A hydraulic cylinder is a tube that produces linear actuation utilizing hydraulic pressure. Basically, the pressure of a hydraulic fluid forces a piston to move in either a pushing or pulling motion...
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This article will take an in-depth look at hydraulic valves and their advantages.
The article will bring more detail on topics such as:
This chapter will discuss what hydraulic valves are and the methods of flow control they use.
A hydraulic valve is a mechanical device that regulates the flow of the hydraulic fluid in a hydraulic system. Hydraulic systems are typically high pressure systems, ranging from 200 Bar averaging 700 Bar upwards. This means they have to be constructed from materials that can withstand these high pressures. The methods of controlling these valves are also vast. They can be controlled physically and mechanically with electrical actuation, hydraulics, and pneumatics.
The various methods of flow control used by hydraulic valves include:
Throttling flow control is when the size of the path of the fluid is adjusted so that one can vary the flow rate. In the image depicted above, varying the cross-sectional area across the valve will help vary the flow rate. Bernoulli’s principle explains this the best.
In Bernoulli’s tapered tube depicted below, varying the diameter of the pipe from d1 to d2 will increase the velocity of the fluid running through it (V1 < V2) whether the pipe is inclined or not. Increasing the tube’s velocity means the flow rate has also been increased, as shown before. So any mechanism that will vary the cross-sectional area across the valve will effectively vary the flow rate.
Pressure compensated flow control valves are designed to maintain a constant volumetric flow rate despite any pressure drops across the valve.
It will consist of a variable orifice and a mechanism that compensates for pressure loss. The fluid will flow a path as illustrated above. It enters through an inlet whose size is varied by the pressure compensator. In this example, the pressure compensator is a compensator spool. The compensator spool is spring loaded such that the resultant force from the spring, the hydraulic load and the incoming fluid will position it to open the inlet to just the right size to maintain a constant volumetric flow rate even with pressure drops in the system.
A variation of pressure compensated flow control valve is a temperature compensated flow control valve. This variation comes because sometimes the temperature of operation may rise such that set tolerances in orifices will become inaccurate. Temperature compensators are installed to cater to these variations.
This is the most basic method of fluid flow control. It consists of a drilled hole in what acts as a passage on an otherwise blocked fluid passage. When employed for flow control, it is usually put in series with the hydraulic pump.
A common adjustable flow control valve is a priority valve. Priority valves will switch flow to only a required outlet at a given time. For instance, if the pressure in a system drops to a certain extent, the priority valve will block other outlets just to supply the crucial outlet. It achieves this by a spring load that responds depending on the pressure applied.
Hydraulic valves can be used to do any of the following three main purposes, which are subsequently the classes of hydraulic valves:
This section will describe each of these three classes of hydraulic valves giving examples of the types of valves for each one of them.
In a hydraulic system, these valves are used to maintain or adjust the flow rate of the hydraulic fluid. They usually have a means to adjust the flow rate. This is usually an opening or port that is able to change the flow area and by altering that flow area, it then affects the flow rate.
A typical example would be controlling the speed of extending or retracting a hydraulic cylinder. It can sometimes be on hydraulic motors or any other hydraulic actuator. The speed of operation is directly related to the flow rate of the hydraulic fluid.
Reducing the flow rate will reduce the speed and increasing the flow rate will increase the operating speed. A greater flow rate equals a greater force acting on the piston, which also means a greater speed at which the cylinder is retracting or extending.
Flow control valves vary depending on the principle method they employ to alter flow rate. Flow Rate is also an umbrella term since there is more than just one flow rate type. There is a volumetric flow rate which is expressed as mm3/sec. This is used to compute linear speeds in hydraulic piston rods.
There is weight flow rate, measured in lb/sec generally used to compute power mainly in imperial units. There is also a mass flow rate measured in kg/sec, usually used to compute forces of inertia when decelerating or accelerating.
