The invention relates to a system as set forth in the preamble of Claim 1, which is particularly intended for use in combination with a system using pressurized medium for employing and controlling a hoisting boom assembly.

A lifting or hoisting boom assembly for applying the method of the invention typically comprises a substantially vertical main boom and an intermediate boom pivotally mounted on the main boom. Further, a terminal boom is pivotally mounted on this intermediate boom, whereby a load, a working machine or the like can be fixed to the intermediate boom to be moved by the hoisting boom assembly. Such a hydraulic hoisting boom assembly is operated with two cylinder-piston mechanisms wherein the first cylinder-piston mechanism connecting the main boom to the intermediate boom is used for effecting a movement substantially in the vertical direction, and a substantially horizontal movement is effected in a corresponding manner by using the second cylinder-piston mechanism connecting the intermediate boom to the terminal boom in association with the first cylinder-piston mechanism. In conventional hoisting boom assemblies, a cylinder- piston mechanism is controlled using a control unit provided with separate operating valves via a hydraulic medium pressurized in a pressure unit and conveyed by independent primary and secondary lines connected to the operating valves.

A hoisting boom assembly of the above-mentioned type is manoeuvred in a way that e.g. when the load is to be moved towards the main boom, the first cylinder-piston mechanism is used for pivoting the intermediate boom upwards and simultaneously the free end of the terminal end is descended with the second cylinder-piston mechanism correspond¬ ingly, wherein the manoeuvre is naturally made in reverse order for moving the load away from the main boom. The weight of the load will induce a certain amount of pressure in each cylinder-piston mecha¬ nism. For carrying the load away from the main boom, no pressure from the pressure unit will be required for controlling the first cylinder-piston mechanism because of the effect of gravity, whereas full pressure will

be required for controlling the second cylinder piston mechanism. Thus, in a hydraulic system where the cylinder-piston mechanisms are linked to the hydraulic system with fully independent flow channels according to the prior art, the operating pressure will always be determined by the maximum pressure required. Consequently, if both of the cylinder-pis¬ ton mechanisms are equal in size, half of the energy produced by the pressure unit will always be turned into dissipation heat. Further, the cylinders are very easily subject to cavitation. This is due to the fact that the pressure caused by the load and by the mass of the hoisting boom assembly itself directs a strong flow to the return ports of the cylinder- piston mechanisms, and it is not possible to provide a pressurized medium replacement through the valve arrangements to the inlets at a sufficient rate. Moreover, it is difficult to use the hoisting boom assem¬ bly for moving a load or the like particularly in the horizontal direction, because simultaneous use of both cylinder-piston mechanisms is nec¬ essary, and these are controlled separately but simultaneously with two operating valves.

The purpose of the method of the invention is to provide a decisive improvement to the problems presented above and thus to improve the prior art substantially. For achieving this purpose, the method of the invention is primarily characterized in what will be presented in the characterizing part of the appended Claim 1.

The most important general advantages of the method of the invention include its simplicity, reliability and effectiveness. Using the method, it is possible to make even very massive constructions having pressure units and cylinder-piston mechanisms with essentially smaller capaci¬ ties than conventional systems. Further, it is possible to reduce the quantity of the pressurized medium to be circulated as much as to the half of the quantity used in current systems. Moreover, particularly in hoisting boom assemblies, the forces and moments directed from the cylinder-piston mechanisms to the hoisting boom assembly are reduced significantly from the present ones; consequently, it is even possible to make the construction of the hoisting boom assembly less complex and less massive, if necessary. Thus, the invention has in this respect also the advantage of significantly better efficiency and output than current methods. Another advantage of the method according to the invention

is easier control of the pressurized medium system, wherein the appli¬ cation of the method most often does not require controlling two inde¬ pendent cylinder-piston mechanisms with separate operating valves. Also, the method of the invention makes it possible to reduce substan- tially problems of cavitation.

Advantageous embodiments of the method of the invention will be pre¬ sented in the dependent claims relating to the method.

In the following description, the invention will be described in detail with reference to the appended drawings, in which

Fig. 1 shows an advantageous operational chart relating to a pressurized medium system applying the method of the invention, and

Fig. 2 shows an advantageous hoisting boom assembly applying the method and pressurized medium system according to the invention, seen from the side.

