Abstract:
A hydrostatic machine having an actuating device for adjusting a displacement volume of the hydrostatic machine is provided. The actuating device has a return element for actuating a control valve as a function of a position of the actuating device. A valve unit is further provided for adjusting an actuating pressure of the actuating device, which is activated mechanically by means of the return element.

Description:
BACKGROUND 
     The invention relates to a hydrostatic machine having a control device for adjusting the displacement volume of the hydrostatic machine, wherein the control device comprises a return element for controlling a regulating valve. 
     When adjusting displacement volumes of hydrostatic machines, e.g., axial piston machines, maximum and minimum displacement volumes can generally be adjusted by way of integrated control devices. These maximum and minimum displacement volumes are fixed within an axial piston machine by mechanical stops. The control of minimum or maximum volumes is thus associated with a control of minimum and/or maximum stop regions. The stressed stop regions have a considerable load placed thereon in the case of extremely quick pivoting actions which extend to the stressed mechanical stop regions. These loads result in signs of wear and material fatigue in the components of the control systems located in the flow of force. 
     The loading of the components of the mechanical limitation located in the flow of force could cause the components to become worn or to change such that the operation and thus quality of operation of the control devices—whose component parts also include the minimum or maximum stop regions—becomes impaired. For example, wear on a mechanical stop changes the displacement volume, wherein the movement of an adjusting mechanism is limited by the stop. A control pressure acting in the control device is adjusted by a regulating valve in dependence upon the position of the control device. 
     It is thus the object of the invention to provide a hydrostatic machine in which the hard stopping of components on the mechanical maximum and/or minimum stop regions is prevented and which thus comprises a displacement volume limitation which is not subject to wear. 
     SUMMARY 
     The axial piston machine in accordance with the invention comprises a control device for adjusting the displacement volume. The control device for adjusting the displacement volume comprises a return element and a regulating valve. The return element and the regulating valve co-operate to control a control pressure of the control device. The mechanical co-operation of the return element and the regulating valve adjusts a control pressure acting in the control device. In accordance with the invention, when a maximum or minimum displacement volume is reached, the control pressure is controlled such that hard stopping against a stop region is prevented. In this respect, an additional valve unit on the hydraulic path prevents further adjustment. When a particular position of the adjustment device is reached, the valve unit is actuated by the return element. The valve unit functions such that further adjustment of the control device in the same movement direction is counteracted. 
     It is advantageous to provide the control device with a first control piston which controls the axial piston machine towards the larger displacement volume and on which the return element is disposed. This return element effects a movement with a directional component along the first control piston axis. Thus, in a simple manner the positional and/or positional change information of the first control piston can be supplied mechanically by the return element to the regulating valve axial piston machine [sic]. 
     By orienting a movement plane of the return element and a longitudinal axis of the additional valve unit at least approximately in a parallel manner with respect to each other, the mechanical co-operation of the return element and valve unit for controlling the control pressure of the control device is simplified on a technical level and is configured in a space-saving manner. By way of the preferred guiding of the return element in a groove of the housing of the axial piston machine, undesired directional components of the movement are prevented. Providing the return element with a control sleeve is advantageous in that within this control sleeve additional elements for exerting a pressure-dependent return force can be guided in a displaceable manner. 
     An arm is preferably formed on the control sleeve for actuating the valve unit. A force is mechanically applied to the valve unit by the arm of the control sleeve in order to control said unit. Providing the valve unit with a valve sleeve permits locking of the valve unit in an adjustable position by means of the positive-locking arrangement. This produces a simple way of adjusting the valve unit. An adjustable positive-locking arrangement for the valve sleeve is used to reliably and adjustably position the valve sleeve in relation to the housing in which it is received. By adjusting the position of the positive-locking arrangement, the valve sleeve can be adjusted in terms of its housing position such that the start of actuation of the valve unit is fixed. 