Since the flow control valves basically regulate the amount of fluid that passes through the valve per unit time, all flow control valves can be used for all types of flow rates. They vary by the mechanism that is employed to alter the flow rate. For example:
So any given flow control valve will operate using a specific mechanism to achieve a certain principle method that regulates the flow rate. The principle mechanism is the science concept that if employed, the flow control will be varied. The mechanism is the tool that you employ to achieve the scientific concept.
Ball valves use a mechanism of a ball that has holes in it. Anytime the holes align an input and an output, hydraulic fluid will flow in that path. As such, there are several ball valve configurations. These configurations vary with the number of inputs and outputs are linked. They can be two way, three way, or four way ball valves.
In a 2-way ball valve, the flow is just between one input and one output. Turning the ball perpendicular to the flow direction will block out the passage completely. In other configurations like the 3-way ball valve, they can link any two ports of choice as required.
Ball valve mechanisms are used as “Switches” to shut out or open the flow. They can also be used as throttle valves when you turn them only partially but they are not well recommended for throttling.
The needle pin valve is used to control flow rates with high accuracy in low pressure applications. They are used to control flow rate, especially in pressure compensated flow control.
Needle valves work with a plunger that sits on a tapered orifice to shut off the flow. Opening and closing the flow is achieved by lifting the plunger.
(A) Is the handle that is fixed to the plunger, which can also be called a stem (F). As the handle is turned, the plunger will move up and down the threads (C) while the Locknut (B) will stop it from fully unscrewing. When the plunger comes down, the tapered end or stem (I) will sit on the valve seat and that fully seals the (H) orifice. G is an inlet port, (D) is the Bonnet, and (E) is the valve housing.
The needle pin can sometimes use an electric or pneumatic actuator to turn the plunger. These can also be remote controlled, especially in a closed-loop circuit with feedback.
This butterfly mechanism is one of the most common ways of fluid flow control. It incorporates a flipped disk to either open or close a pathway. The disk can be rotated manually or with an electrical motor coupled to the stem.
Butterfly valves are a very affordable means of flow control. They are also lightweight and the disk material comes in vast materials to cater to different hydraulic fluid properties. They can be used to shutout flow as well as to throttle flow.
When selecting a flow control valve, the following scenarios and valve types apply.
|Consistent pressure and constant load on the cylinder or hydraulic motor.||Fixed Flow Control Valve (Orifice) Variable flow control|
|Varying Load on the cylinder or hydraulic motor and varying pressure in the system||Pressure compensated valve|
|Varying load on the cylinder or hydraulic motor, varying pressure in the system, and varying temperature in the system||Pressure and temperature compensated|
Pressure-control valves as their name suggests are used to regulate the fluid pressure in a hydraulic system. This is done either by making sure the system pressure does not exceed a certain set point.
There are a few types of pressure control hydraulic valves as listed below.
They keep the system pressure below a set level. They can be used to sustain upstream or downstream pressure from the valve. They also serve as protection to the equipment against pressure spikes or pulses.
These are pressure operated valves, usually come as normally closed valves to open when the fluid pressure rises to a certain level
These valves will allow free flow of the fluid into the actuator but block out reverse flow until a certain pressure is reached.
These valves will stop the pump’s flow or remove it and send it back to the tank, especially when the machine isn’t operating.
These valves will be explained in detail and their variants in the section below.
Many fluid power systems work within a set pressure limit. These limits or ranges are a function of the generated forces required to do the work by the actuator. If these are left unmonitored, there can be excessive damage to the equipment. Relief valves chip in to help safeguard to prevent machine damage and operator damage.
The pressure at which a relief valve will start to allow flow to pass is called the cracking pressure. The difference between the current pressure in the system and the cracking pressure is called pressure differential or pressure override.
The different types of relief valves are listed below:
The direct acting relief valve will have a poppet ball that is directly exposed to the pressure in the system on one end. On the other end it is connected to a spring that pushes it against the system pressure. When a direct acting valve is a normally closed one, the force exerted by the spring will be greater than that of the system.
The spring can also be adjusted in length and thus the cracking pressure can be adjusted on these valves. When the system pressure surges that it becomes greater than the force from the spring, the fluid will unseat the ball such that it opens the flow to let excess fluid flow out until the system pressure is in the accepted range.