The invention relates to a method in a pressurized medium system comprising at least two cylinder-piston mechanisms 1 , 2 (later actua¬ tors) using a pressurized medium; a pressure unit P, such as one or several pumps for pressurizing the medium; and a control unit V having a valve arrangement 4, 5 for operating said actuators 1 , 2 by conveying pressurized medium to their pressure chambers to be filled in at the time and, in a corresponding manner, by displacing pressurized medium from their pressure chambers to be emptied at the time. To improve the efficiency of the pressurized medium system particularly during simultaneous use of the actuators 1 , 2, the actuators 1 , 2 are coupled to operate in a series so that pressurized medium to be dis¬ placed is conveyed from the pressure chamber of one actuator 1 or 2 to be emptied to the pressure chamber of the other actuator 2 or 1 to be filled in. The pressurized medium is returned to the control unit V from the pressure chamber of the second actuator 2 or 1 to be emptied.

The method of the invention is advantageously applied, according to the operating principle shown in Figure 1 , in a pressurized medium

system having two actuators 1 , 2 which are controlled by two operating valves 4, 5 of the control unit V by means of pressurized medium sup¬ plied via a flow arrangement, such as primary 6a, 7a and secondary lines 6b, 7b, connected to the operating valves 4, 5. At least during simultaneous use of the actuators 1 , 2, pressurized medium is con¬ veyed by the control unit V, such as the first or second operating valves 4 or 5, to one actuator, such as the first or second pressure cylinder 1 or 2, from which the pressure medium to be displaced is further con¬ veyed to the next actuator 2 or 1 and returned to the control unit V.

In a particularly advantageous embodiment, further with reference to the principle shown in Fig. 1 , during simultaneous use of actuators 1 , 2 pressurized medium is conveyed by the control unit V, e.g. from the primary port 5A of the second operating valve 5 to the second pressure chamber C2 of the first actuator 1 to be filled up. Next, the pressurized medium is conveyed further from the first pressure chamber C1 of the first actuator 1 to be emptied to the second pressure chamber D2 of the second actuator 2 to be filled up. After this, the pressurized medium is returned from the first pressure chamber D1 of the second actuator 2 to be emptied to the control unit V, e.g. to the secondary port 5B of the second operating valve 5.

In a situation of reverse use of the pressurized medium system, during simultaneous use of actuators 1 , 2, pressurized medium is conveyed via the control unit V, i.e., from the secondary port 5B of the second operating valve 5, to the first pressure chamber D1 of the second actuator 2 to be filled up, whereafter the pressurized medium is con¬ veyed further from the second pressure chamber D2 of the second actuator 2 to be emptied to the first pressure chamber C1 of the first actuator 1 to be filled up. The pressuhzed medium is then returned from the second pressure chamber C2 of the first actuator 1 to be emptied to the control unit V, i.e., to the primary port 5A of the second operating valve 5.

In a particularly advantageous embodiment of the method, reference is made to Fig. 2 showing a pressurized medium system in connection with a so-called hoisting boom assembly. This kind of a hydraulic hoisting boom assembly X is to be placed in connection with a working

machine, such as a tractor, a forest harvester or an excavator, and in its operating situation the hoisting boom assembly comprises a sub¬ stantially vertical main boom X1 with its lower, first end connected to the working machine and its second end its journalled to the first end of intermediate boom X2 which is further, at its second end, journalled to the first end of terminal boom X3, wherein the second end of the termi¬ nal boom X3 is provided with means for fixing a load, working tool or the like to be moved with the hoisting boom assembly. The hoisting boom assembly is preferably operated with two cylinder-piston mecha- nisms 1, 2 (later actuators), wherein the first actuator 1 connecting the main boom X1 and the intermediate boom X2 is used for effecting a movement of the second end of the terminal boom X3 substantially in the vertical direction and, in a corresponding manner, the horizontal movement of the second end of the terminal boom X3 is effected by using the second actuator 2 connecting the intermediate boom X2 to the terminal boom X3 in association with the said first actuator 1. In an embodiment of this kind, the method of the invention is utilized during simultaneous use of actuators 1 , 2, i.e., for example in horizontal trans¬ fers of the end of the hoisting boom assembly, by using the displace- ment energy of the pressurized medium supplied to the actuator 1 for operating the second actuator 2 by coupling the flow channels of the flow arrangement entering them at least partly together as shown in the chart of Fig. 1; the first section 71 of the primary duct 7a of the second actuator 2 to the secondary duct 6b of the first pressure cylinder 1 for conveying pressurized medium to the pressure chamber C2, and the second section 711 of the primary duct 7a to the primary duct 6a of the first pressure cylinder 1 for connecting the pressure chamber 01 with the pressure chamber D2 of the second pressure cylinder 2. Conse¬ quently, the changes in length of the actuators 1 , 2 take place in the same direction.

