HYDRAULIC CIRCUIT ARRANGEMENT WITH RECOVERY OF ENERGY

Abstract

The invention relates to a hydraulic circuit arrangement having a first pump which is driven by an electric motor and is assigned to a lifting cylinder, wherein, in order to recover energy, the electric motor can be operated as a generator and the first pump can be operated as a hydraulic motor in order to recover potential energy of the lifting cylinder, and having a second pump for supplying at least one further load. The electric motor, the first pump and the second pump are embodied as a motor/double pump assembly, wherein in the lowering mode of the lifting cylinder the first pump acts as a hydraulic motor with reversal of the rotational direction while the second pump always operates as a pump with a constant delivery direction. Arranged between the first pump and the lifting cylinder is a priority valve which, depending on requirements, distributes the volume flow which flows in from the first pump to the lifting cylinder on the one hand, and to the at least one further load on the other, and the volume flow which flows in from the lifting cylinder in the lowering mode accordingly to the at least one load and the first pump.

Description

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic circuit arrangement having a first pump which is driven by an electric motor and is assigned to a lifting cylinder, wherein, in order to recover energy, the electric motor can be operated as a generator and the first pump can be operated as a hydraulic motor in order to recover potential energy of the lifting cylinder, and having a second pump supplying at least one further load according to the preamble of Claim 1.

Such a circuit arrangement for battery-driven industrial trucks is known from DE 43 17 782 C2. In said document, a direct current or asynchronous electric motor drives a pump P which makes available the hydraulic pressure for the lifting cylinder. When the load is lowered, the circuit arrangement clears a path on which the fluid can flow back virtually without loss of pressure from the lifting cylinder to the pump P. In this context, the potential energy which is stored in the lifting cylinder is recovered by virtue of the fact that the pump then operates as a hydraulic motor and the electric motor then operates as a generator G for recharging the battery. The particular advantages lie in the fact that the expenditure is relatively low and no pressure losses due to the principle occur.

In the known arrangement, it also possible to connect other loads. For this purpose, a separate circuit with a further pump and separate drive motor is provided. Furthermore, additional functions can be provided by the first-mentioned working circuit. However, the lowering function must then be respectively interrupted for this purpose.

DE 299 11 686 U1 discloses a further electrohydraulic lifting module which can be operated in an energy-saving mode. In this context, a single pump supplies both main loads and secondary loads which can be connected into the circuit via an electromagnetically actuated valve. A separate return line is provided for the lowering process of the lifting cylinder, and when said return line is opened it connects the lifting cylinder to the intake side of the pump, wherein a non-return valve prevents the direct emptying into the reservoir. The working line, otherwise provided for the lifting mode, of the pump then functions as a return line and is connected to the tank via an electromagnetic valve.

SUMMARY OF THE INVENTION

The invention is based on the object of specifying an improved, more cost-effective circuit for recovering energy from a lifting cylinder while simultaneously operating with other loads.

According to the invention, this is achieved in a hydraulic circuit arrangement according to the preamble of Claim 1 by virtue of the fact that the electric motor, the first pump and the second pump are embodied as a motor/double pump assembly, wherein in the lowering mode of the lifting cylinder the first pump acts as a hydraulic motor with reversal of the rotational direction while the second pump always operates as a pump with a constant delivery direction, and wherein provided between the first pump and the lifting cylinder is a priority valve which, depending on requirements, distributes the volume flow which flows in from the first pump to the lifting cylinder on the one hand, and to the at least one further load on the other, and the volume flow which flows in from the lifting cylinder in the lowering mode accordingly to the at least one load and the first pump.

This solution reduces quite considerably the complexity of known circuit arrangements in terms of valve technology for supplying a plurality of loads.

As preference, arranged in the working line of the first pump between the priority valve and the lifting cylinder is an electrically controllable lifting valve which is partially opened during simultaneous operation of the lifting cylinder and of the at least one further load, and in its two limiting positions switches the working line of the first pump to the closed and open positions, wherein when the lifting cylinder and the at least one further load are not operating simultaneously, the lifting valve opens fully. The lifting valve ensures, together with the priority valve, that excess volume flow of the first pump can flow to the further loads, wherein it is added to the volume flow of the second pump. The second pump can therefore be given relatively small dimensions because it is assisted by the first pump.

