A vibratory drive of a vibrating roller includes an unbalanced-mass vibration generator configured to be used in at least one drum of the vibrating roller. The vibrating roller is operated by an external drive device or advancing device such that the unbalanced-mass vibration generator can be rotated relative to the drum in at least one direction. The unbalanced-mass vibration generator is mechanically coupled to a hydraulic motor, which is configured to be supplied with a pressure medium by a hydraulic pump to rotate the unbalanced-mass vibration generator. At least one high-pressure accumulator is provided to accommodate pressure medium pumped by the hydraulic motor in a pushing operation. The high-pressure accumulator feeds stored pressure medium to the hydraulic motor in a driving operation of the hydraulic motor.

This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2010/007884, filed on Dec. 22, 2010, which claims the benefit of priority to Serial No. DE 10 2010 006 993.0, filed on Feb. 5, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to a vibratory drive of a vibrating roller. 
     A vibrating roller is generally a construction machine and is included in the group of compaction devices in this context. With the aid of such devices it is possible to compact cohesive and noncohesive soils, base layers, anti-frost layers and asphalt. The vibrating roller generally has two roller bodies with preferably smooth drums in the interior of which a vibration unit for improving the compaction result is installed. This provides the vibrating roller with the capability of applying, in addition to its own weight, additional energy into the underlying surface. 
     A vibrating roller of this generic type is known from the prior art, for example according to DE 40 33 793 C2. This vibrating roller has a roller frame to which a propulsion unit is attached, and at least one drum in the interior of which an unbalance vibrator, which can make it vibrate, is arranged. The unbalance vibrator is composed of an unbalance shaft which is made to rotate by a further drive motor which is disconnected from the propulsion motor. Both the propulsion motor and the further vibrator drive motor are each embodied as hydraulic motors which are fluidically connected via a hydraulic system to a hydraulic pump which is driven by an internal combustion engine. There are also vibrating rollers in which the propulsion motor is supplied with pressure medium by at least one first hydraulic pump, and the vibrator drive motor is supplied with pressure medium by at least one further hydraulic pump. 
     Furthermore, hydraulic drives with recovery of braking energy in an open or closed hydraulic circuit design are known from the prior art, for example according to DE 10 2006 050 873 A or according to DE 10 2006 060 014 A1. Hydraulic drives of this type have at least one hydraulic pump which is fluidically connected to a hydraulic motor via working lines. The downstream connection of the hydraulic motor can be optionally connected to a high pressure accumulator here. 
     In the case of a driving mode, the hydraulic pump delivers pressure medium to the hydraulic motor, which accordingly outputs a torque to an output shaft in order to drive a machine and/or a vehicle. In the case of an overrun mode, i.e. in the case in which a torque is applied from the output shaft to the hydraulic motor, the hydraulic motor then operates as a pump and delivers pressure medium in the direction of its downstream connection. In this particular case, the high pressure accumulator is connected to the downstream connection of the hydraulic motor in order to temporarily store the pressure medium which is delivered by the hydraulic motor (now acting as a hydraulic pump). 
     As soon as the overrun mode changes over again into the driving mode and therefore the hydraulic motor is intended to output a torque to the output shaft again, the high pressure accumulator is connected to the upstream connection of the hydraulic pump and therefore outputs pressure medium under high pressure to the hydraulic pump. As a result, the energy consumption of the hydraulic pressure pump is reduced. In other known hydraulic drives with energy recovery, the hydraulic accumulator is connected to the upstream connection of the hydraulic motor. 
     Such regenerative hydrostatic drive systems are used in the prior art to recover energy from vehicles in the overrun mode. However, in vibrating rollers of the present generic type this is not possible in this form since vibrating rollers in practical use essentially do not go into an overrun mode which is relevant in terms of energy. 
     In view of this situation, the object of the present disclosure is to make available an energy recovery possibility for vibrating rollers of this generic type. 
