Hydraulic systems for a press, for example a bale press, comprise today pumps, valves, at least one cylinder, press ram and tank for the hydraulic medium, which usually is oil. The different parts are connected by hoses and pipes to bring about the desired function.

The tank usually is located above a main press cylinder, which in its turn is attached to a press ram. To the press ram is also attached a device for returning the press ram. The returning device can be, for example, at least one separate return cylinder alternatively located in the main press cylinder, which thereby is double-acting.

The oil flows from the tank via at least one valve, for rapid filling, into the primary side of the main press cylinder, while the press ram is lowered to the object to be pressed, for example a pulp bale.

The press ram produces a gradually increasing pressure against the object.

In order to create a final press thrust, oil is pumped from the tank into the primary side of the main press cylinder.

When the press ram returns, the oil is pressed from the primary side of the main press cylinder back to the tank.

When the oil is pressurized, air can be solved in the oil. When thereafter the pressure decreases, the air is released. The air in the oil has a negative effect on the hydraulic system. In order to provide for the air a possibility of diffusing out of the oil, there is a large volume of oil in the tank. The oil volume in the tank usually is dimensioned to three times the pump flow per minute. The large oil volume shall give the oil such a stay-time and such a surface exposed to the atmosphere, that air can diffuse out of the oil.

Another object of the tank is to take up variations in the oil level during the movements of the press ram.

In order to obtain pressure balance in the tank at rapid level variations, the tank is via a filter connected to the atmosphere. Large air volumes pass through the filters. The filter is close-meshed, in order to prevent particles from the air from following along into the tank and contaminate the oil. The filter, therefore, must have high flow capacity, because pressure variations in the tank shall not be permitted when the oil level rapidly changes.

The present state of art has the problem, that hydrolysis causes decomposition of sealings. The air passing through the air filter takes along moisture which is absorbed by the oil. The moisture causes hydrolisis damages on the sealings.

Although the tank is large and contains much oil, there is always a certain amount of air in the oil. Therefore, cavitation problems can arise a. o. in pumps and cylinders.

Another problem is, that the oil is disintegrated by oxidation. The oxygen oxidizing the oil originates both from the moisture and from the air in the oil.

The air filter as such also causes problems. Since it must be close-meshed, it easily gets clogged.

The large tank, which must be located above the main press cylinder so that the oil can flow into the primary side thereof, implies a high structure.

According to the present invention, the problems are solved in that a flow amplifier operating in counterphase to the main press cylinder is provided in the hydraulic system.

Owing to the flow amplifier the tank, which according to known technique is located above the main press cylinder, can be eliminated During the lowering of the press ram, the oil is pressed from the device for returning the press ram, for example the return cylinders, to the flow amplifier, which presses the oil into the primary side of the main press cylinder. The dead weight of the press ram can thereby be utilized for amplifying the filling on the secondary side of the main press cylinder, which according to the known state of art is not possible.

The flow amplifier implies, that rapid variations in the oil level, for example in a tank, are avoided. The rapid level variations could result in that moisture, air and particles from the air could get into the oil.

At least the part between the flow amplifier and primary side of the main press cylinder is closed. Since the supply of the oil to the primary side of the main press cylinder thereby takes place under pressure, a higher flow rate and smaller valve size than at the known state of art can be achieved.

According to a variant of the invention, substantially the entire hydraulic system is closed. Necessary pumps are also supplied from the closed part of the hydraulic system. As a result thereof, only a small tank is required for compensating for heat expansion and leakage oil. The total oil amount in the system can be reduced to, for example, one third of the oil volume required by known technique.

If the pumps are fed from the closed part of the hydraulic system, the oil can be de-aired and de-humidified, while substantially no new air and moisture is supplied to the oil.

The characterizing features of the invention are apparent from the attached claims.

The invention is described in greater detail in the following, with reference to the accompanying Figures 1 and 2 illustrating two embodiments used for pulp bale presses.

Fig. 1 shows a hydraulic system for a press comprising a main press cylinder 1 with a primary side 21, a press ram 2, a valve 3 and a device for returning the press ram, where the returning device is located in at least one separate return cylinder 4. The return cylinders comprise each a double-acting piston 5 with a large active piston area acting on the primary side 6 of the return cylinders and a small active piston area acting on the secondary side 7 of the return cylinders.

The hydraulic system further comprises a flow amplifier 8 with a piston arrange- ment 9, for example a double-acting piston or two connected pistons comprising a large active piston area acting on the primary side 10 of the flow amplifier and a small piston area acting on the secondary side 11 of the flow amplifier. The large piston area shall be at least 10 times and preferably 20-30 times larger than the small piston area. A main pump 12 for pumping hydraulic medium, preferably oil, at least one pump 13 for maintaining overpressure, return of oil and replacing possible leaked oil, and a water and air trap 14 working toward an atmospheric tank 15 are provided, in addition to lines 16,17,18,19 and 20, which connect the various components directly or via a valve plate 24.