For applications where huge flows need to be relieved but using a small pressure override or small pressure differential, pilot operated relief valves are used.
The valve works in 2 different stages. The first is called the pilot stage. This is where a small relief valve (depicted as a rod with piston above) is moved so that it actuates the main relief valve (depicted with spring above).
The main relief valve acts as normally closed, especially if the system’s pressure is below that of the force exerted by the main valve spring. It must be noted that the main relief valve is exposed to pressure at both its ends with the front having less surface area in contact with the fluid than the back.
This difference in the surface area will mean that a rise in the system fluid pressure will be multiplied on the smaller surface area (pressure is inversely proportional to the area). This allows the main relief valve to open and empty excess fluids into the tank and thus subside the otherwise surging pressures.
In hydraulic systems, for regulating secondary lower pressure, pressure reducing valves are used. These valves are normally open and they are also two way valves that shut off the flow when subjected to unwanted downstream pressure.
The two variations of pressure reducing valves are: pilot operated ones and direct acting ones.
This pressure reducing valve restricts the pressure in the secondary circuit (the circuit connected to the outlet) without paying heed to the main circuit’s pressure changes. This valve generally acts like the opposite of a pressure relief valve. It will not sense the pressure from the inlet like the pressure relief valve, but rather it will sense the pressure downstream, which is the pressure at the outlet.
In the picture above, as the pressure increases in the secondary circuit, a hydraulic force acts on area A of the valve, thus closing it partially. The spring’s force opposes the hydraulic force such that only sufficient oil flows past the valve to supply the secondary circuit at the preferred pressure. The spring setting is also adjustable.
When outlet pressure equals that of the valve setting, the valve closes. However, a small quantity of the oil bleeds from the low-pressure side of the valve. This frequently happens through an orifice in the spool, then through the spring chamber, and finally to the reservoir.
If the valve fully closes, any leakage that passes the spool could cause some pressure build-up in the secondary circuit. To escape this, a bleed passage to the reservoir keeps it opened slightly, avoiding a rise in downstream pressure above the valve setting. The drain passage returns leakage flow to the tank.
Constant-pressure-reducing valves will give off a stable pre-set pressure, irrespective of main circuit pressure, provided that the pressure in the main circuit is greater than that in the secondary.
These valves will balance the secondary circuit pressure with the force exerted by an adjustable spring that tries to open the valve. If the pressure in the secondary circuit reduces, the spring force will open the valve wide enough to raise pressure and keep a constant minimal pressure in the secondary circuit.
This valve operates by balancing the force applied by the pressure in the main circuit against the sum of the forces applied by secondary circuit pressure and the spring. Since the pressurized areas on both sides of the poppet are equal, the spring exerted a fixed reduction.
The spool is a pilot-operated, pressure-reducing valve that has balanced hydraulic pressures from downstream pressure at both ends. A light spring keeps the valve open. A small pilot relief valve will relieve the hydraulic fluid to the tank when reduced pressure reaches the pilot valve’s spring setting. This fluid flow causes a drop in the pressure across the spool. The pressure differential then shifts the spool toward its closed position against the light spring force.
The pilot valve only relieves enough fluid to position the main valve spool or poppet so that flow through the main valve equals the flow requirements of the reduced pressure circuit. When flow is no longer required in the low-pressure circuit during a portion of the cycle, the main valve closes. The high-pressure fluid leaking into the reduced-pressure section of the valve will then return to the reservoir through the pilot-operated relief valve.
In hydraulic circuits with more than one actuator, it is common to drive the actuators, such as the hydraulic cylinders, in a definite order or sequence. This outcome can sometimes be achieved by sizing cylinders according to the load they must be moving.
In many installations, the limitations in space and the force requirements govern the cylinder size required to do the job. In such scenarios, sequence valves can be utilized to actuate the cylinders in the required order.
Sequence valves are normally closed and 2-way valves. They control the sequence in which several functions in a circuit have to occur. They bear a resemblance to direct-acting relief valves but their spring chambers are usually drained externally to the reservoir, unlike internally to an outlet port, like in the case of the relief valve.