     SUMMARY 
     This object is achieved by means of a vibratory drive of a vibrating roller having the features of the disclosure. Advantageous developments of the disclosure are the subject matter of the dependent claims here. 
     The basic idea of the disclosure is accordingly not to use the overrun mode, which is irrelevant in terms of energy, of the vibrating roller from the propulsion motor for energy recovery but instead the vibratory drive of the vibrating roller comprising an unbalance vibrator, which is inserted, or can be inserted, in a rotatable fashion in at least one vibrating roller drum which is preferably driven by the propulsion motor. The unbalance vibrator is mechanically coupled, or can be mechanically coupled, here to a hydraulic motor (preferably via an output shaft), which hydraulic motor can in turn be supplied with a pressure medium by a hydraulic pump via working lines. According to the disclosure, in this hydrostatic drive of the unbalance vibrator, i.e. in the vibratory drive, at least one high pressure accumulator is provided which serves to accommodate pressure medium which is delivered by the hydraulic motor in an overrun mode, i.e. in a coasting mode of the unbalance vibrator. 
     In other words, the disclosure for recovering energy does not specify the propulsion unit but rather the vibratory drive as the drive which is relevant for the recovery of energy. This vibratory drive can operate independently of the propulsion unit even in the stationary state of the vibrating roller. Said vibratory drive can effectively be used to recover energy. 
     One advantageous refinement of the disclosure provides that the hydraulic pump and the hydraulic motor are arranged in a closed circuit in which, in the overrun mode (coasting mode of the unbalance vibrator) the downstream connection of the hydraulic motor can be fluidically connected to the high pressure accumulator, and in the acceleration mode (starting up of the unbalance vibrator) the upstream connection of the hydraulic motor can be fluidically connected to the high pressure accumulator. 
     As an alternative to this, another advantageous refinement of the disclosure provides that the hydraulic pump and the hydraulic motor are arranged in an open circuit in which the downstream connection of the hydraulic motor can be fluidically connected to a tank or to the high pressure accumulator. 
     In the case of the open hydraulic circuit design, a valve arrangement is provided by means of which the high pressure accumulator can be optionally fluidically connected to the downstream connection of the hydraulic motor or to the upstream connection of the hydraulic motor. In the case of the closed hydraulic circuit design, a low pressure accumulator is preferably provided which, in the overrun mode of the hydraulic motor can be connected to the upstream connection thereof, and in the driving mode of the hydraulic motor can be connected to the downstream connection thereof. 
     In this context, the connection between the downstream connection of the hydraulic motor and the upstream connection of the hydraulic motor is preferably continuously maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be explained in more detail below by means of two exemplary embodiments and with reference to the accompanying figures, of which: 
         FIG. 1  shows in this context a hydrostatic vibratory drive of a vibrating roller with energy recovery according to a first preferred exemplary embodiment of the disclosure in a first open hydraulic circuit variant, 
         FIG. 2  shows a hydrostatic vibratory drive of a vibrating roller according to the first preferred exemplary embodiment in a second open hydraulic circuit variant, 
         FIG. 3  shows a hydrostatic vibratory drive of a vibrating roller according to a second preferred exemplary embodiment of the disclosure in a first closed hydraulic circuit variant, 
         FIG. 4  shows a hydrostatic vibratory drive of a vibrating roller according to the second preferred exemplary embodiment in a second closed hydraulic circuit variant, and 
         FIG. 5  shows a schematic illustration of a vibrating roller. 
     
    
    
     DETAILED DESCRIPTION 
     According to  FIG. 1 , the vibratory drive has a hydraulic pump  1  whose intake connection is fluidically connected to a pressure medium tank  2 , and whose pressure connection is fluidically connected to an upstream connection of a hydraulic motor  6  via a working line  4 . In this context a spring-biased nonreturn valve  8 , which is set here to a working pressure of approximately 2 bar, is connected into the working line  4 . Furthermore, a branch line  10  branches off from the working line  4  to the pressure medium tank  2 , into which branch line  10  a pressure-limiting valve which can be adjusted in proportion to the electricity is connected. Said pressure-limiting valve can be adjusted, for example, between 8 bar and 250 bar. 