     The mode of action can advantageously be selected as hydraulic limitation of the minimum delivery volume V gmin . In order to achieve the mode of action as hydraulic limitation of the minimum delivery volume V gmin , a valve piston can be disposed in the valve sleeve in a displaceable manner and can be mechanically displaced by the application of force by the arm of the control sleeve of the return element. A radially tapered section is formed on the valve piston and forms an annular gap with the valve sleeve. The pressure medium can flow off via this annular gap towards the contact point between the arm of the control sleeve and the valve piston into a housing tank. A control pressure chamber of the control device is thus relieved and the function as hydraulic limitation of the minimum delivery volume V gmin  or hydraulic zero dead stop is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated in the drawing and is explained in detail with the aid of the following description. In the drawing: 
         FIG. 1  shows a perspective illustration of a section of a first exemplified embodiment of an axial piston machine in accordance with the invention; 
         FIG. 2  shows a partial section of a regulating valve of the axial piston machine of  FIG. 1 ; 
         FIG. 3  shows a second partial section of the valve block of  FIG. 1 ; 
         FIG. 4  shows a perspective illustration of the valve block of  FIGS. 2 and 3 ; 
         FIG. 5  shows a perspective illustration of a partial section of the valve block of  FIGS. 2 and 3 ; 
         FIG. 6  shows a third section of the valve block of  FIGS. 2 and 3 ; 
         FIG. 7  shows a fourth section of the valve block of  FIGS. 2 and 3 ; 
         FIG. 8  shows a fifth section of the valve block of  FIG. 1 ; 
         FIG. 9  shows a further perspective illustration of a section of the valve block of  FIG. 1 ; 
         FIG. 10  shows a further section of the first exemplified embodiment of an axial piston machine in accordance with the invention; 
         FIG. 11  shows a further side view of a section of the first exemplified embodiment of an axial piston machine in accordance with the invention; 
         FIG. 12  shows a front view of the valve block of  FIG. 1 ; and 
         FIG. 13  shows a side view of the front part of the valve block of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a hydrostatic machine in accordance with the invention in the form of an axial piston machine  1  having a pivot-out piston  2  on which a return element  3  is disposed. The pivot-out piston  2  forms a first control piston of a control device and can be pressurised in a control pressure chamber by a control pressure in order to adjust the axial piston machine  1  towards the larger delivery volume. On a control sleeve  4  of the return element  3  there is formed an arm  5  of the control sleeve  4 . The control sleeve  4  and the arm  5  of the control sleeve  4  are guided in a housing groove  19  along a valve axis of a valve unit, which is to be described hereinafter, and a pivot-out piston longitudinal axis. 
     The arm  5  is disposed laterally on the control sleeve  4  and extends in the longitudinal direction thereof. The control sleeve  4  is substantially perpendicular to the first control piston. The arm  5  of the control sleeve  4  is thus formed such that it can apply a force to a movable element of the valve unit  6  in order to co-operate therewith as hydraulic limitation e.g., of a minimum delivery volume V gmin  of the axial piston machine  1 . The valve unit  6  is disposed in a valve block  18  which is attached to the housing of the axial piston machine  1 . The valve unit  6  comprises a valve sleeve  7  and a valve piston  8  which can be displaced therein along the valve axis. The valve piston  8  is influenced by a compression spring  9  towards a rest position, which compression spring is supported on a locking screw  10  at its opposite end. 
     At least one channel  24  is formed in the valve sleeve  7 . The channel  24  connects the valve unit  6  to a second control pressure chamber  28  of the control device, which chamber is connected simultaneously (in parallel) to a regulating valve  15 . A hydraulic force can be applied to a second control piston, which is formed as a pivot-in piston  27 , by pressure acting in the second control pressure chamber  28 . The pressure acting in the second control pressure chamber  28  can be adjusted via the regulating valve X. On the side of the arm  5  of the valve sleeve  4  remote from the valve unit  6 , a roller  13  is mounted in a movable manner and applies, in a variable manner, a force dependent upon the delivery pressure of the axial piston machine  1  to a lever  14  of a hyperbola regulator in order to control the regulating valve  15 . A force, which is proportional to the pressure which influences the control device in the first control pressure chamber towards the maximum delivery volume, is applied, in a variable manner, to the lever  14  of the hyperbola regulator by the roller  13  of the return element  3  at a commonly formed contact point. The position of the common contact point between the roller  13  and the lever  14  of the hyperbola regulator depends upon the position of the pivot-out piston along the pivot-out piston axis and thus upon the adjusted delivery volume. The lever  14  of the hyperbola regulator is part of an angled element  30  which is mounted in a rotational spindle  31  in a rotatable manner. The turning moment produced by the application of force at the contact point between the lever  14  and the roller  13  is proportional to the output of the axial piston machine  1  designed as an axial piston pump. 