It is particularly advantageous if the priority valve is embodied in such a way that it is controlled by the pressure difference which occurs at the lifting valve, wherein in a first extreme position of the priority valve, the first pump is connected, on the one hand, to the lifting valve via a throttle and, on the other hand, to the at least one further load, while in a second opposed extreme position of the priority valve, the working line which comes from the first pump is switched to the open position and at the same time is connected to the at least one further load, and the priority valve also has a central position in which the working line of the first pump is switched to the open position and the line to the at least one further load is closed.

Preferably provided in the line leading from the priority valve to the at least one further load is a non-return valve which then opens if in the working line leading from the first pump to the lifting cylinder there is excess pressure above the pressure generated by the second pump.

The supply of further loads can advantageously be carried out by a constant flow system which is of particularly simple design and is sufficient for a large number of practical applications. However, a load-sensing system which permits better pressure adaptation and volume adaptation to the instantaneous requirements of the individual loads can preferably also be provided for this purpose.

In a further advantageous embodiment of the invention, an electrically controllable load-lowering valve is provided, and in the lowering mode said load-lowering valve diverts the volume flow from the lifting cylinder to the reservoir without the recovery of energy and switches in the cases in which the recovery of energy is to be dispensed with. Such a case can occur, for example, when a small lifting load is lowered and there is simultaneous operation of further loads. If, in this context, only a small amount of energy is available for recovering, it is, according to circumstances, preferable to divert the volume flow into the reservoir. Such situations can preferably be detected by means of pressure sensors which are provided in the respective working lines of the first and the second pumps and are used to actuate the load-lowering valve.

Further features and advantages of the invention emerge from the following description of the figures:

BRIEF DESCRIPTION OF THE FIGURES

In said figures:

FIG. 1: shows a first exemplary embodiment of the invention in which the further loads are supplied by a load-sensing system.

FIG. 2: shows the exemplary embodiment from FIG. 1 with a load-lowering valve for optionally bypassing the recovery of energy, and

FIG. 3: shows a further exemplary embodiment in which the additional loads are supplied by a constant flow system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first exemplary embodiment of the invention. The motor/double pump assembly MDP comprises an electric motor M/G which drives a first pump P1 and a second pump P2 which has smaller dimensions, wherein the electric motor M/G and the pumps P1, P2 are arranged on a common shaft. The first pump P1 supplies the hydraulic pressure for a lifting cylinder HZ, and the second pump P2 supplies further loads Z1, Z2.

When the load is lowered, potential energy of the lifting cylinder HZ is recovered, wherein the first pump P1 then functions as a hydraulic motor with reversal of the rotational direction, and the electric motor M/G functions as a generator for recharging an accumulator (not illustrated). The second pump P2 always operates as a pump with a constant delivery direction irrespective of the rotational direction.

The first pump P1 is port-controlled, i.e. its expeller spaces are alternately connected to the high pressure end and to the low pressure end via a valve plate. If the expeller space is connected to low pressure in the intake stroke, that is to say when there is an increase in volume, the unit then acts as a pump, and if it is connected to the high pressure when there is an increase in volume, the unit then operates as a motor. When there is a changeover from the lifting mode into the lowering mode of the lifting cylinder HZ, the unit P1 which first operates as a pump changes its rotational direction with essentially unchanged pressure conditions, and therefore changes simultaneously into the motor mode.

The pump P2 is controlled instead of via a valve plate by virtue of the fact that each expeller space is assigned a non-return valve via which the connection to the high pressure and to the low pressure is made. Irrespective of the rotational direction, such a unit which is controlled via a non-return valve will thus always suck out of the low pressure when there is an increase in volume and feed into the high pressure when there is a decrease in volume. The pump P2 cannot operate as a motor. This dependence on the rotational direction has an advantageous effect on the present invention because the basic supply of the further loads Z1, Z2 is ensured irrespective of the lifting or lowering of the lifting cylinder.

Furthermore, provided in the working line from the pump P1 to the lifting cylinder HZ is a priority valve PRV which, depending on requirements, distributes the volume flow which flows in from the first pump P1 to the lifting cylinder HZ on the one hand, and to the further loads Z1, Z2 on the other, so that the pump P2 is correspondingly assisted. This also occurs in the same way in the lowering mode in which the priority valve distributes the volume flow which is now flowing in from the lifting cylinder HZ to the first pump P1, and, depending on requirements, also to the further loads Z1, Z2.