     The downstream connection of the hydraulic motor  6  can be connected to the pressure medium tank  2  via a two-way/two-position switching valve  14  which can be activated electromagnetically. An energy recovery line  16  branches off between the switching valve  14  and the downstream connection of the hydraulic motor  6 , which energy recovery line  16  leads to a high pressure accumulator  18  which is biased (biasing pressure of, for example 150 bar). In the present case an energy recovery valve arrangement  20  is connected into this energy recovery line  16 . Said energy recovery valve arrangement  20  is composed, according to the present exemplary embodiment, of a three-way/three-position switching valve  22  which can be activated electromagnetically and, in a first switched position which is embodied as a spring-centered center position, blocks off all the connections. In a second switched position, the said switching valve  22  connects the downstream connection of the hydraulic motor  6  to the high pressure accumulator  18 . In a third switched position, the switching valve  22  connects the high pressure accumulator  18  to an energy recovery line  24 , which is connected to the upstream connection of the hydraulic motor  6  downstream of the nonreturn valve  8 . In this feedback line  24 , a spring-biased nonreturn valve  26  is also connected, said nonreturn valve  26  being preferably set to 2 bar opening pressure. 
     Finally, a branch line  28 , which leads to the pressure medium tank  2  and into which a pressure-limiting valve (preferably set to 250 bar) is connected, is connected between the downstream connection of the hydraulic pump  1  and the three-way/three-position switching valve  22 . 
     In the case of a driving mode of the hydraulic motor  8 , which is connected via an output shaft  32  to an unbalance vibrator  34  (illustrated schematically in  FIG. 5 ) which is preferably arranged in a drum  36  of a vibrating roller, for the time being the three-way/three-position switching valve  22  is in its first switched position (shown according to  FIG. 1 ) in which the high pressure accumulator  18  is disconnected from the open hydraulic circuit. The pressure-limiting valve  12  is set to a higher pressure than usually occurs and serves only as a safety valve. In this case, pressure medium is delivered by the hydraulic pump  1  to the upstream connection of the hydraulic motor  6  via the spring-biased nonreturn valve  8 , in order to drive said hydraulic motor  6 . From there, the now relaxed pressure medium passes back into the tank  2  via the two-way/two-position switching valve  14 , which is in the open position in this situation. 
     In order to switch off the unbalance vibrator  34 , the pressure-limiting valve  12  is set to a very small value, with the result that the pressure upstream of the hydraulic motor  6  drops to, for example, 6 bar. The unbalance vibrator  34  vibrates and rotates as a consequence of its moment of mass inertia. In this case, a torque is transmitted via the output shaft  32  to the hydraulic motor  6  which, in this case, now assumes the function of a pump, that is to say the hydraulic motor  6  now feeds pressure medium out of the working line  4  in the direction of the pressure medium tank  2 . At this moment, an electronic controller (not illustrated in more detail) which also transmits the signal for adjusting the pressure-limiting valve  12 , switches the two-position/two-way switching valve  14  into the closed position and the three-way/three-position switching valve  22  into the second switched position, in which the downstream connection of the hydraulic motor  6  is connected to the high pressure accumulator  18 . In this case, the high pressure accumulator  18  is charged, i.e. the pressure medium which is delivered by the hydraulic motor  6  (now in the function of a pump) is conducted into a high pressure accumulator  18 . The residual quantity, which the hydraulic motor does not subtract from the quantity of pressure medium delivered by the hydraulic pump  1  flows to the tank via the pressure-limiting valve  12  when there is a low pressure. What is decisive here is that the output shaft  32  of the hydraulic motor  6  is connected to the unbalance vibrator  34  of the vibrating roller, i.e. the run-on energy of the unbalance vibrator  34  is used to recover energy in the form of hydraulic pressure in the high pressure accumulator  18 . 
