Hydraulic Valves

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What is a Hydraulic Valve and Why Do You Need It?

A hydraulic valve properly directs the flow of a liquid medium, usually oil, through your hydraulic system. The direction of the oil flow is determined by the position of a spool. A hydraulic system can only function - as per requirements - by using valves. Thus, you should always look for the correct type of hydraulic valve to serve your intended purpose.

Hydraulic valves are available in a variety of sizes. The size required is determined by the maximum flow of the hydraulic system through the valve and the maximum pressure in the hydraulic system. Hydraulic valves are available with different mountings: e.g. mounting in pipe lines, threaded connection as cartridges, subplate mounting, etc.

How DTA Can Help You:

DTA has extensive expertise with hydraulic valves and carries a substantial inventory of valves in different sizes and spool configurations. Depending on your requirements, DTA can supply high-quality hydraulic valves, taking into account a wide variety of functional and hydraulic system requirements.

Hydraulic valves are available from several major vendors; our preferred hydraulic valve suppliers are Parker Hannifin and Denison Hydraulics. Additionally, we supply hydraulic valve parts and accessories such as sub plates, seal kits, O-rings, solenoids and hydraulic valve control electronics.

Different Types of Hydraulic Valves

Hydraulic valves are sometimes referred to as hydraulic components. These are subdivided into three main categories: directional control valves, pressure control valves and flow control valves. We have added proportional and servo control hydraulic valves as a separate category below:

Directional Control Valves

  • Check valve;
  • Directional spool valve
  • Directional poppet valve

Pressure Control Valves

  • Control task: variable throttle valve
  • Switching task: fixed throttle valve

Flow Control Valves

  • Throttle valve: flow Δp dependent
  • Control valve: flow Δp independent

Electro Hydraulic Valves

  • Servo hydraulic valve
  • Proportional hydraulic valve

Different valves function in different ways. Check valves permit free flow in one direction and block flow in the opposite direction. The directional control valve is used to pass on the pressure medium (i.e. flow) in an orderly fashion to a particular direction. Pressure control valves switch (or control) at a certain pressure; the switching pressure may be adjusted on the valve. Flow control valves regulate the flow�this is done by adjusting the size of the bores (orifices).

Note that proportional hydraulic valves and servo control valves may be used to perform a directional control, pressure control or flow control function.

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Directional Control Valves

Directional control valves can control the start, stop and change in direction of flow of a pressure medium (i.e. hydraulic oil). For this reason, they are also referred to as switching valves. Directional control valves are available as spool valves, poppet valves or rotary slide valves. Rotary slide valves, however, are no longer commonly used due to low operating pressure (up to 70 bar).

The performance of a directional control valve depends on the following:

  • Dynamic power limit (as a function of maximum flow and pressure)
  • Static power limit (adhesive force may be produced between the spool and housing due to standstill)
  • Resistance to flow (internal resistance, i.e. pressure drop)
  • Leakage (spool valves only)
  • Switching time (time between the actuating force and the completion of the stroke of the control element)

The function of a directional control valve is determined by the number of working ports (excluding control ports) and the number of spool positions. A directional control valve comprises at least two spool (switching) positions and two working ports. The most common directional control valve is a 4/3-way valve, which means the valve has four working ports and three spool positions, in accordance with DIN ISO 1219.

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Check Valve

A check valve is the simplest type of directional control valve used in hydraulic systems. Check valves stop the flow of fluid in one direction and allow free flow in the opposite direction. They are also known as non-return valves. Check valves may be used as:

  • Prefill valves (anti-cavitation)
  • Bypass valves (e.g. throttling points or return-line filters)
  • Stops for flow in one direction
  • Pre-tensioning by creating a certain backpressure
  • Protection of hydraulic components against pressure surges

Most check valves are spring-loaded and use a ball or plate to seal the flow in one direction. Check valves are designed with seats and thus are able to isolate circuits with no leakage. Balls, plates, poppets or poppets with soft seals are used as isolating elements.

There is a special type of check valve that prevents pistons or cylinder plungers from coming down and causing accidents. This is called a line rupture valve. When the line ruptures, the flow through the line rupture valve increases substantially, causing an increased pressure drop. This in turn creates a stronger force on the ball, which will close immediately.

Other special types of check valves are pilot-operated check valves and shuttle valves. A pilot-operated check valve allows flow in either direction by application of an external pilot pressure signal. A shuttle valve permits free flow at the highest operating pressure.

