Abstract:
The invention concerns a hydraulic machine with a toothed set comprising a toothed ring with an inner toothing, whose circumferential surface extends in parallel with its axis, and a gear wheel ( 3 ) with an outer toothing, whose circumferential surface ( 24 ) extends in parallel with its axis, at least one tooth flank having an edge as an axial end of a section, which is located between the circumferential surface and a surface directed radially inwards. It is endeavoured to reduce the wear. For this purpose it is ensured that in the radial direction an edge ( 22, 23 ) ends so as to be offset inwards in relation to the circumferential surface ( 24 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Applicant hereby claims foreign priority benefits under U.S.C. § 119 from German Patent Application No. 10 2004 046 934.2 filed on Sep. 28, 2004, the contents of which are incorporated by reference herein.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a hydraulic machine with a toothed set comprising a toothed ring with an inner toothing and a gear wheel with an outer toothing.  
       BACKGROUND OF THE INVENTION  
       [0003]     In a hydraulic machine with a toothed set comprising a toothed ring with an inner toothing, whose circumferential surface extends in parallel with its axis, and a gear wheel with an outer toothing, whose circumferential surface extends in parallel with its axis, at least one tooth flank having an edge as an axial end of a section, which is located between the circumferential surface and a surface directed radially inwards. Such a machine is, for example, known from EP 0 959 248 A2. This machine has clearances or recesses, which can, for example, be made by milling. They are located on both tooth flanks of each tooth of the outer toothing of the gear wheel and are used for a so-called secondary commutation, during which a brief connection to one of the neighbouring pressure chambers is ensured shortly before any pressure chamber reaches a minimum or a maximum volume. At the time of the commutation it is ensured that no connection exists between the pressure chamber in question with maximum or minimum volume, respectively, and the corresponding neighbouring pressure chambers. Thus, a stable operation with low speeds and high pressures should be achieved. At the axial ends of the clearances and at the front edges, the circumferential surfaces extend over edges into end faces of the clearances or the front side of the gear wheel, respectively.  
         [0004]     Such machines now turn out to have problems with wear in some areas of the toothed set.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The invention is based on the task of preventing wear.  
         [0006]     With a hydraulic machine as mentioned in the introduction, this task is solved in that in the radial direction an edge ends so as to be offset inwards in relation to the circumferential surface.  
         [0007]     It is also advantageous that at a clearance is provided in at least one flank, said clearance extending in the axial direction over a predetermined distance, the clearance having edges at its axial ends.  
         [0008]     In this connection the circumferential surface is the surface, which forms the toothing. Thus, in a manner of speaking, the circumferential surface extends in a wave shape around the gear wheel or inside the toothed ring, respectively. It defines teeth and tooth spaces. Initially, its every point extends in parallel with the axis of the gear wheel or toothed ring, respectively. Until now, this circumferential surface has only been interrupted by the clearance. This clearance causes that a connection occurs between neighbouring pressure chambers between the two toothings in certain relative positions of gear wheel and toothed ring. In the known case, the clearance extends through an edge into the circumferential surface, the edge being located exactly in the circumferential surface. It has now been established that in the axial direction the toothing on both sides of the clearance are loaded by an increased pressure. While, between the clearance and the front side of the gear wheel or the toothed ring, respectively, the active force can be distributed on a relatively large area, this is not possible in the vicinity of the clearance. Here, the same force is thus concentrated on a substantially shorter area in the axial direction. It is now assumed that this increased surface pressing is causing the increased wear. According to the invention, it is ensured that the clearance is no longer located in the circumferential surface, but is offset inwards in relation to the circumferential surface. This offsetting can be very small. It causes that the increased surface pressing is removed from the immediate vicinity of the clearance. This has the advantage that the axial end wall of the clearance is only loaded to a smaller extent. Exactly here, however, the increased risk of wear exists.  