     If switching occurs from the overrun mode into a driving mode, i.e. into a mode in which the hydraulic motor  6  outputs a torque to the output shaft  32 , the two-way/two-position switching valve  14  is switched to the open position, and the three-way/three-position switching valve  22  is switched to the third switched position in which the connection between the downstream connection of the hydraulic motor  6  and the high pressure accumulator  18  is closed and instead a connection is formed between the high pressure accumulator  18  and the upstream connection of the hydraulic motor  6 . In this switched position, the high pressure accumulator  18  therefore outputs pressure medium under pressure to the input side of the hydraulic motor  6 , with the result that the latter accelerates the unbalance vibrator  34  independently of the hydraulic pump. In this phase, the hydraulic pump firstly still rotates with low pressure. After a time period in the range of seconds, which can be determined by trials or by calculation, the three-way switching valve is moved back to its first switched position by switching off the one electromagnet and the proportional-pressure-limiting valve  12  is set to a high pressure value. The hydraulic motor is then supplied with pressure medium by the hydraulic pump. 
     If the rotational speed of the hydraulic motor is detected by a rotational speed sensor, the valves  22  and  12  can also be switched or adjusted as a function of the rotational speed or the change in the rotational speed per time unit. 
       FIG. 2  shows a second variant of a vibratory drive of a vibrating roller according to an open hydraulic circuit design, wherein details will mainly be given below only on the circuitry differences compared to the first variant described above. 
     In the first variant above, as already explained a proportional pressure-limiting valve  12  is arranged in a branch line  10  which leads to a pressure medium tank and which branches off from the working line  4  between the hydraulic pump  1  and the hydraulic motor  6 . This proportional pressure-limiting valve  12  can be adjusted in a range from 8 to 250 bar. With the latter, the hydraulic motor  6  is also optionally deactivated. That is to say when the vibratory drive is to be switched off, the proportional pressure-limiting valve  12  is set to 8 bar, with the result that the hydraulic pump  6  delivers substantially directly into the pressure medium tank  2 . At this moment, the hydraulic motor  6  is switched off as a torque output means. 
     An alternative embodiment to this is presented by the second variant of the open hydraulic circuit design according to  FIG. 2 . Accordingly, the proportional pressure-limiting valve  12  specified above is replaced by a first, permanently set pressure-limiting valve  38 , which is preferably set to 250 bar, a second permanently set pressure-limiting valve  48 , and a direction control valve  46 . In addition, a bypass line  40  is provided which bypasses the hydraulic motor  6  and the spring-biased nonreturn valve  8  which is connected upstream of the hydraulic motor  6 , i.e. connects the output connection of the hydraulic pump  1  to the output connection of the hydraulic motor  6 , and in which a two-position/two-way switching valve  42  is connected. This switching valve  42  is biased into its open position by means of a spring and can be switched electromagnetically into a blocking position. Furthermore, a second branch line  44  branches off from the bypass line  40  upstream of the specified two-position/two-way switching valve  42 , which bypass line leads to the pressure medium tank  2 . The two-way/two-position switching valve  46  is arranged in this branch line  44 , which two-way/two-position switching valve  46  is spring-biased into a blocking position and can be switched electromagnetically into an open position. The pressure-limiting valve  48 , which is preferably preset to a value between 10 and 20 bar, is arranged downstream of this further two-way/two-position switching valve  46 . The rest of the structure of the hydraulic circuit of the open design according to  FIG. 2  corresponds to the hydraulic circuit in the first variant, as has already been described above with reference to  FIG. 1 , with the result that at this point reference can be made to the corresponding references in the text of the description. 
     In the vibratory drive of the vibrating roller according to  FIG. 2 , the hydraulic pump  1  also delivers a pressure medium to the upstream connection of the hydraulic motor  6  via the spring-biased nonreturn valve  8  with the result that said hydraulic motor  6  outputs a torque to an output shaft  32  for driving an unbalance vibrator  34  which is illustrated in  FIG. 5 . The relaxed pressure medium is subsequently conducted away into the pressure medium tank  2  via the two-position/two-way switching valve which is biased into its open position. In this driving phase, the two valves  42  and  46  are in their blocking position. 
