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Electro-hydraulic servo valves are no longer the preserve of the Aerospace and military markets, they are common place in the industrial markets, in machine tool applications. The principle of operation being the hydraulic flow output being directly proportional to the electrical input current. Valve construction commonly consists of a main spool piloted by a double nozzle flapper powered by a torque motor.
The input current is applied to the control coils, the electro-magnetic force is applied to either side of the flapper assembly via a flexure tube. As the flapper moves the nozzle opening is varied. Pilot pressure is applied to each nozzle and the pressure difference through these variable nozzles produce is applied to the ends of the main control spool.
As the main four-way spool moves it varies hydraulic flow to the control ports in a proportional amount dependent upon the spool movement. The flapper assembly is connected to the main spool via a feedback wire. The torque the movement produces is a produces a movement in the flapper, this torque becomes balanced with the electromagnetic torque produced from the input current.
This internal feedback loop ensures the main spool can be designed to give a highly proportional output characteristic.
Using this proportionality Electro-hydraulic servo valves are effective in applications such as steel mills, cargo cranes, flight simulation, construction equipment and mobile vehicles.
With the addition of a sensor such as load cell or potentiometer the actuator performance is feedback into a controller. PID or fuzzy logic closed loop control philosophies can be used to give exceptional control of this equipment and processes.
The purpose of check valves within Hydraulic Power Packs and Systems is to allow fluid to pass in one direction but to prevent it from travelling the other direction, or doing what is known as a reverse flow. The device is usually added to a pipe to prevent oil from flowing backwards. When necessary the valve will close so that all backward movement of fluid is stopped.
The hydraulic check valve has two ports. One is the inlet for the hydraulic fluid to enter and the other is an outlet. They will both operate in combination with the motor, cylinders and hydraulic pump. The valve controls the flow of fluid for the correct operation of equipment.
Hydraulic valves are available in a number of different designs. They may look like a poppet, a disc or one of the ball or plunger types. This will depend on where and how they are being used as to what style and size is used.
Most often you’ll find hydraulic check valves used in application such as braking systems, construction tools, lifting systems and other hydraulic systems. They are installed in systems where the backup of fluid could cause serious issues.
For example, if oil flowed backwards through a pipe, it could empty a hydraulic system back into the equipment reservoir. Even when the machine is turned off the hydraulic valve can prevent fluid from flowing through the system, keeping it full ready for the next time it is operated.
Dual Pilot Operated Check valves (abbreviated P.O.C), are check valves that can be opened by an external pilot pressure. Flow is blocked in one direction as per a standard in line check valve, but it can be opened when sufficient pressure from a pilot line is applied to the third port. The pressure required at the pilot port is normally only 1/3 of the pressure locked within the cylinder. This is determined by the Pilot Ratio (3:1 and 4.5:1) are normally available. They are regularly used with double acting cylinders to lock the system when pressure is switched off, either intentionally or by accident or failure. They can be fitted directly between ports on a ram or incorporated into a power manifold block or module. It is preferable to mount them directly to a ram with “hard” pipework as this increases the integrity of the device. If the pilot check is only required or desired on one side of a cylinder then it can be on the A or B sides, referred to pilot check on A or B.
Regular applications for pilot check valves are rear loading ramps on commercial vehicles. Balers and compactors where the load needs to be held while baling occurs. Security access bollards and blockers to stop the creeping down when the system is at rest. It is important top note that POC are not best suited to applications that have a load that that will over run when they are reversed.
Flow control valves regulate the flow of a fluid and take many forms:
Fixed orifice: Basically a hole in a tube or an insert that fits into the hydraulic line, restricting the amount of fluid that can pass through it for a given pressure.
Adjustable orifice: The size of the effective orifice is adjustable. Common forms are inline and barrel type where the body of the valve is twisted, needle valves for fine adjustment on low flow systems. When set the adjustment can be locked. These are regularly used on lifts or tipper applications where the load is uniform.
Pressure compensating: When a load such as a cantilever passes through an arc the system pressure can vary. This causes the speed of the cylinder to change leading to potentially undesired results. To overcome this pressure compensating valve account for changes in pressure and delivers broadly uniform flow to the hydraulic actuator. In a scissor lift a high pressure is required at the initial raise and decreases as the mechanical advantage increases. The reverse is true when lowering under gravity so a compensating flow control is suited here.
