Hydrostatic drive with braking energy recovery

A hydrostatic drive in an open circuit is provided. The hydrostatic drive has a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor are connected to one another via a feed line. In addition, the hydrostatic drive has a storage element for storing pressure energy. A first valve device, which is arranged downstream of the hydraulic motor, alternately connects the downstream connection of the hydraulic motor to a tank volume or to the storage element.

BACKGROUND

The invention relates to a hydrostatic drive with a device for the recovery of braking energy.

For the recovery of released energy in hydrostatic travel drives, it is known to store the released kinetic energy in the form of pressure energy. For this purpose, a hydrostatic drive in which a hydraulic pump is connected to an adjustable hydraulic motor in a closed circuit is known from AT 395 960 B. The hydraulic pump is connected to the hydraulic motor via a first working line and a second working line in the closed circuit. Connected to the first working line is a high-pressure accumulator and to the second working line a low-pressure accumulator. The second pressure accumulator has to be provided here for volume flow equalisation, since the high-pressure accumulator is filled up in the case of a recovery of released kinetic energy by the hydraulic motor then acting as a pump.

The drive known from AT 395 960 B has the disadvantage that a reversal of the flow direction is required in order that, on the side of the hydraulic motor, the high pressure is always present at the same connection of the hydraulic motor. The arrangement proposed in AT 395 960 B is thus not transferable to travel drives having an open circuit.

SUMMARY

One aspect of the invention is to provide a hydrostatic drive having an open circuit, in which a simple recovery of released energy is possible.

In the case of the hydrostatic drive according to the invention, a hydraulic pump and a hydraulic motor are connected to one another via a delivery line. The hydrostatic drive has an accumulator element for storing pressure energy. The hydraulic pump and the hydraulic motor are arranged in an open circuit and a downstream connection of the hydraulic motor can be alternately connected to a tank volume or to the accumulator element via a first valve device.

In the case of the hydrostatic drive according to the invention, the downstream connection of the hydraulic motor is connected to an accumulator element via a valve device when, for example, a deceleration of the vehicle in the case of a travel drive takes place. The vehicle goes into overrun condition and the hydraulic motor acts as a pump. Since the flow direction does not reverse here, the downstream connection of the hydraulic motor is connected to the accumulator element by the valve device. Instead of a relief to the tank volume, pressure medium is thus delivered to the accumulator element under increasing pressure. The kinetic energy is thus converted into pressure energy and is available again for subsequent acceleration processes.

As a result of the hydrostatic drive being designed with an open circuit, simultaneously the pressure medium delivered into the accumulator element is sucked out of the tank volume on the suction side of the hydraulic pump, so that a volume flow equalisation automatically takes place. Consequently, a second pressure accumulator, as required for the volume flow equalisation in closed circuits, does not have to be provided.

As a result of the arrangement of the valve device and the accumulator in accordance with the invention, a recovery of energy is also possible in an open circuit.

In particular, it is advantageous to provide a second valve device, via which a suction connection of the hydraulic pump can be alternately connected to the accumulator element or a tank volume. While a filling of the accumulator element in overrun condition is effected by the hydraulic motor by way of the first valve device, in the case of a connection of the suction connection of the hydraulic pump the recovery of the stored pressure energy is possible via the suction connection of the hydraulic pump. This results in an energy saving owing to the reduced pressure difference between the suction side and the delivery-side connection of the hydraulic pump.

In particular, it is advantageous when not only the hydraulic motor can be connected to the accumulator and the hydraulic pump to the accumulator element by the first valve device and the second valve device, but also when simultaneously the hydraulic motor and the hydraulic pump can be connected to one another in a closed circuit. If the accumulator element has reached its capacity limit and a further storage of energy is no longer possible, a further braking of a vehicle can then take place through the engine braking effect of the engine connected to the hydraulic pump. For this purpose, a closed circuit of the hydraulic pump with the hydraulic motor is produced, so that the hydraulic motor acting as a pump is supported by a primary driving engine.

According to a further preferred embodiment, the accumulator element is connected to a pressure limiting valve. If a braking torque cannot be produced by the driving engine, a further braking by filling up the accumulator element is, however, likewise not possible, and so the pressure medium delivered by the hydraulic motor is relieved to the tank volume via the pressure limiting valve. The released kinetic energy which can no longer be stored is thus converted into heat. Since the pressure limiting valve is connected to the accumulator, a switching of the first valve device during a braking procedure is not required in this case. Rather, the pressure limiting valve is automatically opened when a maximum pressure is reached in the accumulator element and thus the pressure medium is relieved to the tank volume via the pressure limiting valve while generating heat.