Fig. 2 shows a hydraulic system for a press where, contrary to the embodiment shown in Fig. 1, the return device is located in the main press cylinder 1, which is double-acting and comprises a large active piston area acting on the primary side 21 and a small active piston area acting on the secondary side 22. There is further a line 23, which via the valve plate 24 and the line 16 connects the secondary side 22 of the main press cylinder with the secondary side 11 of the flow amplifier.

The press ram 2 in the hydraulic system according to Fig. 1 is lowered to the object to be pressed, for example a pulp bale, in that the hydraulic medium via the line 17 is pumped into the primary side 6 of the return cylinders, while the hydraulic medium via the line 16 and valve plate 24 by means of the force transferred by the pistons 5 of the return cylinders is pressed into the secondary side 11 of the flow amplifier. The press ram 2 contributes to this force by its dead weight.

The force acting on the secondary side 11 of the flow amplifier is transferred via the piston arrangement 9 of the flow amplifier to the primary side 10. The hydraulic medium is pressed via the line 19 and valve 3 from the primary side 10 of the flow amplifier into the primary side 21 of the main press cylinder.

In order to ensure that the primary side 21 of the main press cylinder will not be without hydraulic medium, the total stroke volume of the return cylinders on the secondary side 7 shall be greater than the stroke volume of the flow amplifier on the secondary side 11.

When the return cylinders 4 cannot any longer lower the press ram 2 toward the pulp bale, i. e. when a predetermined maximum pressure in the primary side 6 of the return cylinders has been achieved, the valve 3 is closed and the hydraulic medium is pumped into the primary side 21 of the main press cylinder via the line 18 while at the same time the hydraulic medium continues to be pumped into the primary side 6 of the return cylinders via the line 17.

When the press ram 2 is to be lifted, the hydraulic medium is pumped via the line 16 into the secondary side 7 of the return cylinder. The oil in the primary side 6 of the return cylinders is thereby pressed by the pistons 5 of the return cylinders via the line 17, valve plate 24 and lines 20 and 19 to the primary side 10 of the flow amplifier.

The hydraulic medium in the primary side 21 of the main press cylinder is pressed, while the press ram 2 is being lifted, via the valve 3 and line 19 to the primary side 10 of the flow amplifier.

When the press ram 2 in the hydraulic system according to Fig. 2 is lowered to the object to be pressed, for example a pulp bale, the dead weight of the press ram, via the force transferred by the small piston areas of the hydraulic cylinder, gives rise to a pressure on the secondary side 22 of the main press cylinder. The hydraulic medium in the secondary side of the main press cylinder is thereby pressed via the line 23, valve plate 24 and line 16 into the secondary side 11 of the flow amplifier.

The force acting on the secondary side 11 of the flow amplifier is transferred by the piston arrangement 9 of the flow amplifier to the primary side 10. The hydraulic medium is pressed via the line 19 and valve 3 from the primary side 10 of the flow amplifier into the primary side 21 of the main press cylinder.

In order to ensure that the primary side 21 of the main press cylinder will not be without hydraulic medium, the stroke volume of the main press cylinder on the secondary side 22 shall be greater than the stroke volume of the flow amplifier on the secondary side 11.

When the dead weight of the press ram cannot any longer lower the press ram 2 toward the pulp bale, i. e. when a predetermined maximum pressure has been achieved in the secondary side 22 of the main press cylinder, the valve 3 is closed, and the hydraulic medium is pumped into the primary side 21 of the main press cylinder via the line 18.

When the press ram 2 is to be lifted, the hydraulic medium is pumped via the line 23 into the secondary side 22 of the main press cylinder.

The hydraulic medium in the primary side 21 of the main press cylinder is pressed, while the press ram 2 is being lifted, via the valve 3 and line 19 to the primary side 10 of the flow amplifier.

It applies to both embodiments shown in Figures 1 and 2 that, in order to ensure that substantially no moisture and air shall be found in the hydraulic medium, part of the hydraulic medium is led via the water and air trap 14 for separating air and water.

If the water and air trap 14 works against atmospheric pressure, the trap 14 is connected to the atmospheric tank 15, from which the pump 13 pumps the hydraulic medium, which is substantially free of water and air, to the hydraulic system.

At the variants shown of the invention substantially the entire hydraulic system is closed and pressurized.

At one variant of the invention, only the part between the flow amplifier 8 and main press cylinder primary side 21 is closed.

Pumps required are fed at this variant from an atmospheric tank For maintaining the pressure in the closed part of the system, the pump 13 is connected to the secondary side 11 of the flow amplifier.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the claims with reference to description and drawings.