A VALVE EQUIPPED WITH COMPENSATED VALVE STEM AND METHOD FOR CONTROLLING A VALVE

The invention relates to a method for controlling a hydraulic valve which is controlling to a hydraulic system belonging actuator, as a cylinder or a hydraulic motor, whereby said valve is controlled by means of pressure compensated valve stem, which belongs and is connected to said valve, and further, the control is brought to the pressure compensated stem by means of load sensing transmitting pressure channels by conveying their pressure to a spring loaded pressure space of the compensated valve stem and the pump of the hydraulic system is controlled by means of pressure existing in the load sensing duct.

It is known that pressure compensated valve stem is used in hydraulic cylinders with the purpose to regulate the quantity of oil floating over the actual valve stem regardless of the load, or pressure of the beside locating blocks or of the system. Always, constant pressure difference effects over the stem, so the quantity of fed oil amount follows the stem position accurately.

The principle of known pressure compensated stems: On the other side of the pressure compensated valve stem 2, there is a spring 21, which strives to open the pressure compensated valve stem, so that the oil of the system can get to the opposite side of the stem and closing the stem against the spring. In addition, the pressure is conveyed from the load side into the spring space. Now the pressure compensated valve stem is directed into a position, where the forces are in balance and thus regulates the oil flow to the stem of the actual valve. Then, this works well, when the pressure causing the load is greater than the pressure of back flow.

If instead, the pressure of the back flow is greater than the load pressure, the present solution does not work properly. The pressure of the back flow can be greater than the pressure of the load, among other things, in cases, where the load is let downwards or its turning motion is stopped, whereby the load strives to continue turning. In these cases the load and especially its turning strives to cause a strong suction in the cylinder or in the hydraulic motor to the inlet side of fluid, when there is not time enough for the fluid to reach the space, while the piston is escaping because of the load. In these cases no load pressure comes to the spring space of the pressure compensated stem but the suction

instead. This causes closing of the pressure compensated stem against the spring. The spring again strives to open the stem and that results in strong vibration. Enough oil does not flow past the pressure compensated stem to the valve stem and the vacuum in the actuator becomes worse. When negative pressure, even a vacuum during the load lowering, generates on the other side of the cylinder piston, it causes disturbance in the system controllability. There will be less or more free piston back motion in the cylinder, which phenomena can even be dangerous. The vacuum state causes cavitation in valves and hydraulic motors and fluid boiling and foaming already by low temperature. Also the working life of the components, such as packings, hoses hydraulic motors of the equipment gets shorter.

In known solutions one has tried to get rid of the problem, among other things by means of separate suction valves, through which the side attempting to become vacuum, gets fluid from the return line. The penetration power of the valves is not always enough, but in order to work this usually still needs that the return line is kept knowingly pressurised. The disadvantage is the need of suction valves and the fact that the operating efficiency of the system drops because of pressurisation of the return line.

Further, among other things, is known load lowering valves in connection with their actuators, by means of which loads are lifted and let down, for instance in connection with double-acting cylinders of lift beams. On using a load lowering valve the load does not go down during the downward motion if there is not a certain pressure in the working pressure space in the cylinder. The load lowering valves are expensive outside mountable separate valves, which increase the costs and deteriorate the control qualities of the actuators.

By means of this invention the above problems can be removed so that the pump is controlled by means of the pressure of a separate load sensing duct, which steers said pump according to the highest pressure directed on the load, and the pressure compensated stem is steered by means of the separate, to the actuator connected canalisation pressure, which pressure there is in the gates of the actuators as the highest, in spite of whether said highest pressure is caused by the force directed to the load or force caused by the load on the return hoses.

Simply, this can be realised so that between the inlet/outlet hoses a change-counter valve is fitted, which among said hoses, steers the pressure from one of said hoses which is in highest pressure to impact into the spring loaded pressure chamber of the pressure compensated stem.

The advantage of the invention is that by means of a very simple change-counter valve the pressure compensated stem can be made to work, whether the load is lifted or let down, in other words, in which situations pressure fluid is needed in order to carry out the motion, and on the other hand, in those where pressure fluid is only let to discharge to the return line. Due to the change-counter valve, always to the spring side of the pressure compensated stem a greater pressure is conducted, either from the load side or from the return fluid side of the load, depending on which one is higher.

In the following the invention is closely disclosed with reference to the enclosed drawing where

Figure 1 presents a known hydraulic system, Figure 2 presents a hydraulic system according to the invention.

Figure 1 shows, included in the system, a pump 1, pressure line 7, pressure relief valve 34, through which the overpressure discharges into return line 26. In addition there is an pressure relief valve 9 for servo control. The described valve assembly includes a pressure end 21, valve blocks 22 and 23 as well as a return end 24. Further, pressure relief/feedback valves 4 of particular blocks, and possible suction valves 10 and counter valves 11 in the load sensing duct. For both blocks 22 and 23 a pressure compensator 2 is fitted. The pressure compensator is furnishes with spring 21. In addition to the spring space of pressure compensator 2 is lead a pilot channel, which is still controlled by the valve stem in connection either with pressure connector A or B.

The valve block 22 is connected to control the lifting cylinder 16 of the loader by means of hoses 12 and 13. The folding cylinder 17 of the loader is controlled with valve block 23, by means of hoses 14 and 15. The loader has a vertical pillar 30, a lift beam 8, folding beam 19 and load 20.

