1- The directional valves
In hydraulics, the directional valve's role is to direct the oil towards the receivers. It can be "all or nothing" or proportional.
The pilot order can be:
- Mechanical by lever or cable.
The 4/3 directional valve (4 ports and 3 positions) can be:
- Open center A and B to T (called center Y or inverted chair).
- Open center from P to T (called tandem center).
- Open center (called center in H).
2- Directional valve 4/3 open center A & B to T resting
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The symbol is representative of the mechanics. We find 4 holes:
- P : Pressure.
- T : Tank.
- A & B: Directional valve Outlets.
The spool has 3 positions (item 7):
- A central position: P closed, A & B open to T.
- A straight arrows position: P to A & B to T.
- A crossed arrows position: P to B & A to T.
The spool is pulled back by 2 springs (items 11 and 10).
The control is electric, the coils are represented (item 5 and item 6).
To note : A pusher (item 8) makes it possible to force the spool manually by means of a peg. This additional command is very useful during troubleshooting. It allows to force the control of the directional valve and to quickly know if the problem is electric or hydraulic.
The pump (item1) driven by a heat engine (item 2) delivers 30l / min. The spool (item 7) of the directional valve isolates the orifice P from the other outlets. The oil from the pump accumulates and returns to the tank by raising the pressure relief valve to 200 bars observed in M1.
It can be seen that the two chambers of the jack (item 9) are decompressed in the tank by the spool (item 7) of the directional valve. The cylinder can enter or leave depending on the mechanical stresses it may suffer. It is says "floating".
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The electric coil (item 6) is powered. The spool (item 7) moves to the left, compresses the return spring (item11) and communicates the port P to the A and the port B to the T. This function corresponds to the straight arrows of the symbol.
The oil from the pump is oriented towards the large cylinder chamber. The pressure is established at the pressure of the load. We find 50 bars in M2 and 52 bars in M1. This pressure difference is due to the pressure drops of the circuit. (Friction of the oil in the piping as well as the baffles of the directional valve).
When the coil (item 6) is no longer supplied, the spring (item 11) repositions the dispenser spool (item 7) in the neutral position. (Central box of the symbol).
4- Directional valve 4/3 open center A & B to T crossed arrows
The electric coil (item 5) is powered. The spool (item 7) moves to the right, compresses the return spring (item 10) and communicates the port P to the B and the port A to the T. This function corresponds to the crossed arrows of the symbol.
The oil from the pump is directed towards the small cylinder chamber. The pressure is established at the pressure of the load. It should be noted that to return we see a pressure of 100 bars in M2 because the bearing surface of the oil is 2 times smaller. (Section ratio of the cylinder).
When the coil (item 5) is no longer supplied, the spring (item 10) repositions the dispenser spool (item 7) in the neutral position. (Central case of the symbol).
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