Abstract:
A hydraulic additional drive includes a hydraulic or having working ports configured to be connected to a pump via a high pressure branch and a low pressure branch. The hydraulic motor further includes a purging port to which a feed pressure is configured to be applied to set a “free-wheel mode” while the working ports are connected to a tank. The pressure at the purging port is limited to a comparatively low value between the feed pressure and the tank pressure by a pre-charging valve integrated into the hydraulic motor. The pre-charging valve is arranged in a free-wheel duct which hydraulically connects the purging port to one of the working ports.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/005670, filed on Nov. 11, 2011, which claims the benefit of priority to Serial No. DE 10 2010 053 105.7, filed on Dec. 1, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The disclosure relates to a hydrostatic drive. 
     Such hydrostatic drives are used, for example, in the case of commercial vehicles as an additional drive for the front wheels, while the rear axle is driven via a conventional mechanical drivetrain. 
     Such a commercial vehicle with conventional and hydraulic drivetrain is explained, for example, in DE 42 12 983 C2. In the case of this solution, the hydraulic drivetrain can, where necessary, be connected via a valve arrangement, wherein each wheel of the front axle is assigned a hydraulic motor which is supplied with pressurizing medium via a variable displacement pump which can be pivoted across zero in order to drive the front wheels. The known system is furthermore embodied with a retarder via which the brake system of the commercial vehicle are hydraulically assisted during braking. 
     One disadvantage of this solution is that both in the case of the retarder function and in the drive function, the pressurizing medium is heated to a significant extent in the hydraulic drivetrain which forms a closed circuit. A further disadvantage of this solution lies in the fact that, in the case of a disconnected hydraulic drivetrain, the hydraulic motors run along with the wheels of the rear axle and thus increase the fuel consumption as a result of the friction inherent therein. 
     In order to avoid the first disadvantage mentioned above, DE 39 26 354 C2 describes a hydraulic motor which is embodied with a flushing valve in order to remove a proportion of the pressurizing medium from the closed circuit and feed it back to a tank from which the removed quantity is balanced out via a feed pump. In this manner, excessive heating of the pressurizing medium can be reliably avoided. 
     In order to avoid the second disadvantage mentioned above, a vehicle with a conventional mechanical drivetrain and a hydraulic drivetrain is explained in U.S. Pat. No. 6,367,572 B1. Here, a hydraulic motor is assigned to both wheels of the front axle and is of a radial piston design. Such a radial piston machine has a plurality of pistons supported on a lifting ring, which plurality delimits a working chamber, wherein the working chambers are consecutively connected to high pressure and low pressure in order to drive the hydraulic motor. When disconnecting the hydraulic drivetrain, it is moved into a “freewheel mode” in the case of which tank pressure or a comparatively low pressure acts on the working chambers, while a feed pressure or another pressure which is greater than the tank pressure acts on the lifting ring—or on the housing side. The differential pressure which results from the higher housing pressure brings about a “retraction” of the pistons so that they lift off from the lifting ring and thus the friction is reduced in the case of a disconnected hydraulic drivetrain. In the case of adjustment of the pressure difference via the piston, it must be ensured that the pressure difference which is active in the lifting-off direction is so large that it holds the pistons in the lifting-off position counter to the centrifugal forces which are active during rotation. 
     It is disadvantageous in the case of this solution on the one hand that a comparatively high pressure is active in the housing if the hydraulic drivetrain is moved into the freewheel mode. Moreover, in the case of this known solution, the pressurizing medium can heat up since in turn no housing flushing is provided. 
     Against this background, the object of the disclosure is to create a hydrostatic drive which enables operation in the “freewheel mode” with minimal effort and in the case of which heating of pressurizing medium can be largely avoided. 
     This object is achieved by a hydrostatic drive with the features of the disclosure. 
     Advantageous further developments of the disclosure are the subject matter of the subordinate claims. 
