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N°20

Understand the directional valve

 4/3 center open from P to T

Hydraulic training

 

 

 

1- Directional valves

Directional valve can direct oil to receivers

(Cylinders, motors ...)

They fall into two types:

     - Poppet: They are used for their seals.

     - Spool: The spool design allows for a greater number of positions than the poppet design. However they are not waterproof.

On the diagrams appear 4 letters:

      - P : pressure.

      - T : tank.

     - A & B: Directional valve Outlets.

4/3 directional valve can have multiple centers:

     - Center Y: P closed, A & B open to T.

     - Closed center: P, T, A, T closed.

     - Open center from P to T, A & B closed.

     - Center in H.

 

2- Directional valve 4/3 center open from P to T at rest

 

 
 
 
 

The above directional valve is a Cetup type mounting (ISO standard).

It is mounted on a drilled block and assembled with 4 screws. The seal between the drilled block and the distributor is provided by 4 O-rings.

This directional valve has 4 holes and 3 positions (4/3).

The diagram is representative of the mechanics. The spool (item 7) is represented by 3 boxes.

The central box corresponds to the spool (item 7) in the rest position. The P communicates to the T, the A & B are closed. (This center is also called tandem center)

The spool moves with 2 electric coils (items 5 & 6). Two springs (item 10 & 11) allow the spool to return to neutral (item 7). A mechanical pusher (item 8) is used to force the spool with a peg. (Used for troubleshooting).

The pump (item 1) is driven by a heat engine (item 2). It delivers a flow rate of 30 l / min which returns to the tank through the center of the directional valve, thus avoiding pass the oil in the pressure relief valve (item 3). We will read 2 bars in M1, pressure created by the pressure drops (friction of the oil in the piping).

The cylinder is hydraulically locked since the A & B holes are closed.

To note : Directional valve spools are not waterproof. If a mechanical force is permanently applied to the cylinder (suspended load for example), the cylinder may drift. It will then use pilot check valves to block the cylinder and change the center of the directional valve (use a center Y to decompress the valves).

 

3-Directional valve 4/3 center open from P to T: crossed arrows

 

 
 
 
 

By supplying the electric coil (item 5), the spool (item 7) moves to the right and compresses the return spring (item 10). The oil from the pump (item 1) is oriented from P to B and moves the cylinder (item 9). The oil from the small chamber of the cylinder returns to the tank through the directional valve from A to T. We read 50 bars in M2,pressure created by the force of the jack (the load). We read 52 bars in M1 (50 bars + 2 bars load loss).

By cutting the power supply of the coil, the return spring (item 10) brings the spool (item 7) to the central position.

4- Directional valve 4/3 center open from P to T: straight arrows

 

 
 

By electrically feeding the coil (item 6), the spool (item 7) moves to the left. The oil from the pump is oriented from P to A. The cylinder returns and the volume of oil from the large chamber returns to the tank by the distributor passing from B to T.

We read 100 bars in M2. The pressure necessary to retract the jack is multiplied by 2 since the bearing surface of the oil is 2 times smaller (section ratio of the cylinder).

By cutting off the power supply of the coil, the return spring (item 11) returns the slide (item 7) to the central position.

 

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