Abstract:
The invention relates to a drive with an energy recovery and retarder function. The drive ( 1 ) comprises a hydrostatic piston engine ( 12 ) which is connected to an accumulator ( 16 ) for storing pressure energy and to a pressure limiting valve ( 19 ) for generating a braking action. Arranged downstream of the pressure limiting valve ( 19 ) is a cooler ( 45 ). The drive ( 1 ) also comprises a hydraulic motor ( 35 ) for driving a cooler fan ( 33 ), wherein the hydraulic motor ( 35 ) is acted on with a pressure medium which is delivered by the hydrostatic piston engine ( 12 ).

Description:
BACKGROUND 
     The invention relates to a drive having an energy recovery and retarder function. 
     In utility vehicles, such as, for example, construction site vehicles or fork lift trucks, the vehicle is often accelerated and subsequently braked again during working operation. In addition to the primary drive, working hydraulic units are provided in most cases and are operated by means of an additional hydrostatic piston engine. For the driving operation, these hydrostatic piston engines can be connected to the drive and consequently to the primary energy source of the drive. In order to store the energy which is released during the braking of either an operating device or the travel drive, it is known from DE 32 47 335 C2 to connect a store to the hydrostatic piston engine. By charging the store, kinetic energy is converted into pressure energy. To this end, the hydrostatic piston engine draws pressure medium from a pressure medium reservoir and conveys it into the store with the pressure being increased. Owing to the increasing pressure as the pressure medium is conveyed, the vehicle or the operating device is subjected to a braking effect. In addition, it is possible to use a pressure limitation valve in order to brake the operating device or the vehicle. The pressure medium conveyed by the hydrostatic piston engine is depressurised via the pressure limitation valve. 
     The drive known from DE 32 47 335 C2 has the disadvantage that, for example, in the event of a relatively long hill start, the capacity of the store is reached. Further braking of the vehicle or only maintaining the selected travel speed by conveying pressure medium counter to the pressure in the store then becomes impossible. In order to achieve an adequate braking effect, the corresponding kinetic energy at the pressure limitation valve must consequently be converted into heat. This leads to a significant rise in the temperature of the pressure medium. 
     SUMMARY 
     The object of the invention is to provide a drive having an energy recovery and retarder function, wherein the retarder function is improved so that it is also possible to make use of a braking effect for a long period of time. 
     The drive according to an aspect of the invention comprises a hydrostatic piston engine which is connected to a store in order to store pressure energy. Furthermore, the hydrostatic piston engine is connected to a pressure limitation valve in order to produce a braking effect. In order to improve the retarder function, a cooler is provided downstream of the pressure limitation valve. The cooling performance of this cooler is increased using a cooler fan. In order to drive the cooler fan, a hydraulic motor is provided which is also acted on with the pressure medium conveyed by the hydrostatic piston engine. 
     On the drive proposed, it is advantageous for energy which is freely available to be used to drive the hydraulic motor. An additional driving operation, for example, using an electric motor, can consequently be dispensed with. The energy for driving the hydraulic motor is produced in each case by means of the hydrostatic piston engine. Each time a braking operation is initiated, there is consequently at the same time sufficient energy provided to drive the hydraulic motor. 
     Advantageous developments of the drive according to the invention are set out in the subsidiary claims. 
     In particular, a simple configuration of the drive is produced if the cooler is connected to the pressure limitation valve by means of a discharge line. Furthermore, a pretensioned non-return valve is arranged in the discharge line. An additional pressure level is therefore provided upstream of the pretensioned non-return valve and is above the pressure level of the completely depressurised tank volume. In a simple configuration, the hydraulic motor may be directly connected to the discharge line upstream of the non-return valve. By acting on the inlet connection of the hydraulic motor with the pressure of the discharge line upstream of the pretensioned non-return valve, the hydraulic motor is always driven when a braking effect is produced by means of the pressure limitation valve. However, if the pressure limitation valve closes because a braking effect is not required, the cooler fan also automatically stops. It is therefore not necessary to have a separate control for the cooler speed or to switch the cooler fan drive on or off. 