     SUMMARY 
     The hydrostatic drive according to the disclosure has at least one hydraulic motor, the working connections of which formed on a motor housing can be connected via a high pressure and a low pressure branch to a pump and the pistons of which can be acted upon on one hand by a housing pressure and on the other hand with low pressure or with pump pressure. On the housing side, a flushing connection which can be connected to a tank is provided. Moreover, the hydrostatic drive has a feed line for conveying pressurizing medium to the low-pressure side and a valve arrangement via which tank pressure can act on the two working connections and a feed pressure can act on the flushing connection via a feed pressure line in order to set a freewheel mode. According to the disclosure, the motor housing is thus embodied with three connections, the two working connections and the flushing connection. A freewheel passage is formed in the motor housing, which passage connects the flushing connection and one of the working passages connected to one of the working connections and in which a counterbalance valve which opens towards this connection is arranged, which counterbalance valve opens in the case of a substantially lower pressure than the feed pressure. 
     On the hydraulic motor side, only three connections must thus be present. The housing flushing during the “freewheel mode” is carried out via the freewheel passage and the counterbalance valve so that heating of the pressurizing medium can be prevented. 
     The solution according to the disclosure is accordingly of an extremely compact design and makes it possible to flush the motor housing, operate the hydraulic motor in the freewheel mode and furthermore restrict the number of incoming and outgoing lines to a minimum, concretely three line portions. 
     In the case of one exemplary embodiment of the disclosure, a throttle for restricting the flow of pressurizing medium from the feed line to the feed pressure line is provided in the feed line. 
     The counterbalance valve has a particularly simple design if it is embodied as a spring-pretensioned non-return valve, wherein the spring rate of the spring defines the pretensioning. 
     The valve arrangement has, in the case of one exemplary embodiment, a sequence valve which, in one position, connects the two working connections to the tank and, in another position, connects the working connections to the high pressure or low pressure side of the pump. 
     The sequence valve can be assigned a shift valve which acts on a control face which is active in one direction of the second position of the sequence valve, in one position, with tank pressure and, in the other position, with feed pressure in order to move the sequence valve. The shift valve connects, in the first position, an outlet connection of the sequence valve to a connection line which itself can be connected to the tank or the feed pressure line. 
     In the case of one variant of the disclosure, the sequence valve is embodied as a 5/2-way valve with an outlet connection connected to low pressure or an inlet connection connected to high pressure, two consumer connections and a tank connection which can be connected to the tank. 
     The shift valve can be embodied as a 4/2-way valve which, in a basic position, connects the control face to the tank and the tank connection of the sequence valve to the connection line and, in one shift position, connects the tank connection to the tank and the control face to the connection line. 
     The drive can be embodied with an activation valve which, in one position, connects the feed pressure line to the tank and, in a different position, connects the feed pressure line to the feed line and the connection line to the tank. 
     It is preferable to embody the activation valve as a 4/2-way valve which is pretensioned into one position and can be switched into the other position. 
     In the case of one variant of the disclosure, the drive has a crossover valve which, in one position, connects the low pressure branch to the high pressure branch and, in a different shift position, blocks this connection. 
     Filling of the hydraulic motor is particularly fast if the feed pressure or low pressure lies significantly above 10 bar, while the spring pretensioning of the non-return valve is significantly below this but greater than the tank pressure. 
     The hydraulic motor of the drive is preferably embodied with a flushing valve integrated into the motor housing, which flushing valve can be moved into an opening position by a pressure difference between the high pressure and the low pressure branch in order to transfer a connection from the outlet-side working connection to the flushing connection and downstream of which a pressure retention valve with a nozzle is connected, which pressure retention valve can be moved into an opening position when a threshold pressure in the low pressure branch is exceeded. 
     The flushing valve can be embodied, for example, as a 3/3-way valve with a spring-pretensioned basic position and two shift positions in which in each case one of the working passages is connected to the flushing connection. 
