BACKGROUND

The invention relates to hydraulic system, and more particularly to a balancing arrangement in a hydraulic system.

In certain kinds of hydraulic systems, such as digital hydraulic systems, the capacity on the high-pressure line and the low-pressure line varies dramati cally and flow demands can be very large. In known solution, accumulators are typ ically used for solving this issue, with independent charging mechanisms for each circuit. This requires pump capacity for each circuit, as well as accumulators. How ever, accumulators have to be very large, especially in solutions where flow de mands are particularly large, such as material handling equipment. The required size of the accumulators makes them impractical, as they require significant space and add a lot of weight and cost.

BRIEF DESCRIPTION

An object of the present invention is thus to provide a new method and an apparatus for balancing a hydraulic system. The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. Some preferred embodiments of the invention are disclosed in the dependent claims.

The solution described in this description is based on the idea of using excess capacity in either of the high- and low-pressure circuits to generate flow and pressure in the other, thus maintaining the proper balance between them at all times.

An advantage of the method and arrangement of the invention is that the space requirement is reduced, as the invention can be submersed in the hy draulic reservoir, the weight is significantly reduced, the pump capacity for the cir cuits is reduced and the energy consumption is balanced, removing peak demands.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 illustrates schematically a balancing arrangement for a hydrau- lie system; Figure 2 illustrates schematically a balancing arrangement for a hydrau lic system according to an embodiment;

Figure 3 illustrates schematically a balancing arrangement for a hydrau lic system according to another embodiment; Figure 4 illustrates schematically a balancing arrangement for a hydrau lic system according to a further embodiment;

Figure 5 illustrates schematically a balancing arrangement for a hydrau lic system according to a further embodiment;

Figure 6 illustrates a method for balancing a hydraulic system; and Figure 7 illustrates schematically a balancing arrangement for a hydrau lic system according to a further embodiment.

The drawings are for illustrative purposes only. For the sake of clarity, not all the similar features have been numbered in the figures.

DETAILED DESCRIPTION Figure 1 illustrates a balancing arrangement 1 for a hydraulic system 2 according to an embodiment. A balancing arrangement 1 for a hydraulic system 2, such as the balancing arrangement of Figure 1, comprises a first rotating unit 3 configured to be connectable to high-pressure line 4 of a hydraulic system 2. The balancing arrangement 1 further comprises a second rotating unit 5 configured to be connectable to low-pressure line 6 of the hydraulic system 2. Both the first ro tating unit 3 and the second rotating unit 5 may rotate in both directions. Thus, both the first rotating unit 3 and the second rotating unit 5 may act as pump and motor.

In the embodiments shown in connection with Figures 2, 3, 4, and 5 the first rotating unit 3 and the second rotating unit 5 are each configured to be con nected to a hydraulic fluid reservoir 7. Thus, the first rotating unit 3 is configured to be connected between the high-pressure line 4 and the hydraulic fluid reservoir 7 and the second rotating unit 5 is configured to be connected between the low- pressure line 6 and the hydraulic fluid reservoir 7 More particularly, the first ro- tating unit 3 and the second rotating unit 5 are configured to be provided in fluid connection with a hydraulic fluid reservoir 7, in such a manner that hydraulic fluid may be provided from the hydraulic fluid reservoir 7 to each rotating unit 3, 5, and similarly hydraulic fluid may be provided from each rotating unit 3,5 to a hydraulic fluid reservoir 7.

In the embodiment shown in Figure 7 the first rotating unit 3 is configured to be connectable to the high-pressure line 4 and configured to be con nectable to the low-pressure line 6. Thus, in the embodiment shown in Figure 7 the first rotating unit 3 is configured to be connectable between the high-pressure line 4 and the low-pressure line 6. In other aspects the embodiment shown in Figure 7 corresponds to the embodiment shown in Figure 2. The embodiments shown in Figures 3, 4, and 5 may also be amended such that instead of the first rotating unit 3 being configured to be connected between the high-pressure line 4 and the hy draulic fluid reservoir 7 the first rotating unit 3 may be configured to be connecta ble between the high-pressure line 4 and the low-pressure line 6. In the balancing arrangement, a displacement of the first rotating unit 3 is configured to be smaller in size than a displacement of the second rotating unit 5.