Provided between the priority valve PRV and the lifting cylinder HZ is a lifting valve HV which is embodied as an electrically actuated constant valve and can be adjusted between the limiting positions of “closed” and “fully opened”.

The pressure difference which occurs across the lifting valve HV is applied to the priority valve PRV. If no additional loads but rather only the lifting cylinder is to be activated, the lifting valve HV opens entirely so that there is no pressure drop across the control edge. The speed of the lifting cylinder HZ is then controlled by means of the rotational speed of the pump P1, and the priority valve PRV is not deflected from its central position in which the connection to the lifting cylinder HZ is fully opened and the branch to the further loads is interrupted. In this situation, the pump P2, which rotates at the same rotational speed, also delivers. The excess volume flow is diverted to the reservoir via the pressure balance DWP. This causes a certain loss of energy which, however, can be kept small if the pump P2 is given correspondingly small dimensions.

If only the further loads Z1, Z2 are activated, the lifting valve remains closed, as a result of which a pressure which pushes the priority valve into the left-hand position in the drawing builds up. In this position, the working line of the pump P1 is connected via the non-return valve RÜV to that of the pump P2 so that the volume flow of pump P1 is added to that of the pump P2. The further loads Z1, Z2 are as a result supplied as if by a single pump. In this case, after there is no excess volume flow which would have to flow off via the pressure balance DWP, no energy losses due to the principle occur in this mode either.

If the lifting cylinder and further loads are activated simultaneously, the lifting valve is opened only partially so that an appreciable pressure drop occurs across its control edge, said pressure drop pushing the priority valve PRV into the right-hand position in the drawing in which the pump P1 is connected both to the lifting cylinder and to the additional loads Z1, Z2 via the non-return valve RÜV, and the pump P2 is therefore assisted.

The working lines of the pumps P1 and P2 are protected with respect to the reservoir T by overpressure valves in a known way. Furthermore, for safety reasons a load-holding valve LHV, which prevents the load from sagging after the ending of the lifting process or prevents excessively fast lowering of the load, is inserted upstream of the lifting cylinder HZ.

The further loads Z1, Z2 can operate in various ways. In the exemplary embodiment in FIG. 1, a load-sensing system LSS is provided for this purpose. The division of the volume flow between the individual loads Z1, Z2 with their different pressure levels occurs in a known fashion with individual pressure balances DW1, DW2, downstream of which the directional control valves V1, V2 are connected. The changeover valve WV connects the maximum pressure to the inlet pressure balance DWP where this pressure is compared with the system pressure which is present in the working line of the pump P2.

During the operation of the hydraulic circuit according to the invention, the following four cases can now be differentiated:

• • a) Lifting, lifting cylinder pressure higher than at the further loads: in this case the pressure difference at the priority valve PRV and at the individual pressure balances DW1, DW2 of the further loads Z1, Z2 is throttled away. The power loss which occurs grows with the speed of these loads.
  • b) Lifting, lifting cylinder pressure less than at the further loads: in this case, the pressure difference is throttled away only at the priority valve PRV. The power loss which occurs grows with the speed of the lifting cylinder.
  • c) Lowering, lifting cylinder pressure higher than at the further loads: in this case the pressure difference is throttled away only at the individual pressure balances DW1, DW2 of the further loads Z1, Z2. The power loss which occurs grows with the speed of these loads. No pressure drop occurs in the priority valve.
  • d) Lowering, lifting cylinder pressure lower than at the further loads: in this case the non-return valve RÜV does not open. The speed of the lifting cylinder is defined unambiguously by the rotational speed of the pump P1. For the further loads this can mean undersupply if the second pump P2 is given such small dimensions that it is reliant on assistance from the first pump P2. However, the function of the circuit, specifically the independent operation of the lifting cylinder and further loads, is basically still provided despite a certain reduction in comfort.

In the last-mentioned case, in which only a small amount of energy is available in any case, it is possible, if appropriate, to dispense with the recovery in the lowering mode by virtue of the fact that the volume flow is diverted via a flow-regulating valve. This possibility is illustrated in FIG. 2. Apart from the load-lowering valve LSV and two pressure sensors D1, D2, this circuit is identical to that in FIG. 1. The reference symbols for the same circuit parts were retained. The previously mentioned operating state d), in which in the lowering mode the lifting cylinder pressure is lower than at the further loads Z1, Z2, is detected with the pressure sensors D1, D2. If appropriate, the volume flow is diverted from the lifting cylinder to the reservoir T via the correspondingly actuated load-lowering valve LSV during lowering.