     In the case of one exemplary embodiment, the drive is embodied as a traction drive for a vehicle axle of a commercial vehicle, wherein a hydraulic motor is assigned to each wheel of this axle. The other axle can here be driven with a conventional mechanical drive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred exemplary embodiment of the disclosure is explained in greater detail below on the basis of schematic drawings. In these drawings: 
         FIG. 1  shows a highly simplified schematic diagram of a commercial vehicle with a hydrostatic additional drive according to the disclosure; 
         FIG. 2  shows a circuit diagram of the hydrostatic additional drive according to  FIG. 1 ; 
         FIG. 3  shows an enlarged representation of a pump assembly and 
         FIG. 4  shows an enlarged representation of a hydraulic motor of the additional drive from  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a highly schematic representation of a truck  1 , rear wheels  2  of which are driven via a conventional mechanical drivetrain with internal combustion engine  4 , transmission  6 , cardan shaft  8  and differential, etc. Truck  1  is embodied with a hydraulic additional drive  10  which can optionally be activated, for example, on heavy terrain. Said hydraulic additional drive  10  has for each of front wheels  12  a hydraulic motor unit  14  which is supplied with pressurizing medium via a pump  16  driven by internal combustion engine  4 . 
     According to the enlarged partial cutout of hydraulic motor unit  14 , this is embodied as an inverse radial piston driving mechanism, wherein a plurality of pistons  18  are supported on a lifting ring  20 . 
     The pistons are guided radially movably in cylinder bores of a cylinder drum  22  and delimit in each case a working chamber  24 , wherein this plurality of working chambers are consecutively connected to high pressure and low pressure so that, as a result of the resultant piston stroke, cylinder drum  22  rotates, wherein the pistons slide via piston shoes  26  on lifting ring  20 . Cylinder drum  22  is connected in a rotationally conjoint manner to a drive shaft  28  which in practice forms the wheel axle of respective front wheel  12 . Hydraulic motor unit  14  forms a type of “wheel bearing” of the respective wheel. 
     In the position shown, the pistons abut against lifting ring  20  so that significant friction losses occur in the case of an unactuated additional drive  10  and an “idly” running hydraulic motor unit  14 . In order to minimize these friction losses, hydraulic motor unit  14  is operated in the freewheel mode described above. To this end, a motor housing  30  which bears lifting ring  20  is acted upon with a pressure explained in greater detail below, for example, a feed pressure, while the working chambers are acted upon with the tank pressure or another lower pressure so that, as a result of the pressure difference, the pistons are retracted in the direction of drive shaft  28  and thus lift off from lifting ring  20  the friction losses are correspondingly significantly reduced. 
       FIG. 2  shows the circuit diagram of hydrostatic additional drive  10  from  FIG. 1 . One can see both front wheels  12  which are driven in each case via a hydraulic motor unit  14   a ,  14   b . The supply of pressurizing medium is carried out via pump assembly  16  which is hydraulically operatively connected via a valve arrangement  32  to hydraulic motor units  14   a ,  14   b.    
       FIG. 3  shows an enlarged representation of pump assembly  16  and valve arrangement  32 . 
     Pump assembly  16  has a variable displacement pump  34  which is pivotable across zero and is driven by internal combustion engine  4 . There sits on the same drive shaft  36  a feed pump  39  via which pressurizing medium is sucked out of a tank T and can be supplied with a feed pressure of, for example, 20 to 30 bar into a low pressure branch of additional drive  10 . In the case of the following explanations, it is assumed that a pressure line at the top in  FIG. 3  is a low pressure line  38 , while a different pressure line connected to a connection of variable displacement pump  34  should be high pressure line  40 . Depending on the actuation of variable displacement pump  34 , the HP and LP branch can change. 
     The adjustment of the pivot angle of variable displacement pump  34  is carried out by means of an actuating cylinder  42 , the actuating piston of which is connected to a pump control valve  44  which is embodied as a proportional valve actuated via a control electronic unit. Pump assembly  16  is connected via control oil connections X 1 , X 2  to a control oil supply. 
     Such pressure and conveyance flow control systems for variable displacement pumps  34  are known from the prior art so a detailed explanation can be omitted. In the case of such a pump control valve  44 , the setpoint value is specified as an electric variable and, as a function of the actuation of pump control valve  44 , adjusts an actuating piston of actuating cylinder  42 , wherein the return of the actual position of the actuating piston can be carried out mechanically—as in the present case—or, however, also electrically. The position of the actuating piston is then compared via the control electronic unit with the specified setpoint value which corresponds to a specific conveyance volumetric flow and the actuating piston is adjusted until the setpoint and actual value correspond and thus the required conveyance volumetric flow is set. According to the explanations above, the pressurizing medium is conveyed into high pressure line and flows back from the consumer, in the present case from hydraulic motor units  14   a ,  14   b  in a closed circuit via low pressure line  38  to the low pressure connection of variable displacement pump  34 . In terms of further details of pump control, reference is made, for example, to DE 10 2004 061 861 B4. 