       FIG. 2  shows a side view of a partial section of a regulating valve X having a control sleeve  4  guided in a groove  19 ′ of the valve block  18 . The housing groove  19  and the groove  19 ′ are formed such that they guide the return element  3  when the valve block  18  is assembled. The valve sleeve  7  is slid into the valve block  18  so as to be displaceable along the valve axis and is held in a desired position relative to the valve block  18  so as to be adjustable by means of a locking element  12 . The valve piston  8  is disposed in the valve sleeve  7  so as to be displaceable along the valve axis and comprises a control edge  11 . The channel  24  of the valve sleeve  7  is either connected to a chamber  29 , which can be in the form of an annular gap, or is separated therefrom depending upon the position of the control edge  11  of the valve piston  8 . The chamber  29  is connected to the housing tank/leak oil chamber. A connection, through which a flow can pass, between the channel  24  and the housing tank is thus created when the valve unit  6  is actuated. In contrast, in the rest position of the valve unit  6 , which position is given by the compression spring  9 , the connection is separated. The pressure in the second control pressure chamber  28 , used to adjust the axial piston machine  1  towards the minimum delivery volume, is then adjusted solely by the regulating valve  15 . The chamber of the compression spring  9  is connected to the housing tank by a longitudinal and transverse bore in the piston  8 . Pressure equalisation in the chamber of the compression spring  9  is ensured when the piston  8  is moved. 
     On the side of the control edge  11  of the valve piston  8  remote from the locking screw  10 , the chamber  29 , in the form of an annular gap, is formed between the valve piston  8  and the valve sleeve  7 , which gap is connected to the housing tank or a leak oil chamber. The opening of the valve sleeve  7  directed towards the valve block outer side is closed by the locking screw  10  which simultaneously forms a spring bearing for the compression spring  9 . The arm  5  of the control sleeve  4  applies a force along the line III-III in  FIG. 2  to the valve piston  8  via a contact point  16  on the end-side end of the valve piston  8  against the resistance of the compression spring  9  towards the locking screw  10 . When the second control chamber  28  is pressurised via the regulating valve  15 , the second control piston  27  is subjected to a force which exceeds the force of the first control piston  2 . The second adjusting piston  27  moves to the left towards Vg min  and, owing to the resulting movement of the pivot cradle, entrains the first adjusting piston  27  to the right. When the pivot-out piston  2  is adjusted to the right in  FIG. 2  and thus the axial piston machine  1  formed as a hydraulic pump is adjusted towards the smaller delivery volume, the valve unit  6  is actuated by the arm  5  when a particular position V gmin  is reached. The second control pressure chamber, which carries the control pressure which influences the pivot-in piston and which is required for the purposes of adjusting towards the minimum delivery volume, is relieved in the housing tank as a result. Excess force against the first adjusting piston  2  is then no longer applied and further pivoting back is prevented. 
       FIG. 3  illustrates, in a further partial section of the regulating valve X from  FIG. 1 , once again the arm  5  of the control sleeve  4 , the valve sleeve  7 , the channel  24 , the valve piston  8 , the compression spring  9 , the locking screw  10 , the control edge  11  and the contact point  16  between the arm  5  of the control sleeve  4  and the valve piston  8 . The groove  19 ′ corresponds to the housing groove  19  in terms of position and size and guides the control sleeve  4 , either alone or together with the housing groove  19 , along the valve axis  20  by means of the guide sections  23 . In order to prevent jamming of the control sleeve  4 , the groove  19  is designed to be wider. Guiding occurs only in one of the two grooves  19 ,  19 ′. Further valves which are disposed in the valve block  18  can additionally be seen in the sectional plane. Such valves can be e.g., pressure or delivery flow regulating valves of the regulating valve X for regulating the axial piston pump. In addition, a transverse bore can be seen on the end of the valve piston  8  oriented towards the return element  3 . This transverse bore is connected to a relief bore incorporated in the valve piston  8  as a blind bore from the opposite end. Volume equalisation is thus possible in the chamber accommodating the compression spring  9  when the valve piston  8  is moved. 
