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» Home » Hydraulic valves

 
Hydraulic valves
 
D&D Ricambi Srl: the leader on supply of hydraulic components and systems
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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.

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.

CHECK VALVES
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.

DIRECTIONAL SPOOL VALVES
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.

 



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.

PRESSURE RELIEF VALVES
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.

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.

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.

PROPORTIONAL VALVES
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.

   
         
 
   
         
 

 

 Hydraulic valves
  PRESSURE CONTROL VALVES
  Sequence valves
  Pressure interception valves
  Pressure relief valves
  Pressure reducing valves
  Pressure control valves
  Pressure compensating valves
  Pressure limiting valves
  High-low pressure relief valves
  SAFETY VALVES
  Absorbing valves
  Load control valves
  Pipe rupture valves
  Piloted stop valves
  Balanced safety valves
  SHUT-OFF VALVES
  Check valves
  Non return valves
  Hydraulic pilot-operated check valves
  Shuttle valves
  Drain valve
  DIRECTIONAL CONTROL VALVES
  Directional valves and solenoid valves
  Stroke end diverter valves
  FLOW CONTROL VALVES
  Differential lock valves
  Throttle valves
  Throttle check valves
  Flow regulators
  Flow dividers
  Knob flow control valves
  Flow divider valves
  MODULAR VALVES
  Bankable valves
  Hydraulic motors flangeable valves
  BLOCK VALVES
  Unidirectional valves
  Double effect pilot operated check valves
  Single effect pilot operated check valves
  Selector valves
  Hose burst valves
  COUNTERBALANCE VALVES
  Single effect overcentre valves
  Double effect overcentre valves
  Cartridge overcentre valves
  INTEGRATED GEAR PUMPS AND MOTORS VALVES
  Built-in valves
 
 Applications
  Dinamic oil applications

 

 

 

 
 
 

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