     According to another preferred embodiment, the hydrostatic piston engine, which draws pressure medium from the tank in order to produce the braking effect, is connected to a supply line. The hydrostatic piston engine conveys the pressure medium into the supply line by means of which the store and the pressure limitation valve are connected. The hydraulic motor can preferably be connected to the supply line. Owing to the possibility of connecting the hydraulic motor to the supply line, a higher input pressure is provided for driving the hydraulic motor so that hydraulic motors having a higher power level can also be used. A toothed wheel motor is preferably used as a hydraulic motor. 
     In order to connect the hydraulic motor to the supply line, a releasable non-return valve is preferably provided. The releasable non-return valve ensures that the hydraulic motor is not automatically switched on by producing a supply pressure in the supply line by means of the hydrostatic piston engine. Instead, the time at which the releasable non-return valve is released is independent of a pressure build-up using the piston engine. 
     The release function is preferably initiated by a pressure present in the discharge line downstream of the pressure limitation valve. To this end, the releasable non-return valve is acted on with a pressure present in a discharge line which is arranged downstream of the pressure limitation valve. The release function of the releasable non-return valve is automatically actuated when a braking effect is produced by the pressure limitation valve. In this instance, downstream of the pressure limitation valve, the pressure in the discharge line increases and actuates the releasable non-return valve. 
     In order to provide increased pressure for actuating the releasable non-return valve, a pretensioned non-return valve is preferably arranged in the discharge line downstream of the pressure limitation valve. Owing to the pretensioned non-return valve, the pressure in the discharge line that is provided to actuate the releasable non-return valve can be increased via the pressure which is produced owing to the cooler which is arranged in the discharge line. 
     In order to be able to make provision for a depressurisation of the discharge line when the releasable non-return valve closes again, a throttle location is provided in addition to the pretensioned non-return valve and is arranged parallel with the pretensioned non-return valve. If the releasable non-return valve closes and the pretensioned non-return valve which is provided in the discharge line also closes owing to the resilient load thereof, the remaining pressure in the discharge line is depressurised into the tank volume via the throttle location. 
     Furthermore, it is advantageous to provide, parallel with the hydraulic motor, a non-return valve which opens in the direction of an upstream connection of the hydraulic motor. Downstream of the hydraulic motor, a throttle is additionally arranged. The combination comprising the throttle which is arranged downstream of the hydraulic motor and the non-return valve which is arranged parallel with the hydraulic motor prevents the formation of a cavitation when the flow of pressure medium to the inlet connection of the hydraulic motor is abruptly switched off as the braking power is decreased. Owing to the provision of the throttle which is arranged downstream, a small amount of pressure is built up downstream of the hydraulic motor and then opens the non-return valve in the direction towards the inlet side of the hydraulic motor. Pressure medium is consequently supplied to the inlet side via the non-return valve which is arranged parallel with the hydraulic motor and the formation of a cavitation is prevented. 
     The hydrostatic piston engine is furthermore preferably connected to a drive train of the drive by means of a gear stage. The provision of the gear stage as a connection between the hydrostatic piston engine and the drive train allows even low speeds, such as, for example, those on an output shaft of a travel drive, to be transmitted in such a manner that the hydrostatic piston engine operates in an efficiency range which is advantageous for it. 