       FIG. 4  is a perspective view of the regulating valve X. The locking element  12 , which is screwed into the valve block  18  to an adjustable depth, locks the valve sleeve  7  in a positive-locking manner in that a part of the locking element  12 , e.g., a head of a socket head cap screw with minimum clearance, engages into a lateral recess  17  of the valve sleeve  7 . The locking ensures that the valve sleeve  7  assumes a fixedly adjustable position relative to the valve block  18  and is not displaced axially along the valve sleeve  7  or rotated thereabout by any forces or turning moments which originate from operation for example. The locking element  12  is firmly fixed in its position in the valve block  18  using a counternut  32 . 
       FIG. 5  shows a partial section of the perspective outer view of the regulating valve X of  FIG. 4 . 
       FIG. 6  shows a section through the regulating valve X of  FIG. 1 . The co-operation of the return element  3  and the regulating valve  15  can also be seen in this Figure. The return element  3  guides a measuring piston  22  in its control sleeve  4 , which piston is displaceable along the control sleeve longitudinal axis. A guide section  23  formed on the outside of the measuring piston  22  is used for guiding purposes in the control sleeve  4  which in turn is guided in the groove  19 ′ of the valve block  18 . As a result the roller  13  is always aligned in the running direction. The measuring piston  22  applies a force, which is proportional to the delivery pressure of the pump, to the roller  13  of the return element  3  towards the lever  14  of the hyperbola regulator. 
     A section of the regulating valve X along the line VII-VII is illustrated in  FIG. 7 , wherein the valve sleeve  7  and the valve screw  10  of the valve unit  6  can be seen beneath the regulating valve  15 . 
     A further section through the valve block  18  is illustrated in  FIG. 8 . A recess  17  for engagement of the locking element  12  is provided on the valve sleeve  7 . The end of the valve piston  8  remote from the locking screw  10  protrudes from the valve sleeve  7  and contacts the arm  5  of the control sleeve  4  at the contact point  16  between the arm  5  of the control sleeve  4  and valve piston  8  when the minimum delivery volume V gmin  is reached. 
     A perspective partial section through the valve block  18  is once again illustrated in  FIG. 9 . The valve piston  8 , which can be displaced along the valve axis  20 , forms a control edge  11  which is used to connect or separate the channel  24  and the chamber  29  for the flow of pressure medium depending upon the position of the valve piston  8  in the valve sleeve  7 . The arm  5  and the guide sections  23 ′ form a chamber for accommodating the head region of the measuring piston  22  with its guide surfaces  23 . The groove  19 ′ of the valve block  18  guides the control sleeve  4  along the axis  20  in that it prevents rotation of the control sleeve  4  about its longitudinal axis and a movement component of the control sleeve  4  perpendicular to the axis  20 . 
       FIGS. 10 and 11  show in each case a longitudinal section of a first exemplified embodiment of an axial piston machine  1  in accordance with the invention. For improved clarity, not all reference numbers have been provided in  FIGS. 10 and 11 .  FIG. 10  shows a side view of the regulating valve X whereas  FIG. 11  shows a section in a side view of the regulating valve X. The valve block  18  of the regulating valve X is attached to the axial piston machine  1  as in  FIG. 1  such that the valve axis  20  and the pivot-out piston longitudinal axis are in parallel with each other in the same plane. The return element  3  is disposed on the pivot-out piston  2  and is guided in the groove  19 ′ of the valve block  18  and the groove  19  of the housing of the axial piston machine  1 . The return element  3  is approximately perpendicular to the pivot-out piston  2  and is in the same plane as the pivot-out piston longitudinal axis and the valve axis. 
       FIG. 12  illustrates a front view of the valve block  18 . In contrast to the previous Figures with only one locking element  12 , an element  26  is formed or fixed on a second valve sleeve  7 ′ and extends radially outwards from the valve sleeve  7 ′ and co-operates with a first and second locking element  12 ′ and  12 ″. The locking screw  10  is screwed into the valve sleeve  7 ′. The first locking element  12 ′ is used to insert the valve sleeve  7 ′ into the valve block  18  and constitutes an adjustable positive-locking arrangement against displacement of the valve sleeve  7 ′ from the valve block  18 . The second locking element  12 ″ is used as a counterpart to the locking element  12 ′ and prevents displacement of the valve sleeve  7 ′ into the valve block  18  in an adjustable and positive-locking manner. The two locking elements  12 ′ and  12 ″ are designed as screws. The first locking element  12 ′ is screwed into a thread incorporated in the valve block  18 . The first locking element  12 ′ penetrates the element  26 . The lower side of the screw head forms a stop for the element  26 . The second locking element  12 ″, likewise formed as a barrier [sic], is screwed into a thread of the element  26 . The end side of the screw then acts as a stop which is supported on the surface of the valve block  18 . 