Preferably, the delivery line of the open hydrostatic circuit can be connected to a first and a second hydraulic motor line via a direction-of-travel valve, so that a change of the direction of travel is achieved by changing that connection of the hydraulic motor which is subjected to high pressure. In this case, it is particularly preferred when simultaneously the hydraulic motor line respectively not connected to the delivery line is connected to a tank line by the direction-of-travel valve. According to a particularly preferred embodiment, the first valve device is arranged in the tank line. In this way, both for forward travel and for rearward travel, the respectively downstream connection of the hydraulic motor can be connected to the accumulator element via the direction-of-travel valve and the tank line and also the valve device. A storage of released kinetic energy is thus possible both during forward travel and during rearward travel.

According to a further preferred embodiment, the first valve device or else the first and the second valve device is connected to the accumulator element via an accumulator line. A throttling point is arranged in this accumulator line to increase a braking effect during the filling of the accumulator element. It is particularly preferred to design the throttling point so as to be controllable. By virtue of the throttling point, a braking effect can be achieved even when the accumulator element is still largely empty and thus the counterpressure for the hydraulic motor acting as a pump is still low.

A seamless transition on discharging the accumulator element to the suction side of the hydraulic pump can be realised particularly in the case of an adjustable throttle. The pressure acting on the suction-side connection of the hydraulic pump is thus adjustable and a reduction of the pressure can be effected towards the end of the recovery process. Simultaneously, the power which is delivered to the pump by the driving engine, is increased and there is no interruption of the tractive force during an acceleration phase, which consists of the recovery of the released kinetic energy and a subsequent normal acceleration procedure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Illustrated inFIG. 1is a hydraulic circuit diagram of a first exemplary embodiment of a hydrostatic drive according to the invention. The hydrostatic drive is a travel drive1of a vehicle driven by means of a hydrostatic transmission. Such vehicles may be wheeled loaders, stackers or refuse vehicles, for example. In the case of such vehicles, particularly intensive driving cycles occur, i.e. acceleration and braking procedures are frequently repeated and as a rule shortly after one another. Therefore, in the case of these types of vehicles, a recovery of the released energy during a braking procedure is of particular interest.

The travel drive1comprises a hydraulic pump2and a hydraulic motor3. The hydraulic pump2and the hydraulic motor3are arranged in an open hydraulic circuit. The hydraulic pump2delivers pressure medium to a delivery line4which is connected to the hydraulic pump3at its delivery-side connection. The delivery line4is connected to a first hydraulic motor line6.

Via the delivery line4and the first hydraulic motor line6, a first connection8of the hydraulic motor3can be subjected to the delivery pressure produced by the hydraulic pump2. On account of the delivery pressure present at the first connection8, the hydraulic motor3produces an output torque which acts on a vehicle drive (not illustrated further). The pressure medium, relieved after flowing through the hydraulic motor3, is supplied, via a second connection9of the hydraulic motor3, to a second hydraulic motor line7which comprises a first section7aand a second section7bin the exemplary embodiment illustrated. The second hydraulic motor line7is connected to a tank volume15via a tank line16.

For reversal of the direction of travel, a direction-of-travel valve5is provided. By means of the direction-of-travel valve5, the delivery line4can be connected either to the first hydraulic motor line6or else to the second section7bof the second hydraulic motor line7. Illustrated inFIG. 1is the starting position of the direction-of-travel valve5, which is defined by the force of a compression spring34. A second end position of the direction-of-travel valve5can be set by means of a first electromagnet35counter to the force of the compression spring34. In the second end position of the direction-of-travel valve5, the delivery line4is connected to the second hydraulic motor line7or the second section7bof the second hydraulic motor line7. Simultaneously, the first hydraulic motor line6is connected to the relief line16.

In a middle position of the direction-of-travel valve5, the delivery line4is directly connected to the tank line16.

If the direction-of-travel valve5is in a second end position during the energising of the first electromagnet35, the flow direction through the hydraulic motor3and thus the direction of rotation of a drive shaft13connected to the hydraulic motor3reverses.

To set the transmission ratio of the hydrostatic transmission, which comprises the hydraulic pump2and the hydraulic motor3, a first adjusting device10and a second adjusting device11are provided. The first adjusting device10acts on an adjusting mechanism of the hydraulic pump2and sets the delivery volume of the hydraulic pump2.

The second adjusting device11, in contrast, correspondingly cooperates with an adjusting mechanism of the hydraulic motor3and sets the absorbing volume of the hydraulic motor3. The transmission ratio of the hydrostatic transmission can be continuously adjusted in dependence on the set delivery volume of the hydraulic pump2and the set absorbing volume of the hydraulic motor3.