     Pump assembly  16  furthermore has two feed valves  46 ,  48  via which pressurizing medium can be fed into the respective low pressure branch. Each of these feed valves has in a manner known per se a non-return valve  50  which opens towards in each case a line  38 ,  40 , a pressure limiting valve  52  being connected parallel to valve  50 , which valve  52  transfers a pressurizing medium connection to the in each case other pressure line in the case of a predetermined pressure being exceeded in assigned pressure line  38 ,  40 . In terms of the concrete structure of such feed valves  46 ,  48 , reference is also made to DE 10 2004 061 861 B4. 
     Both input connections of feed valves  46 ,  48  open out in a feed passage  54  which leads to a filter unit  56 , the input connection of which is connected to the pressure connection of feed pump  39 . A suction connection of feed pump  39  is connected via a tank connection S and via a suction line  58  to tank T. In the case of actuated feed pump  39 , pressurizing medium is conveyed with the feed pressure (20 to 30 bar) through filter unit  56  and feed passage  54  to the inlet of both feed valves  46 ,  48 . Low-pressure side non-return valve  50  then opens so that pressurizing medium is conveyed into the corresponding low pressure branch. The pressure in the high pressure branch is limited via respective pressure limiting valve  52  of feed valve  46 ,  48  so that the pressure can be reduced towards the low pressure side in the event of this maximum pressure being exceeded. 
     A feed pressure limiting valve  60  is provided in the feed passage  54  in order to limit the feed pressure to the 20 to 30 bar described above. 
     Pump assembly  16  furthermore has a pressure cut-off valve unit  62 , via which the pressure in the high pressure branch is tapped by means of a shuttle valve  64  and is guided to a control face of a pressure cut-off valve  66 , which, in the event of a predetermined maximum pressure being exceeded, connects a control line  70  to tank pressure so that variable displacement pump  34  is adjusted in the direction of a lower conveyance volumetric flow. In terms of further details of such a pressure cut-off valve unit  62 , reference is made to DE 10 2004 061 861 B4 which has already been mentioned. 
     Pump assembly  16  has two working connections A, B which are connected via working lines  72 ,  74  to valve arrangement  32  explained in greater detail below and to hydraulic motor units  14   a ,  14   b . Pump assembly  16  furthermore has a feed connection G which is connected via an internal passage to feed passage  54  and to which a feed line  76  with a throttle  78  is connected. A leakage connection T 1  is connected via a leakage line  80  to a tank line  82  which leads to tank T and in which a further filter  84  is arranged. 
     According to  FIG. 3 , both working connections  72 ,  74  are connected via a bypass line  86  in which a crossover valve  88  is arranged. This is embodied as an electrically adjustable 2/2-way seat valve which, in a spring-pretensioned basic position, opens bypass line and thus connects both working lines  72 ,  74  and which can be moved into a blocking position by energizing an electromagnet. 
     Both working lines  72 ,  74  are connected to connections of a sequence valve  90 ; these two connections are provided below with the designations S, P. 
     Sequence valve  90  is embodied as a 5/2-way valve and is pretensioned via a spring into a basic position (a) in which a tank connection T is connected to two output connections A, B. Both connections S, P are blocked. 
     By subjecting a control line  92  to a control pressure, sequence valve  90  can be moved into a shift position (b) in which tank connection T is blocked off and connections S, A and P, B are connected to one another. 