       FIG. 13  likewise shows in a second perspective illustration the valve sleeve  7 ′ slid into the valve block  18  and held in its position by means of the locking elements  12 ′ and  12 ″ and the element  26  formed thereon. In this case, the abutment of the element  26  against the screw head and the support of the end side of the screw on the housing can easily be seen. 
     The regulating valve X includes pressure, delivery flow and power regulators which are connected in series. The series connection consisting of the pressure, delivery flow and power regulators is connected at its first end to the tank volume and at its other second end to the control pressure chamber  28  of the pivot-in piston  27 . The pressure, delivery flow and power regulators are formed in each case such that the control pressure chamber  28  of the pivot-in piston  27  is supplied with pressure or the respective other valves towards the tank are relieved. In the illustrated example, the power regulator is formed by the regulating valve  15 . The second end of the series connection of the pressure, delivery flow and power regulators, which is connected to the control pressure chamber  28  of the pivot-in piston  27 , comprises in parallel a connection to the valve unit  6 , via which the pressure in the control pressure chamber  28  is supplied to the channel  24 . As will be described in more detail hereinafter, a connection of the channel  26  towards the tank thus results in the fact that pressure cannot accumulate in the control pressure chamber  28  or relief towards the tank takes place. 
     During operation of the axial piston machine  1 , the described valve unit  6  co-operates with the return element  3 , which is disposed on the pivot-out piston  2  of the control device of the axial piston machine  1 , such that a hydraulic stop for the limitation of the minimum delivery volume V gmin  is achieved. The control pressure chamber  28  on the pivot-in piston  27  of the control device of the axial piston machine  1  is connected to the channel  24  of the valve unit  6 . If it is pressurised by the supply of pressure medium via the regulating valve  15  such that as a result the pivot-out piston  2  and the return element  3  are displaced along the valve axis  20  towards the valve unit  6 , then the arm  5  of the control sleeve  4  of the return element  3  approaches the valve piston  8  of the valve unit  6 . 
     If the arm  5  reaches a position defined by the position of the valve unit  6  in the valve block  18 , then the arm  5  of the control sleeve  4  and the valve piston  8  form a common contact point  16  and the valve piston  8  is displaced by the arm  5  of the control sleeve  4  along the valve axis  20  towards the locking screw  10  against the resistance of the compression spring  9 . This actuation of the valve unit  6  causes the displacement of the valve piston  8  so far towards the locking screw  10  that a gap is produced between the valve sleeve  7  and the control edge  11  of the valve piston  8 , which gap connects the channel  24  to the chamber, in which the contact point  16  is formed, via the control chamber  29  formed as an annular gap. In this manner, pressure medium can flow off from the control pressure chamber  28  of the pivot-in piston  27  via the channel  24  through the annular gap between the valve piston  8  and the valve sleeve  7  and via the chamber, in which the contact point  16  is formed, and the grooves  19  and  19 ′ into the housing tank. The discharge of pressure medium from the control pressure chamber  28  of the pivot-in piston  27  causes a reduction in the control force of the control device towards the minimum delivery volume. Thus, further adjustment is hydrostatically prevented and hard mechanical stopping of components limiting the movement of the control device when the minimum delivery volume is reached is likewise prevented as a result. 
     Alternatively, limitation towards the maximum delivery volume is also feasible. By means of a valve disposed on the other side of the return element  3  and actuated via the return element  3 , the control pressure chamber of the pivot-out piston  2  could be connected to the housing tank when a maximum delivery volume is reached. 
     As a further alternative, the control pressure chamber  28  of the pivot-in piston  27  could be connected to the operating pressure. 
     The invention is not limited to the illustrated exemplified embodiment. On the contrary, combinations of individual features of the exemplified embodiment are also advantageously possible.