fibres, for example, carbon fibres, glass fibres or plastic fibres, which are enclosed in a plastic matrix. These fibres then preferably extend in the circumferential direction of the toothed ring  3 . The sealing element  7  can also be made of several layers, one layer being, for example, fluid tight and another being particularly tension-proof. This means that at least two layers have different properties. 
         [0041]    If the sealing element  7  is made of steel or another metal, it can be advantageous to make the sealing element  7  as a ring, which is shrunk onto the toothed ring  3 . In other cases it may be advantageous to make the sealing element as a winding, that is, of one or more layers of materials, which are helically wound around the outer circumference of the toothed ring  3 . 
         [0042]    The sealing element  7  should surround the toothed ring  3  as closely as possible. This can be used to give a further effect. Then the toothed ring can namely be made of several sections, which are stacked upon each other in the axial direction. The sealing element  7  then overlaps at least one abutting joint  8 , which is formed between neighbouring sections  9 ,  10  of the toothed ring  3 . As the abutting joint  8  is covered by the sealing element  7 , it is shown with dotted lines in  FIG. 1 . 
         [0043]    The sections  9  and  10  can be connected to each other by means of bolts, not shown in detail, which are guided through through openings  11 . The bolts can tighten the sections  9  and  10  axially against each other. 
         [0044]    All the sections  9 ,  10  can have the same axial extension. Then, a hydraulic machine  1  can be made in a module-like manner, a desired displacement being achieved in dependence of the number of sections  9 ,  10  used. 
         [0045]    Particularly when they are made of a sintered material, the sections  9 , can be inserted in the sealing  7  and pressed against each other by means of a pressure, so that the sections are sintered together. It is also possible to join the sections  9 ,  10  by means of an adhesive, or, if a plastic material is used, to connect them by welding. 
         [0046]    The  FIGS. 2 to 4  show a modified embodiment of a toothed ring  3  with a sealing element  7 , the same elements being provided with the same reference numbers. 
         [0047]    A first modification is that now the through openings  11  are open in the direction of the circumference of the toothed ring  3 . These through openings  11  are then covered by the sealing  7  in the circumferential direction, when the toothed ring  3  and the sealing element  7  have been assembled. This gives an embodiment, in which the through openings  11  have a larger cross-section than the bolts  12 , as can be seen from  FIG. 5 . This will be explained below. 
         [0048]    According to the  FIGS. 2 to 4 , the circumference of the toothed ring  3  further has an axial groove  13 , which is covered by the sealing element  7 . Such an axial groove can be used to form a channel through the toothed ring  3 . Such a channel is, for example, required in an embodiment as shown in DE 102 16 959 B3 or DE 195 11 501 C2. 
         [0049]    Also the through openings  11  can be used as a channel, at least the part of the cross-section, which is not covered by the bolt  12 . 
         [0050]    As the toothed ring  3  is made of a sintered material or of a ceramic material, the design of the toothed ring  3  is relatively free. The through openings  11  can be made so that the toothed ring  3  is only in contact with the bolt  12  on a share of the circumference of the bolt  12 . In  FIG. 5  this share is called “A”. When n bolts  12  are used, the angle A has a size A=360°/n. In this case, a coincidence in the circumferential direction between the bolts  12  and the toothed ring  3  is avoided. The toothed ring  3 , or rather its sections  9 ,  10 , can be mounted stress-free in the circumferential direction. 
         [0051]      FIG. 6  shows a modified embodiment, in which the area, in which the toothed ring  3  is in contact with the bolt  12 , is divided into to partial areas. In this case, each partial area D has a size D=360°/(2 n), if n bolts are used. Also in this case, a coincidence is avoided. 
         [0052]    In both embodiments, the bolt  12  has the described contact face in each through opening; said contact faces in different pressure openings  11  being offset in relation to each other in such a manner that their sum is a circular cylinder. 
         [0053]    Via a pressure connection  14 , which is merely shown schematically, a pressure can be set between the toothed ring  3  and the sealing element  7 , if this is desired. For example, the pump pressure or the tank pressure or an intermediary pressure between the pump pressure and the tank pressure can be set here. 
         [0054]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the pre-sent invention.