         [0009]     Preferably, a permanent transition face is located between the edge and the circumferential surface. Thus, this transition phase has no steps. This has advantageous effects, when introducing the jacking force in the element of the toothed set, which has the toothing provided with a clearance.  
         [0010]     It is particularly advantageous that in the axial direction the transition face has a convex, particularly elliptic, or a straight shape. Both embodiments cause that no additional edge occurs, at which an increased surface pressure could again occur. With a convex shape of the transition face, an “edge” does strictly speaking not exist any more, as here the transition face extends practically in a “rounded” manner into the circumferential surface. In this case, the border between the transition face and the front wall or the end wall, respectively, is regarded as edge.  
         [0011]     Preferably, the edge has, in the axial direction, a distance in the range from 1 to 3 mm to the circumferential surface. Thus, the transition area is extremely short. This has the advantageous effect that “leakages” practically do not exist.  
         [0012]     It is also advantageous that, at least at the largest depth of the clearance, the edge is offset radially inwards in the range from 0.01 to 0.03 mm in relation to the circumferential surface. As mentioned briefly above, the offsetting of the edge radially inwards is extremely small. It has the size of one to a few hundredth of a millimetre. This has the advantage that the compressive stress or the surface load can also extend into the transition area. Otherwise, there is a risk that the wear problem will simply be displaced axially outwards.  
         [0013]     Preferably, a material share between the circumferential surface and the edge is removed by electrolyte-mechanical trimming. This is a relatively simple opportunity of producing a smooth transition between the clearance and the circumferential surface. Only very little material is removed. With the electrolyte-mechanical trimming it is possible to remove this material during the manufacturing of gear wheel or toothed ring, as the electrolyte-mechanical trimming does not take much time. Finally, also grinding tracks or tracks of another purely mechanical material removal are avoided, which could otherwise have an interfering effect on the operation of the hydraulic machine.  
         [0014]     Preferably, the tooth flank has, at least on one front side, an end edge, which ends inwardly offset in the radial direction in relation to the circumferential surface. In fact, the same problem exists at the front side than at the transition between the circumferential surface and the clearance. Therefore, a transition face can also be provided at the front side, said transition face having a convex, particularly an elliptic, or a straight shape. This will also reduce the risk of wear at the control system. In a similar manner than with the clearance, the edge can have a distance in the radial direction of 1 to 3 mm to the circumferential surface and can be offset radially inwards by a few hundredth of a millimetre.  
         [0015]     It is particularly preferred that each of the two front sides has an end edge, which ends offset radially inwards in relation to the circumferential surface. When, thus, both front sides and both axial ends of the clearance do not immediately extend into the circumferential surface by way of an edge, but his edge is slightly offset radially inwards, so that a transition face occurs, the wear problems are drastically reduced. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     In the following, the invention is described on the basis of a preferred embodiment in connection with the drawings, showing:  
         [0017]      FIG. 1  is a longitudinal section through a hydraulic machine;  
         [0018]      FIG. 2  is a section II-II according to  FIG. 1 ;  
         [0019]      FIG. 3  is a perspective view of a gear wheel;  
         [0020]      FIG. 4  is a schematic section along the line IV-IV according to  FIG. 3 ; and  
         [0021]      FIG. 5  is an enlarged view of the detail V in  FIG. 4   b.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     A machine shown in  FIG. 1  has the form of a motor  1 , which has an output shaft  2 . The output shaft  2  is driven by a gear wheel  3 , which has an outer toothing  4  and rotates and orbits in a toothed ring  5 , which has an inner toothing  6  in the form of rolls  7 . The output shaft  2  is connected with the gear wheel  3  via a cardan shaft  8 , which is inserted in an accordingly suitable toothing  9  on the inside of the gear wheel  3 .  
         [0023]     On the side turning away from the cardan shaft  8 , the toothed set consisting of gear wheel  3  and toothed ring  5  is covered by a cover plate  10 . On the opposite side the toothed set is covered by a channel plate  11 , which interacts with a valve plate  12 . The valve plate  12  engages an extension  13  of the output shaft  2 , so that the valve plate  12  rotates synchronously with and in a predetermined angle relation to the gear wheel  3 .  
