Hydraulic system

A hydraulic system with a pressure source controllable via a load sensing signal, a pressure sink, at least two working sections, each having a hydraulic consumer and a control valve with a load sensing signal connection and at least one back-pressure valve arranged in a tank pipe between the control valve and the pressure sink. In such a system the pressure required from the pump in the neutral position must be as low as possible. On the other hand, however, oscillations should not occur, when an external force acts on another hydraulic consumer in the neutral position of a control valve. In the neutral position of the control valves the load sensing signal connection is therefore connected with the pressure sink via an auxiliary tank pipe by-passing the back-pressure valve.

BACKGROUND OF THE INVENTION 
The invention concerns a hydraulic system with a pressure source 
controllable via a load sensing signal, a pressure sink, at least two 
working sections, each having a hydraulic consumer and a control valve 
with a load sensing connection, and at least one back-pressure valve 
arranged in a tank pipe between the control valve and the pressure sink. 
Such a system is known from DE 42 35 762 C2. 
In this case, the pressure source can be a pump with controllable discharge 
capacity. However, it is also possible to provide a pump followed by a 
pressure control valve. 
In many cases a proportional valve is used as control valve. In the neutral 
position of this valve the load sensing signal connection is connected 
with the tank pipe. The load sensing signal can also be called load 
pressure signal. The load sensing signal connections of all working 
sections are connected with each other via shuttle valves in a way that 
the load sensing signal with the highest pressure reaches the controllable 
pressure source. Thus the pressure source can produce the required 
pressure corresponding to the load sensing signal, also called LS-signal. 
The fact that the load sensing signal connection is connected with the 
tank pipe in the neutral position of the control valve should secure that 
without a consumption from a consumer the pump does not produce a higher 
pressure. When the control valve is in its neutral position, the hydraulic 
consumer connected with the control valve, i.e. a motor or a 
piston-cylinder unit, is not influenced and accordingly it needs no 
hydraulic output. 
However, a certain problem occurs because of the back-pressure valve. When 
a hydraulic consumer, e.g. a piston-cylinder arrangement with two working 
chambers, is loaded by an external force leading to a displacement of the 
piston of this piston-cylinder unit, one working chamber must be expanded, 
the other must be reduced. This is for instance the case with front-end 
loaders, the loaded shovel of which must be lowered. In the expanding 
working chamber there is a relatively low pressure, e.g. 0 bar. To avoid 
cavitation damages, additional hydraulic fluid should be supplied at a 
correspondingly low pressure. However, this additional supply should not 
lead to an increase of the force acting on the piston. The additional 
supply takes place through a refill valve arranged between the two working 
chambers of the consumer. To overcome the closing force of this refill 
valve, it is necessary that a certain pressure builds up on the 
corresponding side. The building-up of this pressure is secured through 
the back-pressure valve. The back-pressure, i.e. the pressure before the 
back-pressure valve, is in such cases normally fairly close to a load 
sensing pressure, thus corresponding to the load sensing signal. Due to 
the pressure drop over the refill valve, certain differences will, 
however, occur. This causes that the load sensing pressure on this 
consumer is normally lower than the back-pressure. As the higher pressure 
is always regarded as load sensing pressure, the back-pressure will be 
reported back to the pump control. This leads to an increase of the pump 
pressure. This again affects the back-pressure, which becomes smaller. 
When the back-pressure becomes smaller, the load sensing signal reassumes 
the control of the pump. Hereby the pump pressure becomes lower and the 
back-pressure becomes higher, leading to the initially described 
situation. There is a risk that the system starts oscillating and unstable 
conditions occur. 
SUMMARY OF THE INVENTION 
The task of the invention is to avoid such a situation. 
According to the invention, this task is solved in that in a hydraulic 
system as described in the introduction the load sensing signal connection 
in the neutral position of the control valve is connected to the pressure 
sink via an auxiliary tank pipe by-passing the back-pressure valve. 