The pressure medium delivered to the delivery line4by the hydraulic pump2in dependence on the set delivery volume is sucked in from the tank volume15via a suction line14by the hydraulic pump2. The suction-side connection30of the hydraulic pump2is, for this purpose, connected to the tank volume15via the suction line14and sucks in pressure medium from the unpressurised tank volume15.

For the following statements, it will be initially assumed that the direction-of-travel valve5is in its rest position, in which the delivery line4is connected to the first hydraulic motor line6. The direction of travel thereby selected is referred to hereinafter as forward travel.

If the vehicle goes into an overrun condition during forward travel, for example when it is travelling on a downhill stretch or else undergoing deceleration, the hydraulic motor3is driven via the output shaft13on account of the mass inertia. Owing to the absorbing volume, set by the second adjusting device11, differing from zero, the hydraulic motor3now acts as a pump and sucks in pressure medium at its first connection8and delivers it to the first section7aof the hydraulic motor line7via its second connection9.

According to the invention, there is provided a first valve device17, via which the first section7aof the second hydraulic motor line7can be connected to a first connecting line22. For this purpose, starting from its rest position, the first valve device17is moved to a switching position counter to the force of a spring20by a second electromagnet21. The first connecting line22is connected to an accumulator element18via an accumulator line23. The accumulator element18is preferably embodied as a high-pressure hydraulic diaphragm accumulator. The hydraulic diaphragm accumulator has a compressible volume, so that pressure medium can be supplied to the accumulator element18while increasing the pressure in the compressible volume. In this way, pressure medium can be delivered to the accumulator element18and energy in the form of pressure energy can be stored.

In overrun condition during forward travel, pressure medium is consequently sucked in from the first hydraulic motor line6by the hydraulic motor3and delivered to the accumulator element18via the first section7aof the second hydraulic motor line, the first valve device17, the first connecting line22and the accumulator line23.

While the accumulator element18is being filled in the manner described above, a second valve device29is still in its starting position, illustrated inFIG. 1. This starting position is defined by a further spring31. In this starting position, a throughflow-enabling connection is established in the second valve device29, which connects the suction-side connection30to the tank volume15. A third electromagnet32can move the second valve device29to its opposite switching position counter to the force of the further spring31. In the opposite switching position, the suction-side connection30is connected to a second connecting line33. Simultaneously, the suction line14is interrupted, so that there is no longer a connection between the suction-side connection30and the tank volume15.

If the first valve device17is in its switching position and the second valve device29is in its starting position, the pressure medium delivered by the hydraulic motor3is firstly delivered to the accumulator element18. When the capacity limit of the accumulator element18is reached, a pressure limiting valve24connected to the accumulator element18opens. The accumulator line23can, for this purpose, be connected to the tank volume15via a pressure limiting line27and the pressure limiting valve24. The pressure prevailing in the accumulator element18or the accumulator line23is supplied to a measuring surface of the pressure limiting valve24via a measuring line25. If this accumulator pressure exceeds a critical value, which is set by an oppositely acting pressure limiting valve spring26, the pressure limiting valve24opens and relieves the accumulator line23and thus the accumulator18to the tank volume15.

Thus, if in the event of a prolonged braking procedure a pressure were consequently to be reached, as a result of the hydraulic motor3, in the accumulator element18for which the accumulator element18is not designed, the pressure limiting valve24opens beforehand and the pressure medium delivered by the hydraulic motor3is relieved to the tank volume15. In the process, the released kinetic energy of the braking procedure is converted into heat.

If the accumulator element18is filled on account of a preceding braking procedure, the pressure energy stored there can be utilised for a subsequent acceleration procedure. For this purpose, the first valve device17is moved to its starting position again, which is defined by the second compression spring20. The first section7aof the second hydraulic motor line7is thus connected to the second section7bof the second hydraulic motor line7again. To recover the pressure energy stored in the accumulator element18, the second valve device29is now moved to its switching position. By energising the third electromagnet32, the compression spring31is compressed and the second connecting line33is connected to the suction-side connection30of the hydraulic pump2. The accumulator pressure prevailing in the accumulator element18is thus present at the suction-side connection30of the hydraulic pump2. The energy which has to be produced by a driving engine (not illustrated) connected to the hydraulic pump2via a driving shaft12is reduced on account of the smaller pressure difference between the suction side and the pressure side of the hydraulic pump2.