         [0024]     The channel plate  11  and the valve plate  12  form a valve arrangement, which controls, by means of a connection arrangement  15 , of which merely one connection can be seen in  FIG. 1 , the supply of pressure chambers  14 , which are formed between gear wheel and toothed ring  5  ( FIG. 2 ). The connection arrangement  15  has a high-pressure connection, at which pressurised hydraulic fluid is supplied to the motor, and a low-pressure connection, through which the hydraulic fluid can flow off from the motor.  
         [0025]     In order to ensure the tightness between the valve plate  12  and the channel plate  11 , a balancing plate  16  is provided, which is located on the side of the valve plate  12  facing the channel plate  11 . The balancing plate  16  is loaded in the direction of the valve plate  12  by a pressure spring  17 . This ensures the corresponding tightness between channel plate  11  and valve plate  12  during the start. Later the required force on the valve plate  12  is then ensured by a pressure in a pressure chamber  18 , in which a corresponding oil pressure builds up during operation of the motor.  
         [0026]     By means of a further toothing  19 , the cardan shaft  8  is connected with the output shaft  2 . Neither at the toothing  9  nor at the toothing  19  a play can be completely avoided. Particularly with large loads it is further possible that the cardan shaft  8  is twisted. The sum of these events now contribute to the fact that the supply in the correct position of the individual pressure chambers  14  between the toothed ring  5  and the gear wheel  3  in relation to the valve commutation is no longer ensured in the manner, which would usually be required.  
         [0027]     Problems particularly occur, when the volume of a pressure chamber  14  has reached its maximum value, and when, after passing this maximum value, the pressure chamber starts contracting. In this case it is required that a connection exists between this pressure chamber and the outlet or low-pressure connection. If, however, at this instant, the pressure chamber is connected with the high-pressure connection, pressure surges occur, which have a negative effect on the operation behaviour of the machine. This is particularly the case with low speeds. The same problem occurs, when the volume of the pressure chamber  14  has passed a minimum value and starts expanding. In this case, a connection of this pressure chamber with the high-pressure connection is required. When, however, this pressure chamber is still connected with the low-pressure connection, there is a risk that cavitation may occur. Thus, problems always occur, when a pressure chamber assumes an extreme value of its volume.  
         [0028]     To remedy these problems, the gear wheel  3 , as shown in  FIG. 3 , is pro vided with clearances  20  on the flanks of the teeth  21  forming its outer toothing  4 . In this connection, the clearances  20  are located approximately in the axial centre of the gear wheel  3 . They have an axial extension in the range from 2 to 50% of the axial length of the gear wheel  3 . Their extension in the circumferential direction is described in connection with the  FIGS. 4   a  to  5   b.    
         [0029]     The clearances  20  serve the so-called secondary commutation. They ensure that until a time shortly before reaching a minimum or a maximum volume of any pressure chamber a connection outside the valve arrangement to the respective neighbouring pressure chambers  14  is provided. Depending of the extension of the clearances  20  in the rotation direction it can also be ensured that a short-circuit between neighbouring pressure chambers is generated, when a pressure chamber has an extreme value of its volume.  
         [0030]     In principle, the embodiment of such a machine is known from EP 0 959 248 A2 or DE 102 09 672 B3.  
         [0031]      FIG. 4   a  is a schematic view of a section through the gear wheel  3  in the area of a tooth  21 , or rather, through a flank of the tooth  21 , in which the clearance  20  is located. The section extends through the area of the clearance  20 , in which it has its maximum depth.  
         [0032]     With an edge  22 ,  23 , the clearance  20  extends into the circumferential surface  24  of the gear wheel  3 . Here, the circumferential surface corresponds to the contour line of the gear wheel  3  shown in  FIG. 2 , which has been extruded perpendicularly to the drawing level of  FIG. 2 . In each point in the circumferential direction this circumferential surface  24  extends in parallel to the axis of the gear wheel  3 .  