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Directional Spool Valve

Directional spool valves comprise a moving spool situated in the valve housing. When an actuating force moves the control spool, the annular channels in the housing are connected or separated. Directional spool valves have several unique features, such as:

  • Low cost due to simple design
  • Low actuating force (due to good pressure compensation)
  • High switching power
  • Low losses (even though oil leakage flows continuously from the high pressure to the low pressure side)
  • Wide variety of control functions

Directional spool valves may be direct-operated or pilot-operated. A direct-operated spool valve is either electrically controlled with solenoids, mechanically (e.g. manually) controlled with levers or rollers, or controlled with hydraulics or pneumatics. Whether a directional spool valve is direct- or pilot-operated depends on the actuating force needed to move the spool. Thus, this is dependent on the flow, i.e. nominal size of the directional spool valve.

When valves are operated at higher hydraulic system operating pressures, leakage losses around the spool and the housing should be taken into account, especially at system pressures over 350 bar. The leakage loss is determined by the size of the gap between the spool and the housing, the viscosity of the hydraulic fluid and the hydraulic system pressure.

Typical parameters of spool valves are:

  • Nominal sizes: NG6 and NG10 (up to NG120 for pilot-operated valves)
  • Maximum flow: up to 150 l/min (up to 7,000 l/min for pilot-operated valves)
  • Maximum pressure: up to 350 bar
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Directional Poppet Valve

Directional poppet valves can operate at a maximum operating pressure of up to 1,000 bar. Directional poppet valves are directional control valves that are fitted into housing bores with a threaded connection. This is why they are often referred to as hydraulic cartridge valves. The moveable seating elements may consist of balls, poppets or plates.

Key features of hydraulic cartridge valves:

  • No leakage
  • Long product life
  • High maximum and continuous operating pressures due to zero leakage losses
  • Very good sealing characteristics without additional isolating elements

The design permits the valve to become more tightly sealed when operating pressure increases. The maximum flow of cartridge valves is limited compared to spool valves. Larger flows would require larger cartridges to fit into housing bores; the threaded connection will not hold.

Typical parameters of poppet valves are:

  • Nominal size: NG6 and NG10, SAE, and ¼" - 2" BSP
  • Maximum flow: up to 60 l/min (up to 400 l/min for pilot-operated valves)
  • Maximum pressure: up to 1,000 bar
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Pressure Control Valves

Every pressure control valve switches (or controls) at a predetermined pressure setting. The switching pressure is generally a variable setting on the valve itself. A change in position of the pressure control valve (i.e. open or closed) occurs either gradually (by control) or suddenly (by switch). Pressure control valves can be subdivided into control and switching task:

Control task (variable throttle)

  • Pressure relief valves
  • Pressure reducing valves
  • Pressure drop valves
  • Pressure difference valves
  • Pressure valves with additional electric switch-off

Switching task (fixed throttle)

  • Pressure shut-off valves
  • Pressure sequence valves

Sequence valves are used to direct the flow to a secondary circuit. When the primary hydraulic circuit reaches a preset pressure, flow is permitted to the secondary circuit. Unloading valves are remotely-piloted pressure shut-off valves that direct flow to the tank when the preset pressure in a particular location of the hydraulic circuit has been reached.

The most common types of pressure control valves are the pressure relief valve and the pressure reducing valve. Pressure relief valves control the system pressure by relieving part, or all, of the flow to tank. Pressure reducing valves reduce the pressure supplied to a sub-system of a hydraulic system.

Pressure control valves may be directly or pilot operated. Larger flows require larger spools or poppets, which increases the area of the spring diameter: the spring force increases proportionally. Pilot-operated pressure control valves require only a small spring, are suitable for compact space requirements and have maximum flows of up to 650 l/min.

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Pressure Relief Valve

There are two pressure control valves: one is a pressure relief valve, and the other is a pressure reducing valve. Pressure relief valves have higher flow capacities than pressure reducing valves because pressure relief valves have to be capable of directing all flow to the tank.

Pressure relief valves are used in hydraulic systems to limit the system pressure to a specific set level. If this set level is reached, the pressure relief valve responds and feeds the excess flow from the system back to the tank.

Note: the pressure relief valve is a normally closed pressure control valve.

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Pressure Reducing Valve

In contrast to pressure relief valves, which affect the level of input pressure (i.e. pump pressure) fed into the hydraulic system, pressure reducing valves are used to influence the output pressure of the hydraulic system to a subsystem (e.g. actuator pressure).

Reducing the input pressure (primary pressure) or maintaining a particular output pressure (secondary pressure) is achieved at a set value which is below the varying pressure occurring in the main hydraulic circuit. It is thus possible to reduce the pressure in one part of the circuit to a lower level than the system pressure.

Note: the pressure control valve is normally open.

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Flow Control Valves

Flow control valves manage the flow by decreasing or increasing the opening at the throttling point. This helps to determine speed of movement for the actuators. The simplest design for a flow control valve is a needle or longitudinal slot mounted in the pipeline and connected to a screw that adjusts the opening at the throttling point.