     Furthermore, it is advantageous to connect the gear stage to the drive train by means of a decoupler. The provision of such a decoupler allows the energy recovery function or retarder function which is not required to be completely switched off, for example, during transit. In this instance, the efficiency level of the entire drive is increased since a carrying action or churning losses in respect of the hydrostatic piston engine do not occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantageous configurations of the drive according to the invention are illustrated in the drawings and are explained in greater detail in the following description: 
         FIG. 1  is a first preferred embodiment of a drive according to the invention, and 
         FIG. 2  is a second embodiment having an alternative connection of the hydraulic motor. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       FIG. 1  schematically illustrates a drive  1  according to the invention. The drive  1  according to the invention comprises a primary drive source which is schematically constructed as a drive motor  2  in the embodiment illustrated. The drive motor  2  is, for example, an internal combustion engine of a utility vehicle. The drive motor  2  is connected to a hydraulic pump  4  by means of a drive shaft  3 . The hydraulic pump  4  is preferably constructed for conveying in two directions and can be adjusted in terms of the supply volume thereof. A motor  5  is connected to the hydraulic pump  4  and can be adjusted in terms of the absorption volume thereof. The hydraulic pump  4  and the hydrostatic motor  5  are, for example, hydrostatic axial piston engine of the construction type involving inclined plates or inclined shafts. 
     The pressure medium conveyed by the hydraulic pump  4  in accordance with the selected supply direction in a first operating line  6  or a second operating line  7  flows through the motor  5  and produces an output torque at the ends  8  and  8 ′ of an output shaft. The output shaft may be constructed, for example, as a continuous output shaft which extends through the motor  5 . The end  8  of the output shaft is, for example, connected to a travel drive of a fork lift truck or a construction vehicle. The output shaft may also be connected to a load type gear which is arranged downstream or an additional switching gear. 
     The end of the  8 ′ output shaft is connected to a gear stage  10  by means of a decoupler  9 . By means of the decoupler  9 , the torque produced by the motor  5  can be transferred to the gear stage  10 . It is insignificant whether the motor  5  is driven by the hydraulic pump  4  during a normal travel operation or the torque is instead produced at the end  8  of the output shaft during overrun operation of the vehicle, owing to the mass inertia of the vehicle. Further reference is made below to the various travel situations in the functional description of the drive  1 . 
     A drive shaft  11  is connected to the gear stage  10  and is connected to a hydrostatic piston engine  12 . The hydrostatic piston engine  12  can be adjusted in terms of the displacement volume thereof by means of an adjustment device  13 . To this end, the adjustment device  13  acts, for example, on an inclined plate of a hydrostatic piston engine  12  which is of the construction type involving an inclined plate. The hydrostatic piston engine  12  can be used both as a pump and as a motor. Consequently, on the drive shaft  11 , there may be produced either a drive torque to drive the hydrostatic piston engine  12  which is then operating as a pump or an output torque by means of one of the hydrostatic piston engine  12  which are operating as a motor. 
     The following references are first based on the use of the hydrostatic piston engine  12  as a pump. The hydrostatic piston engine  12  is used as a pump to produce a braking effect. Pressure medium is then conveyed into a supply line  14  by means of the hydrostatic piston engine  12 . The pressure medium is drawn from a tank volume  22  by means of the hydrostatic piston engine  12  via an intake line  15 . The pressure medium conveyed into the supply line  14  by means of the hydrostatic piston engine  12  is conveyed into a store  16 . The store  16  has a compressible volume, pressure medium being conveyed into the store  16  by the hydrostatic piston engine  12  as the pressure in the store  16  increases. During the overrun operation or a braking operation of the vehicle, therefore, the decoupler  9  is closed and the speed of the output shaft  8 ,  8 ′ is converted, by means of the gear stage  10 , to an input speed of the drive shaft  11  suitable for the operation of the hydrostatic piston engine  12 . A braking function is brought about by the hydrostatic piston engine  12  conveying pressure medium into the store  15  via the supply line  14  and the store line  17  counter to the increasing pressure of the store  16 . 
     If additional intake of pressure medium is not possible by means of the store  16 , the maximum permissible pressure of the store  16  must be prevented from being exceeded by the hydrostatic piston engine  12 . An excess pressure line  18  is connected to the supply line  14  and opens at a pressure limitation valve  19 . If the pressure produced in the supply line  14  by the hydrostatic piston engine  12  exceeds a pressure determined by the pressure limitation valve  19 , the pressure limitation valve  19  opens and depressurises the supply line  14  via the excess pressure line  18  in a discharge line  20 . The discharge line  20  connects the pressure limitation valve  19  to the tank volume  22  by means of a return line  21 . 