         [0033]     Also on both front sides of the gear wheel  3  edges  25 ,  26  occur, with which the front sides of the gear wheel  3  extend into the circumferential surface  24 .  
         [0034]     In  FIG. 4   a  a curve  27  shows the course of the compressive stress in the contact areas between the teeth  21  of the gear wheel  3  and the roller set of the toothed ring  5 . It can be seen that the curve  27  of the compressive stress is substantially constant in the axial direction. Merely in the areas of the edges  22 ,  23  compressive stress peaks  28 ,  29  occur. In the area of the front side edges  25 ,  26  smaller compressive stress peaks  30 ,  31  occur.  
         [0035]     In order to avoid these peaks  28  to  31 , the edges  22 ,  23  and  25 ,  26  are slightly offset radially inwards in the embodiment according to  FIG. 4   b . For example between the edge  22  and the circumferential surface  24 , this results in a transition face  32 , which is also shown in  FIG. 3 . With the other edges  23 ,  25 ,  26 , transition faces  33  to  35  occur. These transition faces  32  to  35  are shown excessively large in the  FIGS. 3 and 4   b . Their dimensioning occurs from  FIG. 5 . The transition face  32  in  FIG. 5   a  shows an extension X in the axial direction, which is in the range from 1 to 3 mm. These dimensions apply for the edges  22 ,  23  in the area of the clearances  20 . With the edges  25 ,  26 , the axial extension x can be substantially smaller, for example 10% to 50% of the extension X at the clearance  20 . In the radial direction it has an extension Y, which is in the range from 0.01 to 0.03 mm. The transition face  32  in  FIG. 5   a  has the shape of a straight or plane surface, that is, it is stepless. The transition into the circumferential surface  24  should possibly occur in a rounded manner, that is, without edge, to prevent the generation of new compressive stress peaks. The transition face  32  has a relatively small inclination of approximately 2%. This makes it possible for the compressive stress according to curve  27 ′ in  FIG. 4   b  to extend into the transition face  32 , so that no increased compressive stresses occur on the transition from the transition face  32  into the circumferential face  24 , which compressive stresses could cause wear.  
         [0036]     The transition face  32  according to  FIG. 5   a  can, for example, be made by electrolyte-mechanical trimming. With this working relatively little material is removed. However, the removal of the material takes a relatively short time, so that this working process can easily be integrated in the manufacturing process of the gear wheel  3 .  
         [0037]      FIG. 5   b  shows a modified embodiment of the transition face  32 , which is here concavely curved. The best solution has turned out to be an ellipse-shaped curvature. The dimensioning is similar to that in  FIG. 5   a , that is, the axial extension of the transition face is in the range from 1 to 3 mm and the radial distance Y of the edge  22  from the circumferential surface  24  is in range of a few hundredth millimetres, for example 1/100 mm to 3/100 mm. The transition areas  33 ,  34 ,  35  can of course be made in a corresponding manner, so that the pressure peaks  29  to  31  can reliably be avoided. It has turned out that a gear wheel  3  manufactured in this manner shows substantially less wear than a gear wheel  3  according to FIG.  4   a , in which the edges  22 ,  23 ,  25 ,  26  are practically located in the circumferential surface  24 .  
         [0038]     The distance Y of the edge  22  from the circumferential surface  24  shown in  FIGS. 5   a  and  5   b , refers approximately to the circumferential centre of the clearance  20 . It is obvious that in relation to the circumferential edges of the clearance  20  also smaller distances can be permitted, without interfering with the stress reduction. However, the radial offsetting of the edges  22 ,  23  in relation to the circumferential surface should cover the total width of the clearance  20  in the circumferential direction. The width (in the circumferential direction) of the radially offset edges  25 ,  26  is exactly as large.  
         [0039]     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 present invention.