A sequence valve typically allows the pressurized fluid to flow to a second path only after an earlier priority path or duty has been concluded and satisfied. When normally closed, a sequence valve permits the hydraulic fluid to flow freely to the primary circuit, to do its first intended function until it reaches the valve’s pressure setting.
When the main function is satisfied, the pressure in the primary circuit will rise and then it is sensed in the pressure-sensing channel. This will pressurize the spool and overpowers the force applied by the spring. The spring becomes compressed, the valve spool moves, then hydraulic oil flows to the secondary circuit.
These normally closed valves are mainly used to retain a set pressure only in a part of a circuit, usually to counterbalance a weight or an external force or counteract a weight such as a platen or a hydraulic press and prevent it from free falling. The valve’s main port is directly linked to the hydraulic cylinder rod end. Then the secondary port is linked to the directional control valve. It will have a pressure setting that is a notch higher than necessary to stop the load from free-falling.
The Counterbalance valve will stop the hydraulic fluid flow from its inlet port to its outlet port until the pressure of the inlet port overpowers the spring force. When the pressurized fluid flows into the hydraulic cylinder cap, the cylinder extends, thus increasing the pressure in the rod end. Ultimately this will shift the main spool in the counterbalance valve. This then creates a path that allows the fluid to flow through the secondary port, leading to the directional control valve and finally to the reservoir. As the load gets raised, the check valve will open to allow the cylinder to retract freely.
If the cylinder force must be increased and backpressure must be relieved at the bottom of the stroke, then the counterbalance valve can be operated remotely. Counterbalance valves are typically drained internally. And when the cylinder extends, the valve has to open its secondary port that is connected to the reservoir. If the cylinder retracts, the check valve will always avoid the spool.
These valves are usually used to unload pumps. They control the pump output flow, (usually the output of a singled out pump in a system with many pumps) directly to the reservoir at a low pressure, after the pressure set point of the system has been reached.
The force applied by the spring keeps the valve closed. When a pilot signal, usually an external pilot signal, acting on the opposite end of the valve spool applies a force big enough to surpass that exerted by the spring, the valve spool shifts, directing the pump output to the reservoir at a low pressure.
The unloading valve is spring-loaded so it stays naturally in the closed position. When the system pressure overpowers the force of the adjustable spring, then the valve opens.
Unloading valves also come with a variation for pilot control to the main valve. A port through the main valve plunger allows the system pressure to act on both faces of the plunger.
A light spring plus the system pressure acting on the larger surface at the spring end of the plunger will hold the valve at a closed position.
A built-in check valve will maintain the system pressure. When system pressure drops to the set point value, the pilot valve will then close. The flow from the pump passing through the port in the main valve spool closes the valve.
Piloted unloading valve has a piston with pump pressure at both ends.
Directional control valves are third on our list of main hydraulic valve classification. These valves allow fluid flow into more than one path when the fluid is also coming from multiple paths, or even one source. They have a spool inside that regulates which path is permitted to receive or give flow. The position of the spool is the determining factor on which paths will be active. They can be electronically controlled or manually controlled.
Directional control valves have three functions:
These functions typically operate in combinations.
The are several Types of Directional Control Valves but these are the major ones:
The most common directional control valve is called the 2-way valve. A 2-way valve either allows or blocks flow. A water tap is a good example of a 2-way valve. A water tap allows or stops fluid flow by manual control.
A single acting hydraulic cylinder requires supply to it and exhaust from its port to operate. This needs a 3-way valve. It will permit fluid flow to the actuator in one position and drain the fluid from it in the other position. Some 3-way valves have a 3rd position that blocks flow in all ports.
A double acting actuator needs a 4-way valve. The valve pressurizes then exhausts two ports interdependently. A 3-position, 4-way valve stops the actuator or makes it float. A 3-position, the 4-way valve is common in hydraulic handling.
