Hydraulic Valves

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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 valves 
• Directional spool valves
• Directional poppet valves

Pressure control valves

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

Flow control valves

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

Electro-hydraulic valves

• Servo hydraulic valves
• Proportional hydraulic valves

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 flowthis 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 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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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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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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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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