     The pressure limitation valve  19  is acted on in the direction of the closed position thereof by means of a valve spring  23 . In the opposing direction, the pressure in the excess pressure line  18  is conveyed to a measuring face  25  by means of a measuring line  24 . The pressure in the excess pressure line  18  corresponds to the pressure produced in the supply line  14  by means of the hydrostatic piston engine  12 . An opening pressure of the pressure limitation valve  19  can consequently be adjusted by means of the valve spring  23 . 
     If the store reaches the maximum capacity thereof, at which the maximum permissible pressure of the store  16  is reached, the pressure limitation valve  19  opens and the volume supplied by the hydrostatic piston engine  12  is depressurised in the return line  21  towards the tank  22  via the supply line  14 , the excess pressure line  18 , the pressure limitation valve  19  and the discharge line  20 . Owing to the depressurisation at the pressure limitation valve  19 , a significant quantity of heat is produced. In this instance, the kinetic energy which is decreased by means of the depressurisation at the pressure limitation valve  19 , must be converted completely into heat. A cooler  45  is therefore provided in the discharge line  20 . Using the cooler  45 , pressure medium flowing through the cooler is cooled on the way to the tank volume  22 . Between the cooler  45  and the pressure limitation valve  19 , a pretensioned non-return valve  26  is arranged and, parallel therewith, a throttle location  27  is formed. The pretensioned non-return valve  26  is preferably a resiliently loaded non-return valve. For the parallel arrangement of the pretensioned non-return valve  26  and the throttle location  27 , the discharge line  20  is divided in one portion into a first discharge line branch  20 ′ and a second discharge line branch  20 ″. 
     Owing to the parallel arrangement of the pretensioned non-return valve  26  and the throttle location  27 , in the portion of the discharge line  20  that is produced between the parallel arrangement and the pressure limitation valve  19 , it is possible to adjust a higher pressure relative to the tank volume  22 . This increased pressure is also above the pressure present at the inlet side of the cooler  45  and can advantageously be used to switch on or operate an additional cooling device. 
     As explained above, the store  16  is first filled during a braking operation and the kinetic energy of the vehicle is thus converted into pressure energy which is stored in the store  16 . In order to be able to store the stored pressure energy for subsequent recovery in a manner which involves the fewest possible losses, the store line  17  can preferably be separated from the supply line  14 . To this end, a switching valve  28  is provided in the store line  17 . The switching valve  28  has a spring  29  and an electromagnet  30  which acts on the switching valve  28  in an opposing direction. In place of the electromagnet  30 , it is also possible to use any other form of actuator. For example, it is also possible to provide a measuring face which is acted on with a control pressure. 
     Owing to the force relationship produced between the spring  29  and the electromagnet  30  which acts in the opposing direction, the switching valve can be switched between a first switching position  31  and a second switching position  32 . In the first switching position  31 , the store line  17  is disengaged. If the switching valve  28  is brought into the second switching position  32  thereof by a control signal acting on the electromagnet  30 , a connection through which a fluid can flow is produced in the store line  17  by the switching valve  28 . 