In another advantageous embodiment, pressurized medium is supplied from the primary port 5A of the operating valve 5 to the pressure cham¬ ber 02 of the first actuator 1 to be filled up. In a corresponding manner, pressurized medium is removed from the pressure chamber 01 of the first actuator 1 to be emptied to the pressure chamber D2 of the second actuator 2 to be filled up, whereafter the pressurized medium is first returned from the pressure chamber D1 of the second actuator 2 to be

emptied to the secondary port 5b of the second operating valve 5 The above process corresponds for example to moving a load at the end of the hoisting boom assembly outwards, wherein the intermediate boom X2 pivots downwards and simultaneously pushes the second end of the terminal boom X3 away from the main boom X1. Upon application of the method, the weight of the load induces a substantially equal pres¬ sure on both actuators 1 , 2. Further, in this connection it is possible to use suitable actuator volumes for achieving their suitable movement relations particularly in an endeavour to facilitate substantially horizon- tal movements Thus, when using the pressurized medium system of the invention for moving a load, an optimally low pressure will be needed, because the pressure of the hoisting actuator 1 itself is used directly for operating the second actuator 2 For the same reason, also the required oil quantity is as small as possible, because oil is now supplied to the first actuator 1 only

For moving the load towards the main boom X1 , pressurized medium is supplied in a corresponding manner from the secondary port 5B of the second operating valve 5 to the pressure chamber D1 of the second actuator 2 to be filled up. Pressurized medium is removed from the pressure chamber D2 of the second actuator 2 to be emptied, in a cor¬ responding manner, to the pressure chamber C1 of the first actuator 1 to be filled up. Following this, the hydraulic fluid is returned from the pressure chamber 02 of the first actuator 1 to be emptied to the primary port 5A of the second operating valve 5. Also in this case the required operating pressure is as low as possible, because the intermediate boom X2 induces a pressure in the actuator 2, thus tending to extend the first actuator 1. According to what was presented above, the quan¬ tity of the oil to be circulated is also in this case one half of that required when using conventional methods.

As shown in Figures 1 and 2, the sole hoisting movement, i.e. the pivot¬ ing of the intermediate boom X2 upwards, can be conducted advanta¬ geously by supplying the pressurized medium through the outlet port of the first operating valve 4, i.e., through the primary port 4A, to the first pressure chamber 01 of the first actuator 1 , wherein the second pres¬ sure chamber 02 is connected with the return port of the first operating valve, i.e., with the secondary port 4B. When the second operating

valve 5 is closed, also the first pressure chamber D1 of the second actuator 2 is closed, and this second actuator 2 is prevented from moving although the pressure is effective in its second pressure cham¬ ber D2. In a corresponding manner, the sole descending motion, i.e., the pivoting of the intermediate boom X2 downwards, is conducted in the reverse order by supplying the pressurized fluid through the secon¬ dary port 4B of the first operating valve 4 to the second pressure chamber C2 of the first actuator 1 , wherein, in a corresponding manner, its first pressure chamber 01 is connected with the primary port 4A of the first operating valve. Also in this case, the second operating valve 5 is closed, and the pressure has no effect elsewhere. For balancing the operations described above, the operation chart of Fig. 1 also includes, as a conventional embodiment in this context, a so-called load lowering valve 3 for preventing any unintentional movement of the second actua- tor 2.

It is obvious that the operating valves 4 and 5 can also be used simul¬ taneously, in which case the movement of the hoisting boom assembly is a complex movement comprising dynamic components selected according to the need and produced on one hand by operations of the first operating valve 4 as described above, and on the other hand by operations of the second operating valve 5 as described above. Con¬ sequently, the method according to the invention renders it possible to make a movement in a certain direction with easy control, as well as to make the paths completely (freely/at random), if necessary. These ver¬ satile operations can be controlled with only two operation valves 4, 5.