FIG. 3 shows a further exemplary embodiment in which the additional loads Z1, Z2 are supplied via a constant flow system KSS. On the lifting drive side, the circuit is identical to that in FIG. 1. The further loads Z1, Z2 are connected in series via the valves V1, V2 and are supplied by a volume flow which is independent of the level of the load pressure. Although such a constant flow system does not provide the possibility of adapting the pressure and volume flow to the instantaneous requirements of the loads, it is of significantly simpler design than the load-sensing system illustrated in FIGS. 1 and 2. What has been stated about the method of functioning with respect to FIG. 1 applies in the same way. Likewise, the circuit for the lowering mode with a low load can be extended with a flow-regulating valve as described with respect to FIG. 2.

The proposed circuit with a motor/double pump assembly therefore permits efficient energy recovery from the potential energy of a lifting cylinder with simultaneous operation of further loads, while compared to conventional circuits the expenditure on pumps and on the valve technology is reduced.

Claims

1. Hydraulic circuit arrangement having a first pump (P1) which is driven by an electric motor (M/G) and is assigned to a lifting cylinder (HZ), wherein, in order to recover energy, the electric motor (M/G) can be operated as a generator and the first pump (P1) can be operated as a hydraulic motor in order to recover potential energy of the lifting cylinder (HZ), and having a second pump (P2) for supplying at least one further load,

characterized in that the electric motor (M/G), the first pump (P1) and the second pump (P2) are embodied as a motor/double pump assembly (MDP), wherein in the lowering mode of the lifting cylinder (HZ) the first pump (P1) acts as a hydraulic motor with reversal of the rotational direction while the second pump (P2) always operates as a pump with a constant delivery direction, and wherein arranged between the first pump (P1) and the lifting cylinder (HZ) is a priority valve (PRV) which, depending on requirements, distributes the volume flow which flows in from the first pump (P1) to the lifting cylinder (HZ) on the one hand, and to the at least one further load on the other, and the volume flow which flows in from the lifting cylinder (HZ) in the lowering mode accordingly to the at least one load and the first pump (P1).

2. Hydraulic circuit arrangement according to claim 1, in which arranged in the working line of the first pump (P1) between the priority valve (PRV) and the lifting cylinder (HZ) is an electrically controllable lifting valve (HV) which is partially opened during simultaneous operation of the lifting cylinder (HZ) and of the at least one further load, and in its two limiting positions switches the working line of the first pump (P1) to the closed and open positions, wherein the lifting valve (HZ) opens fully if only when the lifting cylinder (HZ) is activated alone.

3. Hydraulic circuit arrangement according to claim 1, in which the priority valve (PRV) is controlled by the pressure difference which occurs across the lifting valve (HV).

4. Hydraulic circuit arrangement according to claim 3, in which, in a first extreme position of the priority valve (PRV), the first pump (P1) is connected, on the one hand, to the lifting valve (HV) via a throttle and, on the other hand, to the at least one further load.

5. Hydraulic circuit arrangement according to claim 3, in which, in a second extreme position of the priority valve (PRV), the working line which comes from the first pump is switched to the open position and at the same time is connected to the at least one further load.

6. Hydraulic circuit arrangement according to claim 3, in which the priority valve (PRV) has a central position in which the working line of the first pump (P1) is switched to the open position and the line to the at least one further load is closed.

7. Hydraulic circuit arrangement according to claim 1, in which provided in the line leading from the priority valve (PRV) to the at least one further load is a non-return valve (RÜV) which opens when there is excess pressure in the working line leading from the first pump (P1) to the lifting cylinder (HZ) compared to the pressure generated by the second pump (P2).

8. Hydraulic circuit arrangement according to claim 1, in which the at least one further load is supplied by a load-sensing system (LSS).

9. Hydraulic circuit arrangement according to claim 1, in which the at least one further load is supplied by a constant flow system (KSS).

10. Hydraulic circuit arrangement according to claim 1, in which an electrically controllable load-lowering valve (LSV) is provided, and in the lowering mode said load-lowering valve (LSV) diverts the volume flow from the lifting cylinder to the reservoir (T) without the recovery of energy.

11. Hydraulic circuit arrangement according to claim 10, in which pressure sensors (D1, D2) for actuating the load-lowering valve (LSV) are respectively provided in the working lines of the first and second pumps (P1, P2).