     Connection T is connected via a line  94  to a connection C of a shift valve  96 , to whose control connection X control line  92  is connected. Shift valve  96  embodied as a 4/2-way valve furthermore has a tank connection connected to tank T and a connection P which is connected via a connection line  98  to a connection P of an activation valve  100 . Shift valve  96  is pretensioned via a spring into a represented basic position in which connections X, T and C, P are connected to one another. Control line  92  and thus assigned control face of sequence valve  90  in this basic position are correspondingly relieved of pressure towards tank T. Both working connections A, B of sequence valve  90  are connected to connection line  98  in which—as stated in greater detail below —either tank pressure or feed pressure is present. 
     By switching of shift valve  96  counter to the force of the spring, connection P of shift valve  96  is connected to control connection X so that the pressure in connection line  98  is active in control line  92 . In the event that the feed pressure then acts in connection line  98 , sequence valve  90  is moved into its shift position (b). During switching of shift valve  96 , connection C is furthermore connected to tank connection T so that tank pressure acts in line  94 . 
     Activation valve  100  is likewise embodied as a 4/2-way valve and has a working connection D connected to a feed pressure line  102 , a tank connection T and a feed pressure connection Q. Tank connection T is connected to tank line  82 , while feed pressure connection Q is connected to feed line  76 . Activation valve  100  connects, in its represented spring-pretensioned basic position, pressure connection P to tank connection T so that connection line  98  is connected to tank line  82 . In this basic position, connections Q, D are furthermore connected to one another so that a pressurizing medium connection is present between feed line  76  and feed pressure line  102 . Activation valve  100  is switched by energizing a shift magnet. In this shift position, connections T, D and Q, P are connected to one another. Connection line  98  is then correspondingly connected to feed line  76  and feed pressure line  102  is connected to tank line  82 . 
     According to the circuit diagram in  FIG. 2 , consumer lines  104 ,  106  are connected to both working connections A, B of sequence valve  90 , which consumer lines  104 ,  106  are in each case branched and are connected to working connections A or B of hydraulic motor units  14   a ,  14   b . Feed pressure line  102  is likewise branched and is connected to in each case a flushing connection T of both hydraulic motors  14   a ,  14   b.    
     The structure of hydraulic motor units  14   a ,  14   b  is explained with reference to  FIG. 4  which shows an enlarged representation of hydraulic motor unit  14   b . The structure of other hydraulic motor unit  14   a  corresponds with this. 
     Hydraulic motor unit  14  has a hydraulic motor  108  which could be embodied as an adjustable hydraulic machine and—this exemplary embodiment is, however, not represented—could also act as a pump during braking operation in order to recover the braking energy. 
     Both connections A, B of hydraulic pump unit  14  are connected via in each case one working passage  110 ,  112  to inlet connection A or outlet connection B of hydraulic motor  108 . The latter furthermore has a connection S which is connected via a flushing passage  114  to flushing connection T. As explained above with reference to  FIG. 1 , said flushing passage  114  opens out on the housing side so that pistons  18  are subjected to the pressure in flushing passage  114  in the retraction direction. Pump housing  30  is indicated by a dashed line in  FIG. 4 . 
     A freewheel passage  116  which connects flushing connection T within pump housing  30  to one of working passages  110 ,  112 , in the present case working passage  110 , branches off from flushing connection T or from flushing passage  114 . There is provided in freewheel passage  116  a spring-pretensioned non-return valve  118 , the spring of which corresponds to a pressure equivalent of approximately 3 bar and which opens towards working passage  110 . For opening of non-return valve  118 , the pressure which is active upstream of non-return valve  118  must be approximately 3 bar higher than the pressure which acts on the rear side of non-return valve  118 . 
     A passage  120 ,  122 , which is guided to two inlet connections P, P′ of a flushing valve  124 , furthermore branches off in each case from both working passages  110 ,  112 . 
     This is embodied as a 3/3-way valve and has an outlet connection C to which a connection passage  126  is connected which opens out into freewheel passage  116  upstream of non-return valve  118 . 
     Flushing valve  124  has a spring-centered central position ( 0 ) in which the three connections P, P′ and C are blocked off. The pressure in respective passages  120 ,  122  is tapped via control passages  128 ,  130  and guided to two opposite control faces of flushing valve  124  so that flushing valve  124  is moved in the direction of a control pressure shift position (a) or (b) depending on the control pressure difference which is present. In shift position (a), connection P′ is connected to outlet connection C, while in shift position (b) connection P is connected to outlet connection C. A throttle  132  is provided in connection passage  126 . A pressure retention valve  134 , which is pretensioned via a spring into a blocking position and can be moved by the pressure upstream of throttle  132  into an opening position, is arranged downstream of said throttle  132 . 