As a further possibility for carrying out a braking procedure, the first valve device17and the second valve device29can be moved simultaneously to their respective switching positions. For this purpose, the second electromagnet21and the third electromagnet32are energised and thus the two valve devices17,29are each moved to their second switching position. In this second switching position, the second connection9of the hydraulic motor3is connected to the suction-side connection30of the hydraulic pump2and a closed hydraulic circuit results. In a closed hydraulic circuit, the hydraulic motor3acting as a pump is supported by the driving engine connected to the hydraulic pump2via the driving shaft12, given an appropriate setting of the delivery volume of the hydraulic pump2and the absorbing volume of the hydraulic motor3.

If, therefore, the accumulator element18is already full and a development of heat at the pressure limiting valve24is to be prevented, there is also the possibility of actuating the valve device17and the valve device29simultaneously and thus utilising the available braking power of a primary driving engine (not illustrated inFIG. 1).

Furthermore, it is possible to influence the braking performance by arranging a throttling point28in the accumulator line23connecting the first connecting line22and the second connecting line33to the accumulator element18. The throttling point28is preferably embodied as an adjustable throttle. In particular, the flow resistance counteracting the pressure medium delivered by the hydraulic motor3can be adjusted by an adjustable throttle of the throttling point28. This enables a greater braking effect even when the accumulator element18produces a low counterpressure on account of a preceding withdrawal of pressure medium from the accumulator element18.

On the withdrawal of pressure medium from the accumulator element18, too, the adjustable throttle18can be advantageously utilised. In order to enable a seamless and smooth transition to accelerated motion by means of power made available by the driving engine, the pressure medium is withdrawn from the accumulator element18via the throttling point28. A throttling can take place here in particular while a high pressure prevails in the accumulator element18, so that the pressure increase on the suction side of the hydraulic pump2does not lead to a switching jolt. By increasing opening of the throttling point28during the withdrawal of pressure medium from the accumulator element18, it is possible here to make available a constant inlet pressure at the suction-side connection30of the hydraulic pump2.

InFIG. 1a simple exemplary embodiment of the travel drive1according to the invention is illustrated. Here, a storage of energy is possible only in a braking procedure during forward travel. If, in contrast, the direction-of-travel valve5is actuated in such a way that pressure medium flows through the hydraulic motor3in the reverse direction, the released kinetic energy in a braking procedure cannot be stored in the accumulator element18in the form of pressure energy.

InFIG. 2there is illustrated an exemplary embodiment with which a storage of the released kinetic energy is also possible during rearward travel. For this purpose, there is arranged in the first hydraulic motor line6an additional first valve device17′, which produces a throughflow-enabling connection in the first hydraulic motor line6during normal driving. The additional first valve device17′ is constructed like the first valve device17. The corresponding reference symbols are shown as primed reference symbols with regard to the additional first valve device17′. If the additional first valve device17′ is moved to its switching position by the electromagnet21′, the first connection8of the hydraulic motor3is connected to the first connecting line22and thus to the accumulator element18. Otherwise, the procedure for charging the accumulator element18or for braking via the pressure limiting valve24corresponds to the procedure already explained in detail with reference toFIG. 1. Likewise, on account of an energising of the electromagnet21′ of the additional first valve device17′ and the simultaneous energising of the third electromagnet32of the second valve device29, a closed hydraulic circuit can also be produced during rearward travel. The engine braking effect of the driving engine can thus also be utilised during rearward travel.

Through the additional first valve device17′, the first hydraulic motor line6is divided into a first section6aand a second section6b. The first section6ais connected to the hydraulic motor3. The second section6bis arranged between the additional first valve device17′ and the direction-of-travel valve5.

A further possibility of enabling the recovery of released kinetic energy and storage in the form of pressure energy in the accumulator element8is illustrated inFIG. 3. The exemplary embodiment illustrated inFIG. 3makes use of the fact that the same tank line16is used to carry off the pressure medium relieved via the hydraulic motor3in normal driving. The first valve device17″ is therefore arranged downstream of the direction-of-travel valve5in the tank line16. Therefore, when the second electromagnet21″ is energised, the connection from the direction-of-travel valve5to the tank volume15is interrupted and the tank line16is connected to the first connecting line22. The exemplary embodiment ofFIG. 3has the advantage that a recovery of braking energy is possible in a simple manner both during forward travel and during rearward travel. For this purpose, only one first valve device17″ is required and the double design of the valves in the first and in the second hydraulic motor line6,7can be dispensed with.

Instead of using the sliding valves illustrated as the valve devices, seat valves, in particular logic valves, which are particularly inexpensive, may also be used.

The invention is not limited to the exemplary embodiments illustrated. Rather, individual features of the exemplary embodiments may also be combined with one another.