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N° 21

Directional valve open center

all or nothing

Hydraulic training

 

1- The directional valves

The directional valves make it possible to direct the oil towards the receivers.

We distinguish 2 large families

     - Poppet: the advantage of this type of directional valve is to be sealed internally when closed.

     - Spool : the advantage of this type of dispenser is to have a greater number of positions.

The 4/3 directional valve is very common in hydraulic circuits.

As the name suggests, it has 4 holes and 3 positions.

Here are the models that we find most often:

     - Directional valve 4/3 closed center.

     - Directional valve 4/3 center Y.

     - Directional valve 4/3 tandem center.

     - Directional valve 4/3 center H.

Depending on the application, the center of the most appropriate directional valve is chosen.

The H center explained below brings together the advantage of tandem center and Y center.

2- Directional valve 4/3 center in H: central position

 
 

The symbol is representative of the mechanics. We find :

     - The spool (item 7) represented by the 3 boxes.

     - The return springs of the spool (items 10 & 11).

     - The electric coils (items 5 & 6).

The central box (in H) is open at rest. P, A & B communicate with T:

     - P : Pressure.

     - T : Tank.

     - A & B : outlets.

The oil from the pump (item 1) driven by a heat engine (item 2) delivers 30 l / min. All the flow of the pump returns to the tank by the center of the directional valve passing from P to T. The advantage of the center in H is that the oil does not roll (does not heat) in the pressure relief valve when the directional valve is resting. We have 2 bars in M1 (only pressure drops: oil friction in the pipework).

The cylinder (item 9) is said to be floating, since it can come in or out if a mechanical force is applied to it.

3- Directional valve 4/3 center H: straight arrows

 

By feeding the coil (item 6), the spool (item 7) moves to the left and compresses the return spring (item 11), orienting the oil from P to A and B to T, corresponding to the straight arrows box of the symbol.

The oil from the pump moves the cylinder, the pressure is created by the effort to be overcome (the load). We read 50 bars in M2 and 52 bars in M1.

The oil from the small cylinder chamber returns to the tank through the directional valve from B to T.

When the electric coil is no longer powered, the spring (item 11) returns the spool in the central position.

4- Directional valve 4/3 center H: crossed arrows

 
 
 
 
 

By feeding the coil (item 5) the spool (item 7) moves to the right and compresses the return spring (item 10). The flow rate of the pump is oriented from P to B and moves the cylinder. There are 100 bars in M2 and 102 bars in M1. The pressure is multiplied by 2 when the cylinder returns. This is due to the bearing surface of the oil which is 2 times smaller. (Section report).

The oil from the large chamber of the cylinder returns to the tank through the directional valve from A to T.

When the electric coil is no longer powered, the return spring (item 10) brings the spool (item 7) to the central position.

 

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