     If the vibratory drive is to be switched off, the two-way/two-position switching valve  46  is switched out of its closed position into the open position. In this case, the hydraulic pump  1  delivers pressure medium into the pressure medium tank  2  via the branch line  44  which branches off from the bypass line  40 . The energy recovery from the coasting unbalance vibrator  34 , which in this state applies a torque to the hydraulic motor  6  via the output shaft  32 , takes place in accordance with the vibratory drive according to  FIG. 1 , which is described above. 
     The acceleration also takes place in accordance with the exemplary embodiment according to  FIG. 1 . For this purpose, the directional control valve  22  is moved into the switched position in which the hydraulic accumulator is connected to the upstream connection of the hydraulic motor  6  via the nonreturn valve  26 . After a certain time period or as a function of the rotational speed or as a function of the degree of change in the rotational speed, the directional control valve  22  is moved into its central position, and the directional control valve  46  is moved into its blocking position. Because of the blocking position of the directional control valve  46 , the pressure-limiting valve  46  is switched to an inactive setting and a pressure can build up in the working line  4 . 
     The directional control valve  42  is in its opened switched position only if the vibratory drive is to be switched off entirely but the hydraulic pump  1  is still being driven by a primary unit. The hydraulic pump then delivers to the tank with a very low circulation pressure via the valves  42  and  14 , with the result that only very low energy losses occur. 
     At this point it is also to be noted that the drive of the hydraulic motor  6  in the case of the vibratory drive according to  FIG. 1  and also according to  FIG. 2  has to be configured in such a way that when the hydraulic motor  6  starts the hydraulic pump  1  overcomes the moment of mass inertia of the unbalance vibrator  34 . That is to say for the starting of the hydraulic motor  6  at least for a short time an excessively increased power level is demanded of the hydraulic pump drive. In order to provide this power level, the drive of the hydraulic pump  1  must generally be configured in such a way that the starting peak power is applied thereby. In this respect, the hydraulic pump drive is over-dimensioned for the normal operating state of the vibratory drive. 
     As a result of the arrangement of the high pressure accumulator  18 , which within the scope of an energy recovery process is charged by the coasting of the unbalance vibrator  34  and the drive of the hydraulic motor  6  which is connected thereto, said high pressure accumulator  18  can, for the purpose of starting the hydraulic motor  6 , briefly feed energy into the system and therefore relieve the hydraulic pump  1 . As a result, the hydraulic pump drive can be correspondingly reduced in terms of its maximum power. 
     In the text which follows, a second preferred exemplary embodiment of the disclosure will be described in more detail with reference to two variants in accordance with  FIGS. 3 and 4 . 
       FIG. 3  shows a vibratory drive of a vibrating roller in a closed hydraulic circuit design. While the open hydraulic circuit design described with reference to  FIGS. 1 and 2  is predominantly provided for more lightweight vibrating rollers, a vibratory drive of the closed hydraulic circuit design is generally provided for heavy vibrating rollers with corresponding heavy unbalance vibrators. 
     Furthermore, with a hydraulic drive in a closed circuit it is possible to drive the unbalances easily by reversing the delivery direction of a pump, pivotable over zero, in both rotational directions. Different frequencies and amplitudes of the vibration are often implemented by means of the reversal of the direction of rotation. 
     The vibratory drive according to  FIG. 3  has a hydraulic pump  1  which can be adjusted over zero and which is mechanically connected to a drive unit M, for example an internal combustion engine. The hydraulic pump  1  delivers fluid medium via a working line  4  to at least one hydraulic motor  6  which is coupled via an output shaft  32  to an unbalance vibrator  34  (shown in  FIG. 5 ). Further hydraulic motors can optionally be inserted into the working line in a serial fashion with respect to the hydraulic motor mentioned above, as is illustrated, for example, in  FIG. 3  by the second hydraulic motor shown there by dashed lines. 