     If a vehicle which is driven by means of the drive  1  illustrated, for example, drives a relatively long distance down an incline, it is, on the one hand, possible to bring about a braking effect by means of a corresponding adjustment of the hydrostatic gear. The hydraulic pump  4  is supported on the drive motor  2 . In addition, it is possible to switch on the device for energy recovery and the retarder (brake function by means of the pressure limitation valve  19 ) using the decoupler  9 . During overrun operation of the vehicle, the hydrostatic piston engine  12  is driven by means of the drive shaft  11  and first conveys pressure medium into the store  16  when the electromagnet  30  is supplied with electrical power. If the capacity limit of the store  16  is reached, the control signal of the electromagnet  30  is reset and the connection to the store  16  disengaged. The supply pressure produced by the hydrostatic piston engine  12  in the supply line  14  is depressurised in the tank volume  22  via the pressure limitation valve  19 , with heat being produced. The heat is at least partially discharged again into the surrounding air at the cooler  45 . In order to increase the cooling capacity of the cooler  45 , a cooler fan  33  is provided. The cooler fan  33  is driven by means of a hydraulic motor  35  via a shaft  34 . The hydraulic motor  35  is acted on with pressure medium at the inlet side having a hydraulic motor connection line  36 . To this end, the hydraulic motor connection line  36  is connected to an inlet connection  38  of the hydraulic motor  35 . The pressure medium conveyed by the hydraulic motor  35  is conveyed via a hydraulic motor return line  37  to the return line  21  and thus to the tank volume  22 . To this end, the hydraulic motor return line  37  is connected to an outlet connection  39  of the hydraulic motor  37 . 
     In order to drive the hydraulic motor  35 , the pressure medium conveyed by the hydrostatic piston engine  12  is used. In the first embodiment illustrated in  FIG. 1 , there is provision for the hydraulic motor  35  to be acted on with the pressure produced in the supply line  14 . To this end, a releasable non-return valve  42  is provided in the hydraulic motor connection line  36 . The non-return valve  42  is arranged in the hydraulic motor connection line  36  in such a manner that it opens in the direction towards the supply line  14 . Consequently, when pressure is produced in the supply line  14  or the excess pressure line  18 , the releasable non-return valve  42  is loaded in the closing direction. A release line  43  is provided for releasing. If the release line  43  directs sufficient pressure, the releasable non-return valve  42  is brought into the open position thereof, regardless of the pressure relationships in the excess pressure line  18  and the hydraulic motor connection line  36 . The release line  43  is connected to the discharge line  20  downstream of the pressure limitation valve  19 . Preferably, the release line  43  connects the releasable non-return valve  42  to a location of the discharge line  20  upstream of the parallel arrangement of the pretensioned non-return valve  26  and the throttle location  27 . 
     Owing to the pretensioned non-return valve  26  and the throttle location  27 , a higher pressure can be provided in the portion of the discharge line  20  that is formed upstream of this parallel arrangement. This higher pressure relative to the tank volume  22  is supplied to the releasable non-return valve  42  by means of the release line  43 . The pressure in the line portion of the discharge line  20  upstream of the parallel arrangement is maintained only with the pressure limitation valve  19  open, owing to the throttle location  27 . Consequently, the releasable non-return valve  42  is then in each case brought into the released position thereof when a braking effect is produced by means of the pressure limitation valve  19 . However, if the pressure produced in the supply line  14  and the excess pressure line  18  by means of the hydrostatic piston engine  12  drops below the opening pressure of the pressure limitation valve  19 , the release line  43  is depressurised in the direction of the tank volume  22  via the throttle location  27 . Consequently, the releasable non-return valve closes. 
     As long as the releasable non-return valve  42  is in the released position thereof, part of the pressure medium is removed from the supply line  14  via the excess pressure line  18  and supplied to the hydraulic motor  35  via the hydraulic motor connection line  36 . 
     The hydraulic motor  35  is preferably constructed as a toothed wheel motor and preferably provided for only one flow direction. Owing to the fact that the inlet connection  38  is acted on with the pressure in the supply line  14 , the hydraulic motor  35  is driven and transfers a torque to the cooler fan  33  via the shaft  34 . The cooler fan  33  is arranged in such a manner that an air flow is produced by the cooler  45  and the cooling performance of the cooler  45  is consequently increased. 
