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Valving Technology
  
Valve types and functions

Hydraulic valves can be divided into 3 groups:


1. Directional Control
2. Pressure Control
3. Flow Control



1 Directional control valves
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Advance, hold and retract

The direction of the oil flow can be controlled depending on valve type, valve positions and port functions.

Single-acting cylinder
Controlled by a 3-way, 3-position valve.
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Double-acting cylinder
Controlled by a 4-way, 3-position valve.
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2 Pressure control

Relief valve


Valving Technology The most common type of pressure control valve is the pressure relief valve. This valve is used to limit the maximum pressure in the hydraulic circuit. This valve should always be included in any hydraulic system to limit the circuit to a maximum safe pressure. When used in a system, design considerations should be made since the valve does not act
instantly. As the pressure approaches the set point the valve will at first only permit a very small amount of oil to pass. It is only when the valve opens farther that the full flow will pass through the valve.

From a practical standpoint, don’t set the relief valve with a hand pump and then use it with a power pump and vice versa. The point of operation will vary. Also because of this action, when used in application with a pressure switch, the pressure setting on the pressure switch should be set at least 500 psi (35 bar) lower than the point at which the relief valve opens. This will prevent rapid cycling of the motor on the pump because of the slight pressure loss through the relief valve. If the pressure settings must be closer than that the pressure switch should be monitoring the system pressure and a check valve should be added between the pump and the system. This will permit the pressure to bleed down on the pump through the relief and yet the check holds the pressure in the system, which is monitored by the pressure switch.

Sequence valve

Valving Technology This valve controls the order in which various branches of the hydraulic circuit operate. It sequences the order of the actions. In practice, one part of the circuit will reach a preset pressure at which point the sequence valve will open and permit oil to flow to the secondary part of the circuit. When the flow to the secondary part of the circuit begins,
the pressure in the first part of the circuit will remain at the set point permitting for example a work support to stay at its rated pressure as the swing cylinder clamps.

Enerpac sequence valves have a free flow return check meaning that there is no sequence action when the circuit is unclamping. There is however a small bias spring that will open at about 30 psi (2 bar). This will ensure a positive seal when the valve must provide sequence action in the forward direction. When multiple sequence valves are used they should be used in parallel and not in series. If used in series, these 30 psi (2 bar) bias springs will restrict the flow in an accumulative effect.

For example, if three valves are used, there would be about 3 x 30 psi = 90 psi (6 bar) of backpressure on components after the sequence valve in the system. While on a 5000 psi (350 bar) system this pressure may not seem like much, it is enough to prevent a single-acting swing from unclamping all the way or possibly cause a work support to not fully release and not properly readjust for the next part.

Pressure reducing valve

Valving Technology As the name implies, this valve will reduce the pressure to a lower value for a secondary part of the circuit. This is useful, for example, when you must reduce the capacity of a swing cylinder that might be clamping over a work support. The pressure reducing valve will automatically make-up pressure loss after the valve by permitting a very small
amount of oil to the secondary circuit.

This pressure difference from when the valve first closes to the point it re-opens for pressure make-up is referred as the “deadband” of the valve. For example, on the Enerpac pressure reducing valve, this deadband is about 5% of the system pressure. If your system pressure is 3000 psi (210 bar) and the reduced pressure is 2000 psi (140 bar), the pressure in the secondary part of the circuit would need to drop 5% of the system pressure, [3000 x .05 = 150 psi (10 bar)] before the valve would open.

In this case the secondary part of the circuit would drop to 1850 psi (127.5 bar), before the valve would open and permit oil to flow to the secondary part of the circuit to return the pressure to 2000 psi (140 bar). This valve provides this function in only one direction with free flow in the reverse direction to allow cylinders to unclamp or work supports to unlock.

Pressure limiting valve

Valving Technology This valve, like the pressure reducing valve, will limit the pressure in a secondary part of the circuit to a preset lower setting than the system pressure. This valve functions differently in that once the valve closes, the secondary part of the circuit will not receive any make-up oil for any pressure loss. The system pressure must drop to zero
pressure before the valve will open and permit oil to flow to the secondary part of the circuit. There is no pressure make-up capability with a pressure-limiting valve.

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3 Flow control

Flow control valves


Valving Technology Flow controls permit the change of speed of a hydraulic component through the use of an adjustable orifice. Unlike a regular flow control that provides the same flow restriction in both directions, these flow controls provide a free flow reverse check. This allows restricted flow in one direction and unrestricted flow in the other. This is a very
important feature when using a flow control to regulate the speed of a single-acting swing cylinder or work support. The cylinder requires the clamping speed be regulated to a safe value through the use of a flow control to prevent damage to the cylinder. When unclamping, the spring in the cylinder will develop only a small amount of pressure. To ensure rapid unclamp time, back pressure, or resistance, must be minimized. Free flow reverse checks allow you to minimize this resistance.

Pilot operated check valves

Valving Technology A check valve only permits the flow of oil in one direction. The pilot operated check valve works the same as a regular check valve but also has an additional port for a pressure signal. Pressure to this extra port will mechanically open the check valve to permit the oil to flow in both directions. The pilot operated check is useful in
holding pressure over a period of time in a remote part of a circuit, but allowing the pressure to be released using a pressure signal to the extra port on the valve. Usually this pressure is much lower than the system pressure you are holding back. Enerpac pilot operated check valves only require 15% of the system pressure you are clamping with to open the check valve, permitting the oil to return from the fixture and unclamp the part.

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