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types of direction control valves

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Fluid Power Schematics

Directional Control ValvesFluid Power Schematics1ISO Valve Actuator Symbols

Directional Control ValvesUsed to direct the supply of oil to the actuator in a hydraulic system.The valve body is drilled, honed and heat treated. The inlet and outlet ports are drilled and threaded. The valve spools are heat treated, ground to size and polished, some chrome plated. The body and spool are then mated in assembly. After assembly the spool is the only moving part.

Three Position ValveHere we show thee ISO symbols of the three position valve. In the three position valve, the center position is the neutral or hold position. When the valve is not doing work, the valve is placed in the hold position. The valve on the top is a closed center valve. When in the hold position the center spool blocks all oil flow. The middle valve is a tandem center valve. When in the hold position ports A and B are blocked but the valve connects the pump (P) to the Tank (T).The bottom valve is an open center valve. In hold the spool connects all ports to the tank.

Direction Control Valves

5.3.1 Two-way directional control valves [64.1] 2/2-way valve (1)The 2/2-way valve has a working port A, a supply port P and a leakage-oil port L. In the case of the valve shown here, of slide design, flow from P to A is closed in the normal position.A relief line leading to the leakage-oil port is provided to prevent a build-up of pressure in the spring and piston chambers.

7[64.2] 2/2-way valve (2)The 2/2-way valve is actuated and the passage from P to A is open.

2/2-way valves are also available which are normally open from P to A.

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5.3.2 Three-way directional control valves [69.1] 3/2-way valve, poppet principle (1)The 3/2-way valve has working port A, a supply port P and a tank port T. Volumetric flow can be routed from the supply port to the working port or from the working port to the tank port. The third port in each case is closed. In the normal position shown, P is closed and flow released from A to T.

10[69.2] 3/2-way valve, poppet principle (2)The 3/2-way valve is actuated; flow is released from P to A, the outlet T is closed.3/2-way valves which are normally open from P to A and T closed are also available.

11[71.1] 3/2-way valve, slide principle (1)The 3/2-way valve has a working port A, a supply port P and a tank port T. The volumetric flow can be routed from the supply port to the working port, or from the working port to the tank port. The third port in each case is closed. In the normal position shown, P is closed and flow is released from A to T.

12[71.2] 3/2-way valve, slide principle (2)The 3/2-way valve is actuated; flow is released from P to A, and the outlet T is closed.3/2-way valves which are normally closed from P to A and T are also available.

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5.3.3 Four-way directional control valves [73.1] 4/2-way valve, two pistons (1)The 4/2-way valve has two working ports A and B, a supply port P and a tank port T. The supply port is always connected to one of the working ports, while the second working port is routed to the tank. In the normal position, there is flow from P to B and from A to T.In contrast to valves with three pistons, 4/2-way valves with two pistons do not require a leakage-oil port (see topic 74).

15[73.2] 4/2-way valve, two pistons (2)The 4/2-way valve is actuated, and there is flow from P to A and from B to T.4/2-way valves are also available which are normally open from P to A and from B to T.

16[74.1] 4/2-way valve, three pistons (1)This 4/2-way valve has two working ports A and B, a supply port P and a tank port T. The supply port is always connected to one of the working ports, while the second working port is routed to the tank. In the neutral position, there is flow from P to B and from A to T.4/2-way valves with three pistons require a leakage-oil port, since hydraulic fluid would otherwise be trapped within the valve.

17[74.2] 4/2-way valve, three pistons (2)The 4/2-way valve is actuated, and there is flow from P to A and from B to T.

18[5a] Interaction of componentsThe animations show the sequences in a basic hydraulic circuit in simplified form - the actuation and spring return of the final control element (4/2-way valve), the advance and return of the drive component (double acting cylinder) and the opening and closing of the pressure relief valve.