     At this point it is to be noted that vibrating rollers of the heavy embodiment frequently have two drums into which an unbalance vibrator  34  according to the disclosure is respectively inserted. In this case, at least two hydraulic motors are necessary for the drive of said drums. 
     An output connection of the at least one hydraulic motor  6  is fluidically connected to an input connection of the hydraulic pump  1  via a feedback line  50 . This results in a closed hydraulic circuit. Of course, in the case of a reversed delivery direction of the hydraulic pump  1 , the line  50  is the working line, and the line  4  is the feedback line. An energy recovery line  16  is arranged parallel to the at least one hydraulic motor  6 , which energy recovery line  16  bypasses the input connection and the output connection of the hydraulic motor  6 . A biased high pressure accumulator  18  is connected to the recovery line  16  at a branching point. Furthermore, in the recovery line  16 , which actually connects the working line  4  and the feedback line  50  to one another, two 2-way/2-position switching valves  52 / 54  are inserted in such a way that the connection point of the high pressure accumulator  18  to the recovery line  16  is located between these two switching valves  52 ,  54 . The two switching valves  52 , are each spring biased into a blocking switched position and can be switched into an open position electromagnetically independently of one another. A feed line  56 , which also starts from the working line  4  and the feedback line  50 , is arranged parallel to the recovery line  16 . A 3/3-way switching valve  58 , to which a low pressure accumulator  60  is connected, is inserted into the feed line  56 . The switching valve  58  is embodied here in such a way that it optionally fluidically connects the low pressure accumulator  60  to the working line  4  or to the feedback line  50  in the lateral switched positions, and shuts off the hydraulic accumulator  60  and the lines  4  and  50  from one another in the spring-central position. 
     In a second switched position of the switching valve  58 , the low-pressure accumulator  60  is fluidically connected to the working line  4  via the feed line  56 . In a third switched position, the low pressure accumulator  60  is fluidically connected to the feedback line  50  via the feed line  56 . 
     Control lines are connected to two control sides of the switching valve  58 , said control lines being fluidically connected to the working line on one side and to the feedback line on the other side. Finally, the low pressure accumulator  60  has a pressure relief line  62 , which leads to the pressure medium tank  2  and into which a pressure-limiting valve  64  is connected. 
     Finally, the vibratory drive according to  FIG. 3  is provided with an equalizing pump  66  which is connected to the hydraulic circuit of the closed design in order to equalize oil leakages. 
     Specifically, the equalizing pump  66  is fluidically connected to the pressure medium tank  2  via an intake duct. The outlet connection of the equalizing pump  66  opens into an equalizing line  68  which fluidically connects the working line  4  and the feedback line  50  parallel to the feed line  56  or the recovery line  16 . A nonreturn valve  70  is connected between the junction of the equalizing pump  66  with the equalizing line  68  and the working line  4 , which nonreturn valve  70  only permits a flow from the equalizing pump  66  to the working line  4 . The pressure-limiting valve  72 , which, in the case of an excessively high pressure in the working line  4 , opens in the direction of the junction between the equalizing pump  66  and the equalizing line  68 , is arranged parallel to the nonreturn valve  70 . 
     A comparable structure can be found in the equalizing line  68  between the junction and the feedback line  50 . 
     That is to say between the junction of the equalizing pump  66  with the equalizing line  68  and of the feedback line  50 , a nonreturn valve  74  is also connected, which nonreturn valve  74  only permits a flow in the direction of the feedback line  50 . A pressure-limiting valve  76  is arranged parallel to this nonreturn valve  74 , which pressure-limiting valve  76  opens in the direction of the junction in the event of an excessively high pressure being present in the feedback line  50 . A pressure of 25 to 30 bars is maintained in the respective low pressure line  4  or  50  by the equalizing pump  66 . 