The high-pressure line 4 has a pressure and the low-pressure line 6 has a pressure lower than the pressure of the high-pressure line 4. According to an em- bodiment the pressure of the high-pressure line 4 is in the range of 240 to 350 bar. According to an embodiment the pressure of the low-pressure line 6 is in the range of 16 to 23 bar. The pressure of the hydraulic fluid reservoir 7 is lower than the pressure of the low-pressure line 6. According to an embodiment the pressure of the hydraulic fluid reservoir 7 is 5 bar or less. According to an embodiment, the first port of the first rotating unit 3 is configured to be connectable to the high-pressure line 4 and the second port is con figured to be connectable to the hydraulic fluid reservoir or to the low-pressure line 6. More particularly, when rotating, the first port having a higher pressure than the second port. According to an embodiment, the port line connection of the first and the second rotating unit does not change during operation. According to an embodiment, the port pressure hierarchy does not change during operation. Ac cording to an embodiment, the sign of torque or direction of torque produced by port pressures of the rotating units does not change during operation. Thus, if the first port is connected to a pressure higher than the second port, during operation this does not change but during operation the first port is always connected to a pressure higher than the second port.

According to an embodiment, the displacement size ratio between the first rotating unit 3 and the second rotating unit 5 may be configured to be defined on the basis of the difference in desired pressures in the high-pressure line and the low-pressure line respectively. For instance, in an embodiment, in which the high- pressure line 4 is designed to operate in a pressure range of 240 to 350 bar and the low-pressure line is designed to need a pressure in the range of 16 to 23 bar, the displacement ratio between the first rotating unit 3 and the second rotating unit 5 may thus be configured to be 15. It is clear for a person skilled in the art that this is an example provided to further describe the solution, and the balancing arrange- ment 1 and the method described in this description may be used in connection with hydraulic systems of different types and varying pressure ranges and ratios.

In the balancing arrangement 1, the second rotating unit 5 is coupled to the first rotating unit 3. More particularly, the first rotating unit 3 and the second rotating unit 5 are coupled to one another in such a manner that a pressure ratio between the high-pressure line 4 and the low-pressure line 6 resulting from the displacement ratio between the first rotating unit 3 and the second rotating unit 5 is configured to cause the rotating unit 3, 5 connectable to a line with excess capac ity to start rotating by a movement of hydraulic fluid, thereby forcing the other ro tating unit to act as a pump, until balance is restored. In other words, the displace- ment ratio between the first rotating unit 3 and the second rotating unit 5 causes the balancing arrangement to restore, without a need for additional control com ponents, a predetermined balance of the energy consumption between the high- pressure line 4 and the low-pressure line.

In other words, in a situation where there is excess capacity in the high- pressure line 4, the pressure ratio between the high-pressure line 4 and the low- pressure line 6 resulting from the displacement ratio between the first rotating unit 3 and the second rotating unit 5 is configured to cause the first rotating unit 3 con nectable to the high-pressure line to start rotating by a movement of hydraulic fluid, thereby forcing the second rotating unit 5 to act as a pump, until balance is restored. Similarly, in a situation where there is excess capacity in the low-pressure line 6, the pressure ratio between the high-pressure line 4 and the low-pressure line 6 resulting from the displacement ratio between the first rotating unit 3 and the second rotating unit 5 is configured to cause the second rotating unit 5 connect able to the low-pressure line to start rotating by a movement of hydraulic fluid, thereby forcing the first rotating unit 3 to act as a pump, until balance is restored. In a situation, where there is no excess capacity in either one of the high-pressure line 4 or the low-pressure line 6, but the energy requirement in both pressure lines 4, 6 exceeds the energy available for the hydraulic system 2, the pressure ratio be tween the high-pressure line 4 and the low-pressure line 6 of the balancing ar- rangement 1 resulting from the displacement ratio between the first rotating unit 3 and the second rotating unit 5 levels, or balances, the energy consumption between the high-pressure line 4 and the low-pressure line 6 to a ratio defined by the ratio of the displacements of the first rotating unit 3 and the second rotating unit 5.

The first rotating unit 3 and the second rotating unit 5 are preferably coupled mechanically to provide the balancing effect described above. The first ro tating unit 3 and the second rotating unit 5 may be mechanically coupled to one another in such a manner that one of the rotating units 3, 5 may be, in each situa tion, arranged to drive the other rotating unit. According to an embodiment, the second rotating unit 5 may be mechanically coupled to the first rotating unit 3 by being joined to the first rotating unit 3 by a common shaft 8. According to another embodiment, the second rotating unit 5 may be mechanically coupled to the first rotating unit 3 in such a manner that the first rotating unit 3 is connected to a first shaft and the second rotating unit 5 is connected to a second shaft and the first shaft and the second shaft are mechanically coupled to one another. Although such an embodiment is not shown in the Figures, different ways of coupling shafts to one another are widely known in the art and it is clear for a person skilled in the art how this kind of a coupling can be done.