Thus the load sensing signal of a control valve in the neutral position 
always has the lowest value. Undesired pressure increases of the load 
sensing signal are avoided, as a pressure increase before the 
back-pressure valve can no longer affect the load sensing signal. Thus the 
desired effect is reached: The pressure source receives a signal saying 
that the consumer, the control valve of which is in the neutral position, 
has no pressure demand. On the occurrence of external forces on another 
hydraulic consumer, however, this hydraulic consumer can be controlled so 
that the refilling of its working chamber is under control, to avoid 
cavitation damages. In this connection the back-pressure valve secures 
that hydraulic fluid displaced from another working chamber does not 
immediately flow back to the tank, but is led to the first working chamber 
again. However, as mentioned, an influencing of the load sensing signal is 
not involved in this. The fitting of an additional pipe, namely the 
auxiliary tank pipe, is relatively simple. As practically only pressures 
must be passed on in this auxiliary tank pipe, without requirement for 
large transports of fluids, the dimensions of this pipe can be kept 
correspondingly small. 
Preferably, the auxiliary tank pipe has a non-return valve closing towards 
the control valve. This secures that possibly occurring pressure 
oscillations of the pressure sink will not influence the load sensing 
signal system or a possible electrical activation of the control valves. 
In this connection it should be noted that the pressure sink is not 
absolutely kept at a pressure of 0 bar or atmospheric pressure. In some 
cases pressures of for example 2 to 6 bar can prevail. In case of cold 
hydraulic fluid there may be a temperature dependence, by which the 
pressure can be about 10 bar. However, such an influence is kept away from 
the load sensing signal connection by the non-return valve in the 
auxiliary tank pipe. 
Alternatively or additionally, the auxiliary tank pipe in a preferred 
embodiment may comprise its own pressure sink connection, which is 
separated from that of the back-pressure valve. Thus, pressure 
fluctuations, which may occur on the outlet of the back-pressure valve 
under adverse conditions, can no longer be transferred to the auxiliary 
tank pipe. When the auxiliary tank pipe has its own pressure sink 
connection, the non-return valve is no longer required in all cases. 
It is especially advantageous to arrange the control valve in a valve block 
having a through-going auxiliary tank pipe next to the through-going tank 
pipe. Normally, several valve blocks are arranged next to each other and 
flanged together side by side, by which the corresponding pipes pass 
through all valve blocks. This is especially the case with the pressure 
pipe, which is often also called pump pipe, the tank pipe, the load 
sensing pipe and, as in this case, the auxiliary tank pipe. In this case 
one single back-pressure valve will be sufficient, however, it is secured 
that a pressure build-up before the back-pressure valve can no longer 
influence the load sensing signal. 
Preferably, a refill valve arrangement is provided between the tank pipe 
and the consumer. This refill valve arrangement enables the decanting of 
hydraulic fluid from one working chamber of the hydraulic consumer to the 
other on the occurrence of external forces. In this connection, the 
back-pressure valve secures, however, that this hydraulic fluid is not 
flowing back to the tank.

DESCRIPTION OF EXAMPLES EMBODYING THE BEST MODE OF THE INVENTION 
A hydraulic system 1 has a controlled pressure source, consisting of a pump 
2 and a pressure control valve 3 arranged after the pump. The pump 2 takes 
hydraulic fluid from a tank 4 and supplies it via a pump pipe 5, branching 
between the pump 2 and the pressure control valve 3, into at least two 
working sections 6, 7. 
The working section 6 has a hydraulic consumer 8, in this case a steering 
motor. The hydraulic consumer 8 is connected with the working connections 
of a proportional valve 9. 
Via a pump branch pipe the proportional valve 9 is connected with the pump 
pipe 5. Further, the proportional valve has two tank connections 11, 12, 
which are connected with a tank pipe 14 via a tank branch pipe 13. Between 
the tank branch pipe 13 and each working connection A, B of the 
proportional valve 9 a refill valve 15, 16 is arranged. 
The proportional valve 9 has two load sensing signal connections 19, 20. 
When the proportional valve 9 is not in the neutral position, but is 
supplying hydraulic fluid to the consumer 8, one of the load sensing 
signal connections 19, 20 is connected with the working connection A, B, 
which is connected with the pump connection 10. Then this pressure is 
passed on to a load sensing signal pipe (LS-pipe) via a change-over valve 
21, which always passes on the higher of the pressures on its inputs, the 
LS-pipe being connected with a control input of the pressure control valve 
3. Thus it is possible always to control the pressure in the pump pipe 5 
in dependence of the required pressure. 
In the neutral position the two load sensing signal connections 19, 20 are 
connected with an auxiliary tank pipe 23. 
Apart from having a different consumer, 18, the working section 7 has 
exactly the same embodiment. The parts corresponding to those of the 
working section 6 are therefore provided with crossed out reference 
numbers. Thus the working connections A, B of the working section 6 
correspond to the working connections C, D of the working section 7. 