     During normal operation, the motor-driven hydraulic pump  1  delivers a working medium via the working line  4  to the upstream connection of the at least one hydraulic motor  6  in order to drive an unbalance vibrator  34  via the output shaft  32  of said hydraulic motor  6 . The relaxed pressure medium is subsequently fed back from the downstream connection of the at least one hydraulic motor  6  to the input connection of the hydraulic pump  1  via the feedback line  50 . During normal operation, the two switching valves  52 ,  54  are in their blocked position. Owing to the pressure in the working line  4 , the switching valve  58  is in its third switched position and connects the low pressure accumulator  60  to the feedback line  50 . 
     If the at least one hydraulic motor  6  is now to be switched off, the expulsion-variable hydraulic pump  1  is reduced with respect to its delivery capacity (set to zero), with the result that the hydraulic motor  6  no longer outputs any torque to the output shaft  32  any more. As a result of the mass inertia of the at least one unbalance vibrator  34 , the latter, however, temporarily (coasting process) outputs a torque to the hydraulic motor  6  via the output shaft  32 , as a result of which said hydraulic motor  6  temporarily assumes the function of a pump. That is to say the hydraulic motor  6  now feeds pressure medium into the feedback line  50 . 
     In this case, the switching valve  54  is opened electromagnetically between the high pressure accumulator  18  and the feedback line  50 , with the result that the pressure medium which is temporarily delivered by the hydraulic motor  6  is fed into the high pressure accumulator  16 . As soon as this run-on or overrun mode of the hydraulic motor  6  is ended, the switching valve  54  between the high pressure accumulator  18  and the feedback line  50  closes. Since pressure medium is consequently removed from the closed hydraulic circuit and buffered in the high pressure accumulator  18  under pressure during the overrun mode, a lack of pressure medium (partial vacuum) arises in the hydraulic circuit and, in particular, in the working line  4 . This is equalized by corresponding switching of the three-way/three-position switching valve  58  in the feed line  56 , which switching valve  58  is switched, as a result of a pressure difference occurring between the working line  4  and the feedback line  50 , to its second switched position in which the low pressure accumulator  60  is fluidically connected to the working line  4 . That is to say the pressure medium which is temporarily stored in the high pressure accumulator  18  is equalized in the closed hydraulic circuit by means of the low pressure accumulator  60 , but also additionally by means of the equalizing pump  66 . 
     As soon as the normal driving mode of the at least one hydraulic motor  6  is started, the two-way/two-position switching valve  52  between the high pressure accumulator  18  and the working line  4  is opened, as a result of which the pressure medium buffered under pressure in the high pressure accumulator  18  is fed into the working line  4 . In this way, the power necessary to start the hydraulic motor  6  and to overcome the mass inertia of the unbalance vibrator  34  is provided by the high pressure accumulator  18 . Once the acceleration process from the hydraulic accumulator  18  is terminated, the valve  52  is moved into its blocking position and the hydraulic pump  1  is pivoted out from zero and adjusted to the delivery volume which corresponds to the desired rotational speed of the hydraulic motor. The adjustment can in turn take place here in a time-dependent fashion or as a function of the rotational speed or the change in rotational speed of the hydraulic motor. A corresponding rotational speed sensor is shown in  FIG. 4 . Accordingly, the hydraulic pump  1  and the drive M thereof are able to be configured only for an average operation and not for the expected peak powers which can occur during the starting of the hydraulic motor  6 . The pressure medium which is now additionally fed into the closed hydraulic circuit from the high pressure accumulator  18  leads to a situation in which the three-position/three-way switching valve  58  in the feed line  56  moves into a switched position in which the low pressure accumulator is now fluidically connected to the feedback line  50 . That is to say the excess of pressure medium, which comes about as a result of the relaxation of the high pressure accumulator  18  in the closed hydraulic circuit, is tapped via the low pressure accumulator  60  and buffered there. 
     If a pressure medium leak occurs in the closed hydraulic circuit system, this leads to a situation in which the hydraulic pressure which is built up by the equalizing pump  66  at the respective nonreturn valves  70 ,  74  brings about opening of the one or other nonreturn valve in the direction of the working line  4  or of the feedback line  50 , as a result of which the corresponding leak is equalized. 