According to an embodiment, each of the first rotating unit 3 and the second rotating unit 5 may comprise a fixed displacement hydraulic pump/motor. An example of such an embodiment is shown in Figure 2. According to other em bodiments, at least one of the first rotating unit 3 and the second rotating unit 5 may comprise a variable displacement hydraulic pump/motor. Some examples of such embodiments are shown in Figures 3 to 5. In the embodiment of Figure 3 the first rotating unit 3 comprises a variable displacement hydraulic pump/motor and the second rotating unit 5 comprises a fixed displacement hydraulic pump/motor. In the embodiment of Figure 4 the first rotating unit 3 comprises a fixed displace ment hydraulic pump/motor and the second rotating unit 5 comprises a variable displacement hydraulic pump/motor. In the embodiment of Figure 5 both the first rotating unit 3 and the second rotating unit 5 comprise a variable displacement hydraulic pump/motor. The balancing arrangements 1 may otherwise be similar to, or different from, one another.

According to an embodiment, the displacement size ratio between the first rotating unit 3 and the second rotating unit 5 may be configured to be fixed. Such an embodiment may be implemented using any one of the balancing arrange- ments 1 of Figures 1 to 5. According to another embodiment, the displacement size ratio between the first rotating unit 3 and the second rotating unit 5 may be configured to be adjustable, such that the ratio can be selected to provide a desired ratio between the pressure in the high-pressure line 4 and the low-pressure line 6, respectively. Such an embodiment may be implemented using a balancing arrange ment of Figures 1, 3, 4 or 5, for example. According to an embodiment, the balancing arrangement 1 further com prises at least one compensator 9 provided in connection with at least one of the first rotating unit 3 and the second rotating unit 5. Such a compensator 9 may be configured to protect the respective rotating unit 3, 5 from over-speeding by limit ing the amount of flow, more particularly the hydraulic fluid flow, that can pass the compensator 9 towards the rotating unit 3, 5. According to an embodiment, such as the embodiments of Figures 2 to 5, a compensator 9 maybe provided in connec tion with each of the first rotating unit 3 and the second rotating unit 5. Each com pensator 9 may be configured to protect the respective rotating unit 3, 5 from over speeding by limiting the amount of flow that can pass the compensator 9 towards the rotating unit 3, 5, namely towards the first rotating unit 3 or the second rotating unit 5 in connection with which the compensator 9 is provided. In addition to pro tecting the rotating unit 3, 5 from over-speeding, compensator(s) 9 in such embod iment may also protect the rotating unit 3,5 from exceeding its self-priming capa bility. According to an embodiment, the compensator 9 may comprise a limit ing valve limiting hydraulic fluid flow towards the rotating unit 3, 5 and a non-re- turn valve arranged in parallel with the limiting valve to provide free flow of the hydraulic fluid directed away from the rotating unit 3, 5 and preventing a by-pass of the flow towards the rotating unit 3, 5 thus directing the flow towards the rotat- ing unit 3, 5 through the limiting valve. In another embodiment, the compensator 9 may consist of a specific component providing a similar function.

It should be appreciated that while the embodiments of Figures 2 to 5 show compensators 9 provided in connection with both the first rotating unit 3 and the second rotating unit 5, this is not necessary in all embodiments. According to an embodiment, the balancing arrangement 1 may be ar ranged inside the hydraulic fluid reservoir 7. This is beneficial, since the balancing arrangement does not require extra space, which reduces space requirements, and the need for suction and discharge lines is minimized, which decreases installation costs. According to further embodiment, the balancing arrangement 1 is sub- merged inside the hydraulic fluid reservoir 7. This is particularly beneficial, since in such embodiments no suction and discharge lines are needed at all, but the suction and the discharge may be done directly from and to the hydraulic fluid in the hydraulic fluid reservoir 7.

According to another embodiment, the balancing arrangement 1 may be provided as a separate stand-alone unit. Such balancing arrangement 1 may be con- nected to a hydraulic system 2 using any known type of hoses, pipes or the like.