Reverse flow check: On a single acting power pack the pump and motor combination are optimized to give the desired lift speed of the hydraulic cylinder. The flow control valve has an integral bypass line that allows full flow in the out direction, through a built-in check valve. When lowering the full flow oil path is checked and forced to go through the flow restriction allowing controlled descent of the cylinder.
This consists of two valves in one block. When operating a double acting ram the extend and retract speeds will differ, due to the different fluid volumes. From our control valve full flow is permitted through in one direction whereas the other side is flow controlled and/or vice versa, in this way the different valve settings will optimize the actuator speeds. A common example of this valve configuration would be a rear door on a horsebox where the door will need to close much more slowly to prevent shock and noise.
A relief valve is an important control device in virtually every hydraulic system. They protect the overall system from generating a pressure that could cause mechanical failure. It is a mechanical valve that requires no external input other the applied pressure. When this excess pressure is relieved it re-seats to allow normal operation to resume. The most common type comprises a spring and plunger pushing onto a seat. If the pressure exceeds that of the spring force the oil is spilled to a volume usually the oil reservoir. The springs have adjustment ranges for example 20-100 bar and the valves can be housed in cartridge, module or designed directly into an aluminum or steel hydraulic manifold.
A hydraulic circuit may have multiple relief valves, one at the power pack end to protect the pump, another may be fitted onto a control valve circuit to relieve an induced load caused by external mechanical forces. If a hydraulic cylinder requires different relief valve settings on it full bore or annulus side then a dual relief valve module can be set to handle these needs. On the annulus side the area the oil is acting upon is smaller requiring higher pressures to exert the same force as the full bore side hence two relief valve settings are needed. One example of this is a hydropower generation sluice gate operation where something jammed in the gate such as log stops it closing.
Some terms associated with relief valve operation:
Overshoot: The pressure reading when a relief valve operates to bypass fluid. (It can be two times the actual setting.)
Hysteresis: The difference in pressure when a relief valve starts spilling some flow (cracking pressure) and when full flow is passing.
Stability: pressure fluctuation as the relief valve is bypassing at its set pressure.
Reseat pressure: The pressure a relief valve closes at after it has been operating.
Counterbalance valves are fundamentally a relief valve that is fitted in an application to generate back pressure in a system. They are normally used for ‘counterbalancing’ a load to stop it from running away during lowering. The valve is usually set at 30 percent higher than the pressure induced by the load.
Figure 1 Counterbalance valve circuit.
A built in check valve allows flow in the reverse direction (i.e. to by-pass the counterbalance valve when lifting the load). It should be noted that both sides of the valve will be subjected to full pressure, this is not possible on all relief valve designs. In Figure 1 the counterbalance valve has an integral check valve. When counterbalancing the return path must have a low back pressure to tank, as this will be additive to the valve setting.
The speed of a system actuator is regulated by its flow rate, with the rate of flow also determining energy transfer rates at various pressures. Hydraulic actuators respond to changes in pressure by opening or closing the valve. Automatic flow valves do not require an external power source to operate, so they use fluid pressure to enable them to work effectively.
There are a number of different types of valves which control flow rate using various different methods with the most common examples described below:
Other methods of flow control – Flow dividers are another popular technique for dividing flow between circuits and include examples such as rotary dividers and proportional flow logic valves which all essentially perform similar functions but in different ways.
To correctly measure the flow rate of hydraulic fluid, three principles need to be taken into account - Mass flow, weight flow and volumetric flow:
Weight flow – Commonly measured in Ib/sec or min, the weight flow calculates the power created
Volumetric flow – This looks at linear speeds and rotational speeds of pistons and motor shafts and is normally calculated using in3/sec or min
At Hydraproducts we stock a large range of hydraulic valves as part of our New Components Division so feel free to browse our full range here.
Is it just about “three times flow rate”?
The widespread use of the “three times flow” rule of thumb serves well but current pressures on space, economics and environmental issues warrant a closer examination of this rule.
So what are the factors to be considered?
• Hold enough oil for system function
• Sufficient surface area to dissipate heat to the surroundings
• Large enough volume so turbulence is minimized allowing entrained air to escape and contaminations to settle
• Separating the suction from the return areas.