The tank pipe 14 extending through all working sections 6, 7, is connected 
with an inlet of a back-pressure valve 17, the outlet of which is 
connected with a tank connection T. 
The working section 6 has a valve block 24. The working section 7 has a 
valve block 25. A supply block 26 is flanged onto the valve block 24. The 
valve block 24 is flanged together with the valve block 25 and an end 
block 27 is flanged onto the other end of the valve block 25. Of course 
more than two working section 6, 7 can be provided. The valve blocks 24, 
25, the supply block 26 and the end block 27 are only to be understood as 
functional here. Of course, all blocks can also be placed in a common 
housing, resulting in a monoblock. Thus, the valves of several working 
sections can be placed in the same block. Naturally, this procedure also 
permits more such monoblocks to be connected, e.g. two such monoblocks, 
each with four valves (corresponding to four working sections), could be 
built together to one section with eight valves. 
The auxiliary tank pipe 23 is led through the supply block 26 with a pipe 
section 29. Thus it bypasses the back-pressure valve 17, i.e. it flows 
into tank 4 together with the outlet of the back-pressure valve 17. 
To keep interferences, which might occur on the outlet of the back-pressure 
valve 17, away from the auxiliary tank pipe 23, a non-return valve 28 is 
arranged in the pipe section 29 of the auxiliary tank pipe 23 to the tank 
T. This non-return valve 28 opens in the direction of the supply block 26. 
It can also be arranged in the supply block 26. 
When, e.g., the hydraulic consumer 18, made as a piston-cylinder-unit, is 
loaded by an external force F, by which the piston in the drawing should 
be displaced to the right, the pressure on working connection D increases 
and the pressure on working connection C decreases. When now the 
proportional valve opens correspondingly, hydraulic fluid flows through 
the working connection D and the tank connection 11' to the tank pipe 14. 
Due to the back-pressure valve 17 a pressure arises here, which will 
finally be high enough to open the refill valve 15'. Thus the hydraulic 
fluid displaced from the working chamber connected with the working 
connection D can flow into the other working chamber of the consumer 18 
via the working connection C. However, there will be no simultaneous 
pressure increase on the load sensing signal connection 19' or 20'. As 
there is no connection between the tank pipe 14 and the load sensing 
connection 19, 20 on the proportional valve 9 of the first working section 
6, there will not be any influence on the load sensing signal here either. 
Correspondingly, the pressure source 2, 3 are not activated, i.e. its 
pressure is not increased with this embodiment. The refill of the consumer 
18 can take place at a correspondingly low pressure. 
As usual, the pump pipe 5 and the tank pipe 14 are made through-going for 
all valve blocks 24, 25 arranged next to each other. In this case, the 
auxiliary tank pipe 23, 23' of the two valve blocks 24, 25 flow into an 
auxiliary tank pipe 23", which is also made through-going for all valve 
blocks 24, 25, i.e. for all working sections 6, 7. 
For reasons of clearness, the transition between the left valve block 24 
and the supply block 26 is made so that the auxiliary tank pipe 23" is not 
going direct through the supply block 26, but is connected with the tank T 
via a pipe 29 provided in the supply block 26. However, it is of course 
also possible to let the auxiliary tank pipe 23" go right through the 
supply block 26. 
FIG. 2 shows a modified embodiment of the hydraulic system 1'. The same 
parts have the same reference numbers. 
What has been changed, however, is that the pipe 29' is no longer led to 
the outlet of the back-pressure valve 17. On the contrary, it is led into 
the tank 4 via an auxiliary tank connection TH, i.e. has a separate tank 
connection. With this embodiment the non-return valve 28 can be spared. 
However, it can also be provided as an additional feature. 
The embodiment, in which the auxiliary tank pipe 29' is no longer led to 
the outlet T involves the advantage, that here a real disconnection 
between the back-pressure valve 17 and the load sensing signal takes 
place. Normally, it takes a considerable effort to make non-return valves 
tight. However, as long as some fluid can pass the non-return valve 28, 
the influence on the load sensing signal cannot be prevented. The 
embodiment according to FIG. 2, however, involves the advantage that using 
a pump with constant displacement output will give energy savings when 
idling, as this embodiment completely prevents a pressure from being built 
up in the load sensing signal system.