1- The directional valve
The directional valves can direct oil to receivers (cylinders, motors ...).
There are 2 types of directional valves:
- A poppet valve: They have the distinction of being waterproof. On the other hand, their number of positions remain limited.
- With spools: They can be 3/2 - 4/3 - 6/3 ... On the other hand they are not waterproof.
The directional valve pilot can be:
- Electric.
- Hydraulic.
- Pneumatic.
- Manual (pedal, lever, cable ...)
Pilots can be:
- All or nothing (on- off)
- Or proportional.
The 4/3 type directional valve (4 ports and 3 positions) can be:
- Closed center.
- Open center from P to T (called tandem center).
- Open center (called center in H).
2- Directional valve 4/3 closed center: at rest
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The symbol is representative of the mechanics. The 4/3 directional valve means that the block has 4 holes and the spool (item 7) 3 positions.
The spool moves when the coils are energized (items 5 and 6). It resumes its initial position thanks to the return springs (items 10 and 11) located on each side of the spool.
At rest the spool is closed (P, T, A, B closed).
- P : Pressure.
- T : Tank.
- A & B: Directional valve outlets
The pump (item 1) driven by a heat engine (item 2) delivers 30l / min which are directed towards the tank by raising the pressure relief valve (item 3) at a pressure of 200 bars.
The cylinder ports A and B are closed and lock the cylinder in position.
To note : Directional valves spool are not waterproof. Despite a closed spool, it is possible that a slight leak appears between P to A & B thus causing drift of the cylinder if it is not loaded.
3- Directional valve 4/3 closed center: straight arrows
By feeding the electric coil (item 6), the spool (item 7) moves to the left.
The spool opens the path from P to A & B to T.
The oil from the pump (item 1) is oriented towards the large cylinder chamber (P to A).
The small chamber of the cylinder returns to the tank through the distributor from B to T.
The pressure is created by the load of the receiver, we find 50 bars in M2 and 52 bars in M1 (The 2 bars of difference of pressure are due to the losses of loads (friction of the oil in the piping).
4- Directional valve 4/3 closed center: crossed arrows
By feeding the electric coil (item 5), the spool (item 7) moves to the right.
It opens the passage from P to B & A to T.
The oil from the pump (item 1) is oriented towards the small cylinder chamber (P to B).
The large chamber of the cylinder returns to the tank through the distributor from A to T.
The pressure is created by the load of the receiver. At the same load, more pressure is required to retract the cylinder. In fact, the oil is applied on a surface twice as small (section 2 ratio).
We find 100 bars in M2 and 102 bars in M1. (The 2 bars of pressure difference are due to pressure drops (friction of the oil in the pipework).