     Said pressure retention valve  134  accordingly only opens if the pressure in connection passage  126  and thus the pressure difference across both working passages  110 ,  112  has exceeded a threshold value which corresponds to the spring of pressure retention valve  134 . The fundamental structure of such a flushing valve is explained in EP 1 443 220 B1, wherein in the case of this variant yet another pressure switch-off valve is provided. 
       FIGS. 2 ,  3  and  4  show the respective elements of hydrostatic additional drive  10  in the free-wheel mode, in which, as explained, pistons  18  are lifted off from lifting ring  120 . 
     To this end, the valves of valve arrangement  32  are located in each case in their spring-pretensioned basic position. Both working connections A, B of sequence valve  90  are accordingly connected via its tank connection T, shift valve  96  and activation valve  100  to tank T. Working lines  72 ,  74  are furthermore connected to one another via crossover valve  88  and are thus hydraulically bypassed. In the basic position, activation valve  100  furthermore connects feed line  76  embodied with throttle  78  to feed pressure line  102  connected to flushing connection P of pump housing  30  so that the feed pressure correspondingly acts on flushing connection T and thus in flushing passage  114  and actuates the pistons in the retraction direction. As explained above, working passages  110 ,  112  are relieved of load towards tank T so that the tank pressure is present at the rear side of non-return valve  118 . The pressure upstream of non-return valve  118  is correspondingly limited to 3 bar or more precisely to the pressure which corresponds to the pressure equivalent of the spring of non-return valve  118 . The pressure difference which is set via piston  18  is sufficient in order to hold these in their retracted position counter to the centrifugal force. In this freewheel mode, drive shaft  36  is continuously driven so that feed pressure is correspondingly also available. Variable displacement pump  34  is retracted here. 
     In order to switch on additional drive  16 , activation valve  100  is initially switched into its shift position in which connections D, T and Q and P are connected to one another. The feed pressure is correspondingly built up in working passages A, B since these are connected via sequence valve  90  and shift valve  96  as well as switched activation valve  100  to feed line  76 . 
     As a result of the switching of activation valve  100 , feed pressure line  102  is furthermore connected to tank line  82  so that the housing pressure is relieved towards the tank and pistons  18  abut against lifting ring  20 . Both working lines  72 ,  74  are furthermore hydraulically bypassed via crossover valve  88 . This switching on can also be carried out during travel of the traction drive. 
     In a following sequence, the pivot angle of variable displacement pump  34  is then set in such a manner that the conveying flow of the pump corresponds to the displacement flow of both hydraulic motor units  14   a ,  14   b  plus a predetermined displacement flow difference. 
     As a result, shift valve  96  is then switched so that control line  92  is connected to feed line  76  and thus the feed pressure acts on sequence valve  90  in the switching direction. Sequence valve  90  correspondingly switches so that both working lines  72 ,  74  are connected to consumer lines  104 ,  106  and working passages  110 ,  112  and thus the hydraulic motors are connected to the hydraulic network of the pump. The ultimate switching on is then carried out by switching of bypass valve  88  into its blocking position. 
     In order to switch off the hydrostatic drive, valves  88 ,  96 / 90  and  100  are correspondingly de-energized in this sequence and variable displacement pump  34  is pivoted back so that the “freewheel mode” is in turn set. 
     A hydraulic additional drive with a hydraulic motor, the working connections of which can be connected via a high pressure branch and a low pressure branch to a pump, is disclosed. The hydraulic motor furthermore has a flushing connection, on which a feed pressure can act for setting of a “freewheel mode”, while the working connections are connected to a tank. The pressure at flushing connection is limited via a counterbalance valve integrated into the hydraulic motor to a comparatively low value which lies between the feed pressure and the tank pressure. The counterbalance valve is arranged in a freewheel passage which hydraulically connects the flushing connection to one of the working connections.