       FIG. 4  now illustrates a second variant of a hydraulic circuit of the closed design according to the second preferred exemplary embodiment. For reasons of simplification, more details will be given below only on the refinements which are different from the variant according to  FIG. 3 . 
     As is apparent from  FIG. 4  compared to  FIG. 3 , the three-way/three-position switching valve  58  which is provided according to  FIG. 3  is replaced by two separately electromagnetically switchable two-way/two-position switching valves  78 ,  80 . Specifically, the low-pressure accumulator  60  is fluidically connected to the feed line  56  at a junction point. Between the junction point and the working line  4 , the two-way/two-position switching valve  78 , which is spring-biased into a blocking position, is connected. Furthermore, the other two-way/two-position switching valve  80  is connected between the junction point and the feedback line  50  and is also spring-biased in the blocking position. The methods of functioning of the two valves  78 ,  80  correspond here to that method of functioning of the three-way/three-position switching valve  58  according to  FIG. 3 . That is to say in an overrun mode of the hydraulic motor  6 , in which the hydraulic motor  6  feeds pressure medium into the high pressure accumulator  18 , the switching valve  78  between the working line  4  and the junction is open with the result that a corresponding quantity of pressure medium can flow out of the low pressure accumulator  60  to the working line  4 . In the case of renewed starting of the unbalance vibrator  34 , the switching valve  80  between the junction and the feedback line  50  is opened in order to allow the now excess pressure medium whose pressure is consumed to flow back from the downstream connection of the hydraulic motor  6  into the low pressure accumulator  60 . All the other functions of the closed hydraulic circuit according to  FIG. 4  correspond to those in  FIG. 3 , with the result that at this point reference can be made to the corresponding references in the text. 
     The vibratory drive according to the disclosure which has been described by means of the two exemplary embodiments above allows the following advantages to be achieved: 
     Energy recovery in a vibrating roller is now possible through storage of the rotational energy from the vibratory drive or from the unbalances. 
     The stored energy can preferably be used to accelerate the rotating masses of the vibratory drive in the case of vibrating rollers, in order to equalize power peaks. 
     There is no hydraulic connection/coupling between the vibratory drive and the locomotive drive of a vibrating roller. 
     There is no use of the translatory energy of the vehicle which is irrelevant in terms of energy since vehicles such as the vibrating roller in the working medium without a drive are immediately decelerated automatically. 
     The storage and release of the energy fed back can also take place in the stationary state of the vehicle. 
     Implementation of the vibratory drive with energy feedback is possible with simple commercially available valves. 
     The vibratory drive permits the maximum drive power which is to be installed in an internal combustion engine as a drive unit of the hydraulic pump to be reduced. 
     Less insertion space is required as a result of a relatively small internal combustion engine. 
     The consumption of fuel by an internal combustion engine which is made smaller is reduced. 
     The hydraulic pressure accumulators (high pressure/low pressure accumulator) can be freely arranged or integrated in/on the vehicle frame. 
     The necessary switching valves and the hydrostatic vibratory drive can be electrically or electronically actuated in the system. 
     During the acceleration of the unbalance vibrator, the pressure medium supply of the hydraulic motor from the hydraulic accumulator and from the hydraulic pump can preferably occur sequentially. 
     A vibratory drive of a vibrating roller is disclosed, comprising an unbalance vibrator which can be inserted into at least one drum, operated by an external drive unit or propulsion unit, of the vibrating roller so as to be rotatable relative to said drum in at least one direction. The unbalance vibrator is, according to the disclosure, mechanically coupled to a hydraulic motor which can be supplied with a pressure medium by a hydraulic pump in order to rotate the unbalance vibrator. In addition, at least one high pressure accumulator is provided for accommodating pressure medium which is delivered by the hydraulic motor in an overrun mode. Furthermore, the high pressure accumulator feeds pressure medium stored in a drive mode of the hydraulic motor to the hydraulic motor.