According to an embodiment, a hydraulic system 2 may comprise a bal ancing arrangement 1 according to any one of the embodiments described in this description and/or accompanying figures or a combination thereof, a high-pres sure line 4 connected to the first rotating unit 3, and a low-pressure line 6 con- nected to the second rotating unit 5. In other words, a balancing arrangement 1 described in this description and/or accompanying figures may be connected to the hydraulic system 2 via the high-pressure line 4 and the first rotating unit 3, as well as the low-pressure line 6 and the second rotating unit 5, respectively. In other words, the balancing arrangement 1 may be connected to the high-pressure line 4 and the low-pressure line 6 of the hydraulic system 2 via the first rotating unit 3 and the second rotating unit 5, respectively.

According to an embodiment, the hydraulic system 2 may be a digital hydraulic system. The balancing arrangement 1 and the method may be particu larly beneficial in connection with digital hydraulic systems, since imbalance be- tween available flows in the high-pressure line 4 and/or the low-pressure line 6 is common in digital hydraulic systems. In digital hydraulic systems typically the ca pacity on the high-pressure line 4 and the low-pressure line 6 varies dramatically and flow demands can be very large.

In this description, a digital hydraulic system refers to hydraulic sys- terns, in which discrete valued components, such as discrete output valve systems, are used and controlled by control code to control the output of the hydraulic sys tem. Such digital hydraulic valve systems may be formed of simple and robust valves, such as on/off valves or proportional valve, and programmed by control code to implement versatile features and functions. It is also typical for digital hy- draulic systems that the control edges of the digital hydraulic valve systems may be implemented and controlled independently from one another, while connected to one or more common pressure lines. The illustrations show a two-pressure sys tem with a high-pressure and a low-pressure line, but the invention is applicable for other types of hydraulic systems as well. According to an embodiment, the hydraulic system may be a hydraulic system of a material handling device, such as a material lifting, handling and/or moving device. According to another embodiment, the hydraulic system may be a hydraulic system of a machine, such as a fixed or mobile machine, that may be con figured to lift, handle and/or move material and/or pieces.

Figure 6 illustrates a method for balancing a hydraulic system 2. In such a method, a balancing arrangement 1 according to any one of the embodiments de scribed in this description and/or the accompanying figures, or a combination of such embodiments, may be used to balance the hydraulic system 2.

Such a method may comprise providing 61 a balancing arrangement 1 according to any one of the embodiments described in this description and/or the accompanying figures, or a combination of such embodiments. The method may further comprise providing 63 the first rotating unit 3 in connection a the high- pressure line 4 of the hydraulic system 2, providing 65 the second rotating unit 5 in connection with the low-pressure line 6 of the hydraulic system 2, and providing 67 the first rotating unit 3 in connection with a hydraulic fluid reservoir 7 or in connection with the low-pressure line 6 and providing the second rotating unit 5 in connection with a hydraulic fluid reservoir 7. The method may further comprise configuring 69 a pressure ratio between the high-pressure line 4 and the low-pres sure line 6 resulting from the displacement size ratio between the first rotating unit 3 and the second rotating unit 5 to cause the rotating unit 3, 5 connectable to the line 4, 6 with excess capacity to start rotating by a movement of hydraulic fluid, thereby forcing the other rotating unit 5, 3 to act as a pump, until balance is re stored.

A benefit of the described solutions is that the balancing arrangement 1 and the method enable maintaining a balance between the high-pressure line 4 and the low-pressure line 6, in such a way that cavitation is always prevented and en ergy recovery is improved. The first and second rotary units are of different size, more particularly displacement size, with a ratio that that may be defined by the difference in desired pressures in the high-pressure and low-pressure lines respec tively. The balancing arrangement is self-balancing and bi-directional and it de- creases, or can even eliminate, the need for accumulators for the low-pressure lines.

Figure 7 shows a high-pressure accumulator 10 in connection with the high-pressure line 4 and a low-pressure accumulator 11 in connection with the low-pressure line 6. In all the other embodiments, also, the high-pressure line 4 may be provided with a high-pressure accumulator 10 and the low-pressure line 6 may be provided with a low-pressure accumulator 11. However, because of the balancing arrangement especially the low-pressure accumulator 11 may be smaller than in prior art solutions and in some embodiments the low-pressure accumulator 11 may be totally left out. It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.