     Downstream of the hydraulic motor  35 , a throttle  44  is formed in the hydraulic motor return line  37 . The throttle  44  ensures higher pressure relative to the tank volume  22  during the operation of the hydraulic motor  35  in the portion between the throttle  44  and the outlet connection  39  of the hydraulic motor  35 . An auxiliary line  40  is formed parallel with the hydraulic motor and connects the hydraulic motor connection line  36  to the hydraulic motor return line  37 . A non-return valve  41  is arranged in the auxiliary line  40 . The non-return valve  41  opens in the direction towards the hydraulic motor connection line  36 . Owing to the throttle  44  and the non-return valve  41 , the production of cavitation is prevented. Cavitation may be produced when the releasable non-return valve  42  returns to the closed position thereof at the end of a braking operation. The pressure at the inlet side of the hydraulic motor  35  then abruptly breaks down with the result that a cavitation can be produced. In order to prevent this, the throttle  44  is provided downstream of the hydraulic motor  35 . Upstream of the throttle  44 , a higher pressure is provided which leads to the non-return valve  41  opening and pressure medium being conveyed back to the hydraulic motor connection line  36  via the auxiliary line  40 . Consequently, the formation of reduced pressure and ultimately the cavitation is effectively prevented. 
     The descriptions above are based on a drive  1  which has a hydrostatic gear that is preferably part of a drive train. The gear stage  10  is therefore connected to the drive train of the drive  1  by means of the decoupler  9  in order to produce a braking effect. For example, a coupling to an end  8 ′ of the output shaft is illustrated. In a travel drive of this type, the energy recovery is brought about by pressure medium being removed from the store  16 . To this end, the switching valve  28  is brought into the second switching position  32  thereof by means of the electromagnet  30 . The pressure medium from the store  16 , which is in a state of high pressure, is supplied to the hydrostatic piston engine  12  by means of the store line  17  and the supply line  14 . The hydrostatic piston engine  12  itself now operates as a motor and, with the pressure being decreased, a torque is produced on the drive shaft  11  by means of the hydrostatic piston engine  12 . This torque of the drive shaft  11  is supplied, via the gear stage  10  and the decoupler  9 , to the output shaft at the end  8 ′ thereof. The torque produced by the hydrostatic piston engine  12  is consequently available for driving the vehicle. 
     It can be envisaged that the coupling using the decoupler  9  may also be brought about at any other desired location of the drive train. In particular it is also possible to provide a connection to the drive shaft  3  and thus carry out the energy recovery at the gear inlet side of the hydrostatic gear. In place of a travel drive, a drive of a drive device may also form the basis for the drive  1  according to the invention. 
       FIG. 2  illustrates an alternative connection of the hydraulic motor  35 . Identical reference numerals refer to identical elements, a further description of the individual elements being dispensed with wherever this is not required. 
     The hydraulic motor  35  according to the second embodiment is not acted on with pressure medium from the supply line  14  directly by means of the hydrostatic piston engine. Instead, an increased pressure is produced upstream of the pretensioned non-return valve  26  in the discharge line  20  by means of the pretensioned non-return valve  26 . This pressure which is higher relative to the tank volume  22  in the discharge line  20  is supplied to the inlet connection  38  of the hydraulic motor  35  by means of a hydraulic motor connection line  36 ′. In this particularly simple configuration, the releasable non-return valve  42  which produces the connection of the hydraulic motor  35  to the supply line  14 , may be dispensed with. In the discharge line  20 , upstream of the pretensioned non-return valve  26 , there is in each case a higher pressure relative to the tank volume  22  when a braking effect is achieved by means of the pressure limitation valve  19 . Only when producing a braking effect by means of the pressure limitation valve  19  is the pressure limitation valve  19  in the open position. If a braking effect is no longer produced by the pressure limitation valve  19 , the pressure limitation valve  19  is brought into the closed position thereof again owing to the force of the valve spring  23 . The discharge line  20  upstream of the pretensioned non-return valve  26  is depressurised via the hydraulic motor  35  in the embodiment illustrated. 
     The invention is not limited to the embodiments illustrated. Instead, combinations of individual features of the illustrated embodiments are also possible.