• Access for maintenance and cleaning
• Air space conditions, pressure, dryness and cleanliness
• “Real-estate” for fitting of main system components
Basic features of a traditional oil reservoir:-
Our latest blog - containing useful technical information from https://www.hydraulicspneumatics.com/ - looks at hydraulic valves and the common differences between the various types of load control valves available as well as how these valves operate to control loads.
There are two main types of load control valve, the counterbalance and pilot-operated check valve. The counterbalance valve’s main duty is to hold suspended loads and also sort out the over-running of loads. It is also referred to as a brake valve, when it is used in conjunction with a hydraulic motor.
A big advantage counterbalance valves have over their Pilot-operated check valve rivals, is that they can control an over-running load whereas as the pilot-operated valve does not have this functionality.
The counterbalance valve deals with overrunning by preventing a load from dropping and this process is carried out when there is no pressure in the line that goes to the cap-end port of the cylinder. So, in simple terms, if you have issues with an over-running system, it is best to opt for a counterbalance valve.
The process explained…
With no pressure applied to the cylinder end containing the cap end, fluid pressure is maintained by the counterbalance valve. The pilot lines in the counterbalance valve act on the various surface areas within the valve. The common ratios of this surface area are around 3:1 to 4:1.
Assuming that the ratio used in this instance is 3:1; the counter balance and cylinder rod-ends connection line acts on a piston area located inside the valve. The pressure in the valve would have to total 1,800psi to successfully counter a spring force of the same amount - 1,800Ibs.
As only 1,500psi is produced in our example, the force is substantially lower and therefore, the valve will remain closed. In order to lower the load amount, the volume at the cap end of the cylinder must be pressurised in order for the counterbalance valve to open. This is achieved as the surface area is around 3 times more than the internal pressure acts on.
The external pilot pressure also has less work to do as it only has to exert 300Ibs of extra force added to the 1500Ibs to get the valve to open with pressure rising to 100psi.
The weight and pressure combined will allow the valve to open, thus allowing the load to lower. If the load drops too fast a pressure drop would occur in the external pilot line. An uncontrolled drop of the load would be prevented as the spool of the counterbalance valve would be partially closed.
So, to summarise our blog; if you are looking for a solution for basic load holding applications, pilot-check valves often suffice and would be a cost-effective solution. However, if you add motion control to the equation, a counterbalance valve is essential.
As part of Hydraproducts New Components Division, we now stock a range of hydraulic valves and accessories and you can find out more - click here and Visit our Hydraulic Components Page.
There are four main types of hydraulic valves that Hydraproducts offer, all of which are compliant with CETOP standards. Hydraulic valves that are recognised by CETOP are interchangeable within most types of hydraulic equipment, making it easy to find a supplier that can provide valves suitable for use in your hydraulic equipment.
Valves in hydraulic equipment control and regulate the flow of hydraulic fluid through a system, but as there are different requirements for fluid flow in certain applications it is important that the right valve is used for each purpose. Flow control valves can be configured to regulate the flow of fluid in two directions and are adjustable to allow for fine tuning of the flow rate. They control the movement of fluid in one direction, but in reverse the flow is free and cannot be regulated in the same was as it is when flowing forwards. A pilot operated check valve is used to control the flow of hydraulic fluid to a cylinder, stopping it when needed to prevent unwanted movement of the cylinder. A pressure relief valve is often used in conjunction with a pilot operated check valve or a solenoid valve to release the pressure contained by a pilot operated check valve, or to limit pressure in a control line leading to a solenoid valve. Our solenoid valves have a four-landed control spool and are used to change the direction of fluid flow. They are all rated to ISO4401-03-02-0-94 and DIN 240 340-A6 and can handle a flow rate of up to 60 litres per minute and pressures up to 320 bar. With a distribution network covering the UK, Germany, Australia, New Zealand, Finland, Lithuania and Bulgaria our hydraulic valves can be delivered to a wide range of locations very quickly.