Check valves are 2-port valves. The one is for fluid to enter and the other one for fluid to leave. There are multiple types of check valves made from different materials such as polymers, metal, and rubber. The most common designs comprise a swing or flap. In the swing or flap check valve, a metal disc pivots on a hinge or trunnion to prevent reverse flow. Bigger check valves are normally of the swing or flap type. Then there are spring and ball check valves. These feature a ball that mounts in an appropriately profiled seat.
Normally, the check valve closes due to the force from the action of the spring force. When the fluid is flowing and the fluid pressure is bigger than the cracking pressure, the spool will move off of its seat and then open the valve.
Duckbill check valves utilize a rubber diaphragm which creates a normally closed valve unless +ve pressure is applied. But unlike metal swing or flap check valves, rubber duckbill check valves are pretty reliable. They do not seize, rust, or bind. Rubber check valves do not have problems with mechanical wear that is associated with the metallic ones.
An important value to note in check valves is the cracking pressure, which is the minimum upstream pressure at which the valve will operate. Usually, the check valve is intended for controlling fluid flow in one direction; therefore, it can be specified for a specific cracking pressure.
This is a solenoid controlled directional valve that is used in hydraulic systems for opening, closing, or changing the direction of the flow of the liquid. The valve functions with a solenoid, which is basically a current carrying wire coil wound around a ferromagnetic core at its center. The valve consists of many chambers that are called ports. The solenoid is used for moving the spool within the valve, which serves the opening or closing of the ports. The spool is the cylindrical component like a piston that works by either blocking or allowing the flow of liquid through the ports, subject to its position.
The valve employs solenoids denoted by X and Y in the picture above. These solenoids are placed on either side of the valve for actuation. The valve comprises a cylindrical spool denoted with Z. The spool has “land” which is the larger diameter and grooves which are the smaller diameter. The land functions to block the flow, while the groove functions to allow flow through the valve.
Cartridge valves internally control the pressure, direction, and/or flow control of the hydraulic fluid. They are a kind of inline valves, which means they act parallel to the fluid flow, and are best used when high flow rates and non-leak control are necessary.
They are regarded as bodiless valves, as they are implanted into a cavity and do not have their own integral housing. They can act as many valve types in a single cartridge. Common arrangements include relief, sequence, pilot-operated, flow control, or counterbalance. They are light in weight, offer easy installation, are inexpensive, leak-resistant, and easy to repair.
There are two major types of cartridge valves: slip in and screw mounted.
This is also called a panel type cartridge valve. It will need additional pilot control to function. This type of valve is pressed against the manifold’s cover plate and it will hold on there.
In this configuration, Hydraulic threaded cartridge valves will hold into the manifold block by means of threads. Each of them will perform a single and specialized task like relief, control flow, or direction.
Some of the advantages of hydraulic valves include:
Below is a table that summarizes advantages of cartridge valves.
|Increased power density||Smaller size|
|Several functions from a single mounting position||Lower system cost|
|Fast acting||Improved system response|
|Mounted within a manifold||Lesser chance of oil leakage|
|Soft switching||Fewer system pressure spikes|
|Higher permissible working pressures||Cost effective in the control in high flow systems|
|Low pressure drop||Reduced energy consumption|
|Large flow range||More cost effective control|
|Not sensitive to contamination||Long Service Lifespan|
|Very long holding time||Perfect for safety circuits|
|Not sensitive to water based fluids||Greater stability across all operating conditions|
|Not Sensitive to high pressure drops||Can be utilized in high flow systems and hazardous environments|
What function you want to control: this is what you would want the valve to achieve for your system. Either Controlling pressure, flow rate or changing direction.
How you want to control the function: would you like it to be electrically controlled, or automatically with the mechanical systems in the hydraulics or manually.
Hydraulic fluid type: this will mean you have to choose material of the valve that works well with your chosen hydraulic oil.
Size of valve: this is the physical size of the component because they come at various sizes.
Pressure rating: this is the maximum pressure that the valve will perform in.
Ports and connection type: this is the number of interface ports you have, number of inputs and number of outputs.
Working temperature: this is the extreme end of the working temperature for the hydraulic system.
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