These are called throttle valves and they are regularly used in combination with a check valve, i.e. the throttle check valve for speed control in one direction of flow. A disadvantage of throttle valves is that at varying loads a change in pressure drop will change the flow; thus, the speed of the moving actuator will also be affected. Flow control valves are divided into two types:

Throttle valves (flow depends on Δp)

  • Viscosity dependent
  • Viscosity independent

Flow control valves (independent of Δp)

  • 2-way flow control valves
  • 3-way flow control valves

The difference in pressure before and after the throttling point, i.e. the pressure drop (Δp), determines the rate of flow through the throttle valve at a particular setting. If the pressure drop over the throttling point remains the same, the flow of oil that passes the throttling point remains the same as well. This allows operation at constant speeds, regardless of the load.

Flow control valves are used to influence the speed of movement of actuators by changing the opening for the flow (decreasing or increasing it) at the throttling point. In fact, these are two flow control valves placed in series and built together. The throttling point operates at a fixed setting, but the throttle opening before the throttling point varies with pilot pressure of the load.

Note that flow dividers have a certain special standing: they divide an oncoming flow into two or more flows. Usually it is used to distribute the flow of a single pump to two or more sub-systems that have different operating pressures.

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Throttle Valve

Throttle valves come in piston and needle-valve structures, but we also know them as turn plugs with axial or radial grooves. The smothering of grooves is more accurate but provides more variability than those with piston structures. The needle throttle has one disadvantage: a great deal of power is required to move the needle only a slight amount.

The pressure difference (Δp) determines the amount of flow that passes the throttle. In systems with varying loads (and therefore varying pressures), the flow passing the throttle will always be different.

You can find non-adjustable (fixed) smothering in situations where it is needed to gauge against excessive pressure surges. Also, one can find non-adjustable throttles in control systems.

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Flow Control Valve

A pressure-compensated flow control valve has a fixed throttling flow at all pressures. The two-way pressure-compensated flow control valve is also called a series valve. The pressure reducing valve and the smothering of this valve are placed in series with each other. At a three-way pressure control valve, the pressure relief valve and the throttle in this valve are parallel to each other.

The two-way and three-way pressure compensated flow control valves are quite similar. The only difference is that in the latter type, the excess oil flow is not drained through the pump�s safety valve but through the flow control valve itself. The pump pressure for a three-way pressure compensated flow control valve is always equal to the pressure exerted by the actuator plus the spring pressure.

Note that using three-way flow control valves increases efficiency because the system pressure will never be higher than as required for moving the load. With two-way flow control valves, this is not the case because the pump keeps feeding the system with oil until the preset pressure of the pump's safety - pressure relief - valve has been reached.

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Electro Hydraulic Valves

When your hydraulic application requires a very accurate control of the flow, hydraulic valves may be equipped with advanced control electronics. This allows the use of inductive path measuring devices that monitor the position of the spool continuously to ensure optimum position of the spool.

Proportional hydraulic valves are able to control the opening to flow proportionally instead of gradually, as is the case for most standard hydraulic valves. The simplest type of proportional hydraulic valve is a variable throttle controlled by a manual lever, as illustrated below:

Proportional and servo hydraulic valves are usually classified as high-performance valves. This distinction gives an expected indication of performance, which tends to generalise the true differences between various types of servo and proportional hydraulic valves. Each type has its own unique performance characteristics either in controlling pressure or controlling flow.

Servo hydraulic valves use closed-loop control to monitor and feed back the main-stage spool position to a pilot stage (hydraulic/pneumatic) or driver (electronic). Proportional hydraulic valves, on the other hand, move the main-stage spool in direct proportion to an external piloted control signal and do not have any means of feedback within the valve.

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Servo Hydraulic Valve

The main stage spool of a servo hydraulic valve is equipped with a path measuring system for accurate adjustment. This system measures the exact position of the main-stage spool and feeds its position directly to the electronic control unit. If sworn position is not consistent, the control signal corrects the position of the main-stage spool.

Servo hydraulic valves are widely used in aerospace and industrial applications.

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Proportional Hydraulic Valve

The most common proportional hydraulic valves are directional control valves, pressure relief valves, flow controllers and adjustable throttling. Proportional hydraulic valves convert an incoming mechanical or electrical signal directly proportional to a shear mode. The movement follows a continuously incoming signal. Different types of available proportional hydraulic valves are:

  • Various directional control valves
  • Flow control valves
  • Pressure relief valves
  • Pressure reducing valves
  • Counter balance valves

Typical applications of proportional hydraulic valves include cranes and industrial applications such as injection moulding.

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