19[5.1a] Interaction of components (Animation)

20[5.2a] Interaction of components (Animation)

21[5.6a] Interaction of components (Animation)

22[5.7a] Interaction of components (Animation)

23[5.8a] Interaction of components (Animation)

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4-way, 3-position directional control valves

Application of 4-way, 3-position directional control valve extension iddle(brake) retraction 4/3 Direction Control Valve

[78.1] 4/3-way valve with closed mid-position (1)From the logic point of view, 4/3-way valves are 4/2- way valves with an additional mid-position. There are various versions of this mid-position (in the mid-position in the example shown, all ports are closed in the mid- position, see next illustration). In the switching position shown, there is flow from P to B and from A to T.

30[78.2] 4/3-way valve with closed mid-position (2)The 4/3-way valve is in its mid-position; all ports apart from the leakage-oil port are closed.

In this mid-position, the pump is operating against the system pressure set on the pressure relief valve.

31[78.3] 4/3-way valve with closed mid-position (3)The valve is in its left-hand switching position; there is flow from P to A and from B to T.

32[76.1] 4/3-way valve with pump bypass (1)From the logic point of view, 4/3-way valves are 4/2- way valves with an additional mid-position. There are various versions of this mid-position (in the mid-position in the example shown, the supply port P is directly connected to the tank T, see next illustration). In the switching position shown, there is flow from P to B and from A to T.

4/3-way valves are easy to construct as slide valves and of complex design as poppet valves.

33[76.2] 4/3-way valve with pump bypass (2)The 4/3-way valve is in its mid-position; there is flow from P to T, while A and B are closed. Since the output from the pump flows to the tank, this switching position is called pump bypass or also pump recirculation.

In the case of pump bypass, the pump needs to operate only against the resistance of the valve, which has a favorable effect on the power balance.

34[76.3] 4/3-way valve with pump bypass (3)The valve is in its left-hand switching position; there is flow from P to A and from B to T.

35[77a] 4/3-way valve with pump bypass (Animation)The animations show the switching of the 4/3-way valve into the three switching positions and the corresponding cylinder movements. During the advance stroke, movement can be halted by switching to the mid- position.As appropriate to the application in question, a circuit of this kind must be equipped with a brake valve to prevent damage to the installation when the valve is switched to the mid-position (see also topic 53).

36[77.1] 4/3-way valve with pump bypass (4)The circuit shows the 4/3-way valve in functional representation as a final control element of a double acting cylinder. The valve is in its mid-position; the pump delivery flows via the by-pass line within the pilot piston to the tank.The non-return valve protects the pump in cases where the piston rod of the cylinder is subject to an external load.

37[77.1a] 4/3-way valve with pump bypass (Animation)

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5.3.4 Center positions in three-position, four-way valves

a. open center b. closed center c. tandem The open-center type connects all ports together. The pump flow can return directly back to the tank at essentially atmospheric pressure, little horsepower is consumed. (pump unloaded ) The actuator can be moved freely by applying an external force.(actuator floating)

a. open center b. closed center c. tandem The closed-center design has all ports blocked. The pump flow can be used for other circuit. (not unloaded) The actuator is hydraulically locked. This means it cannot be moved by the application of an external force.(actuator braking)

a. open center b. closed center c. tandem The tandem design also unloads the pump at essentially atmospheric pressure. (pump unloaded ) It also results in a locked actuator. (actuator braking) Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only.43Directional Control DevicesTypical two-position, four-way valve

Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only.44Directional Control DevicesTypical two-position, four-way valve

Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only.45Directional Control DevicesSymbols for four-way valve center position

Check ValveThe purpose of the check valve is to readily permit oil flow in one direction but prevent oil flow in the opposite direction. The check valve is often referred as a one-way check valve.Most consist of a spring and a tapered seat, but some are free floating. For the valve on the left, when the pump oil overcomes the spring tension the seat opens and allows oil to flow. The valve on the right, when the pressure on the implement side becomes less than the pump side the seat will open allowing oil flow.

Check Valve ISO Symbols

Fluid Power SchematicsCheck Valves

48[83.1] Non-return valve (1)Non-return valves block flow in one direction a