A known method shown by figure 1

Lets' assume that if the load of the loader shown in figure 1 is moved toward the base of the loader, whereby the lifting beam 18 is moved upward and the folding beam 19 is let downward.

1. The lifting beam 18 is moved upward

To the pre-control valve 5A of the valve block 22 control current is brought, which hydraulically increased moves stepless way the block stem 35 so that square 6 moves to the middle and steers the pressure through the gate A along hose 12 into cylinder 16. At the same time the gate B connects the line 13 that runs from the cylinder 6, with return line 26. Simultaneously the valve stem shuts LS line 31 to the return line 26 and the pressure caused by lifting beam 18 connects through counter valve 11 to LS channel for controlling of the production and the pressure of the pump 1. In known systems the same pressure is also steered to the side of the spring 21 of the pressure compensated stem 2. On the other side 27 of the pressure compensated stem 2 the pressure fed by the pressure compensator, which strives to close the stem, is affecting. The pressure compensated stem 2 drifts to a position, where the forces are in balance. When the pressure from the load affects in the spring space of the pressure compensated stem 2, it feeds oil to the valve stem with pressure given by the spring regardless of the counter pressure caused by the load. When to the valve stem coming pressure difference in regard to the load remains the same when the load or its motion speed changes, it is possible with the valve stem to steer accurately the quantity of oil going to the cylinder 16 and thus to regulate the speed of cylinder 16 motion. This works well, when the pressure from valve 22 to cylinder 16 is greater than the pressure of oil returning from cylinder 16 to the valve 22. It is assumed that for the lifting motion of lifting beam 18 a pressure of 180 bar is needed. With the same pressure also the pump 1 is controlled. Usually the pressure is 20 bar greater, so in this case the pressure of the system is 200 bar.

2. Lowering of the Folding Beam 19

At the same time the folding beam 19 is let down and it is assumed that weight 20 in the end of the beam 19 causes a pressure of 180 bar on the piston rod side of cylinder 17 and in hose 15. The valve block 23 is controlled so that in the valve diagram square 8 moves to the middle, whereby the pressure side gets through connector A along hose 14 connected to the piston side of the folding cylinder 17. In the same way the piston rod

side gets through hose 15 connected through B gate to the return line 26. Thus a pressure of 180 bar is directed on the B gate.

Thus, when the valve 23 stem opens, a pressure difference of 180 bar is active between B gate and the return channel. It is assumed that the return pressure of the system is 0. When the load 20 is pulling the folding beam 19 downwards, neither pressure is coming into the piston side of the folding cylinder 17 nor into the hose 14, also no pressure is coming to the spring 21 side in the pressure compensated stem 2, but vacuum is produced. This causes closing partly for the pressure compensated stem and thus is throttling oil which is going to the valve stem. Accordingly, this is not sufficient to fill the folding cylinder 17 of its piston side, so a vacuum will be formed there. This can result also in strong vibration of the pressure compensated stem. In the practice the piston side of the cylinder 17 will remain almost empty. This appears in the folding beam 19 lowering so that the folding beam stops in the lower dead point and goes on only when the cylinder 17 piston side is filled with oil.

Turning of load

The same problem appears on turning the load, when in the turning gear a hydraulic motor is used. On stopping the turning the power of load mass strives to continue the turn motion and the hydraulic motor works as a brake. The pressure of return oil from motor to valve gets greater than the pressure fed to the motor. The pressure compensated stem begins to vibrate and the motor to cavitate.

The above presented disadvantages appear in all situations, where the pressure from the valve fed to the actuator, is due to the outside negative load, smaller than the pressure oil from actuator to valve.

Valve arrangement according to the invention shown by figure 1 Folding beam 19 down :

When, at the same time together with the lifting motion of the lifting beam 18, the folding beam 19 is let downwards and assuming that the weight 20 in the beam end causes for instance a pressure of 180 bar on the cylinder 17 piston side and into the hose 15. The valve block 23 is controlled so that in the valve diagram square 8 moves to the middle, whereby the pressure side gets through the gate A along the hose 14 connected to the

piston side of the folding cylinder 17. In the same way the piston rod side 15 gets connected through the gate B to the return line 26. Thus a pressure of 180 bar is directed on the gate B.

Thus, when the valve stem 23 opens between the gate B and the return channel 26, a pressure difference of 180 bar is acting. Since the return pressure to valve 23 is greater than the pressure from the valve, a change-counter valve 3 according to the invention steers this pressure to the pressure compensated stem 2 to its spring space 21. The pressure compensated stem 2 is now steered to get open by a pressure of 180 added with the spring force of the spring 21. This makes sure that the pressure compensated stem 2 is fully open and does not vibrate. The valve stem feeds oil to the gate A with system pressure, no negative pressure arises in the hoses 10.

Construction

To the spring space of the pressure compensated stem pressure is conducted from cylinder connections A or B, from the one with higher pressure. From cylinder connections A and B there is connection to the change-counter valve 3. The change-counter valve still conducts the highest pressure to the spring state 21 of the pressure compensated stem 2 and closes the pressure coming from another connector steered by the pressure compensation. The pump 1 is in its turn steered by LS pressure of such load sensing ducts, which run from the hydraulic valve and get pressure when the valve stem opens the pressure, which is formed to affect in the load sensing duct LS. Because of this, for instance, to the load sensing duct does not come such a pressure, which in case of lowering the load, would not due to the pressure caused by the load, steer greater pressure or volume flow to the pump 1.