Solenoid valves are often chosen over flow control valves for their ability to regulate the directional flow, instead of only being able to regulate in one direction. A flow control valve is suitable for applications where movement of a cylinder in one direction needs to be carefully regulated, but when it returns to the starting position no control is needed over the flow and speed of movement. They can be mounted in either direction, so it is up to the user which direction requires the control. For applications where control is needed over the flow rate in both directions (for example, in technologies that open and close gates or doors at a set speed) a solenoid valve is better as it allows for that fine control of movement and speed in both directions.
In most cases a hydraulic valve will be replaced with one of the same type, and buying from Hydraproducts ensures you will get a valve that is ideal as a replacement even if your equipment is made by a different manufacturer. Sometimes a hydraulic valve needs to be replaced with one of a different type when machine specifications or usage changes, an example being a piece of machinery that used to use a flow control valve, but where there are operational advantages to having better control over the hydraulic flow in two directions.
Choosing the right type should be easy when replacing a valve, as you can use the same type as before, but when designing a new piece of hydraulic machinery, the decision is a little harder. Understanding the basic function the machinery is required to perform is a must, but going a little further into the other functionality that may be required, or thinking about performance improvements that could be made with a different type of valve can inform a better design and result in hydraulic machinery that is more user friendly or that can fulfil more than the basic functions specified at the design stage.
The directional-control valve is one of the principle components of any hydraulic systems. In this post we look at the different types and how they operate:
Discrete valve: They are sometimes described as ‘bang-bang’ as they make a noise when they move from being fully open to being fully closed. This happens almost instantly, and is what controls the rapid acceleration and deceleration of fluid. The valves will move through several positions such as retract, extend and neutral to move the fluid. Under particular conditions, the valve will create a situation known as ‘fluid hammer’. This is when there is a sound like a hammer being used inside the hydraulic system.
Digital valve: Another valve that is more basic than the directional-control valve is the digital valve. These valves operate in the mode of either being on or off. Whereas the discrete valve will achieve their positions through the use of a spool, the digital valve will make use of a poppet, plunger or a ball to create a seal with an internal seal. The seal will ensure that there is no leakage between ports.
Check valve: A check valve is a digital directional-control valve that will allow for fluid to flow in only one direction. Fluid will be stopped from flowing in the opposite direction. They usually work by having a spring-loaded check valve which will not open unless the pressure from downstream becomes more than the spring force pressure.
Spool valve: The way the spool type valve works is through a sliding action. A spool will slide between passages opening and/or closing different flow paths as the fluid is routed from or to the work ports. Spools are adaptable to a variety of spool shifting schemes, which makes it possible for them to be used in a wide range of applications.
Metering or throttling is often required by mobile applications in order for the operator to control a load slowly. It’s in these situations that the spool may have V notches so that just a small amount of fluid can flow for a smoother and slower movement. This notch or bevel is often used in industrial equipment and can be known as a soft-shifting feature.
Ports and positions of valves in hydraulic systems
When it comes to selecting a direction control valve, the amount of ports and directional states of the valve are a key decision point. Valve ports are what make it possible for fluid to flow between components. The number of positions available refers to how many distinct flow paths the valve offers.
If we take a 4 port, 3 position spool valve as an example, the ports will operate like this; One port will receive fluid from the pump that is under pressure, and another will route that fluid back to the reservoir. The final two ports will be referred to as work ports and will send fluid back and forth to the actuator.
The advantage of using spool-type valves is that they can be shifted to 2, 3 or even more positions for routing purposes. A single valve can extend, retract or be in neutral position. It’s also possible for digital valves to provide the same functions.
When it comes to naming conventions, the USA refer to the number of ports. For example, they will say 2-way, 3-way, 4-way etc. Although international standards use the word ports, they would use the term 2 way-2 position which would be known as a 2-port, 2-position elsewhere. It may be abbreviated with 2/2.
This is the name of components such as spools, plungers and poppets. They each will apply force that shifts the elements of a valve to effect flow direction. For the best fluid system power performance, features such as timing, sequence and frequency are key factors and need to be considered and adjusted when possible.
These operators will be mechanical, electrical, electronic or pilot or may use a combination of these.
The world of hydraulic system valves is far more complex than it may initially appear, but by taking time to get to know the ins and outs of the ports and the flow, it’s possible to perfect the performance of your equipment for the ultimate results.
If you’re interested in enjoying the ease and convenience of a hydraulic power pack, contact us today.
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