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Overcentre valves can be described as a type of pilot assisted relief valve, with the only difference between the two being the check valve will open fully when sufficient pressure is applied with pressure in the cylinder port being the only restrictive force, whereas the overcentre valve has to overcome force from the spring mechanism which is reduced by load pressure.
There are 3 main areas of load based functions the overcentre valve provides, which are applicable to both rotary and linear load motion. These areas comprise:
Controlling load – This involves the valve ensuring that the actuator doesn’t run ahead of the pump, thus reducing the risk of cavitation by controlling load induced energy and preventing loss of control.
Ensuring load safety – This safety measure controls movement and ensures that load is under control when a component malfunction occurs, such as a hose failure.
It also ensures that people, equipment and property remain safe when heavy machinery is used, such as a crane with a boom, which has the potential to cause substantial damage if control is lost.
Holding load – Working with the directional valve when it is situated in the neutral position, the load holding function of the overcentre valve prevents any movement of the load and also prevents leakage past the directional valve while it is the closed position.
Pilot ratios explained
When a system is in the design stage, pilot ratios are a main factor that needs to be taken into account as different systems will benefit from different pilot ratios. For example, a system that runs stable, constant loads will normally use a high pilot pressure, while a low, unstable load will benefit from a lower pilot pressure.
The pilot open pressure drop is a good measure of system performance and efficiency, as system pressure typically runs much higher than the pilot pressure needed to open the valve fully.
The two-stage overcentre valve
An addition to the overcentre valve family, the two-stage overcentre valve aims to tackle problems that long unstable booms suffer from, especially those with large capacity cylinders such as telescopic handlers which can suffer from instability issues.
Runaway conditions are encountered in these applications when pilot valves are opened too quickly, due to heavy loads on the cylinder. The two-stage overcentre valve uses two springs with the outer spring being affected by the pilot piston with the inner used as a pressure counterbalance, thus overcoming potential instability issues.
Which type of overcentre valve should you get?
When looking for the correct overcentre valve, you have to ensure you cater for the pressures the hydraulic unit will need to work with. In a system with high back pressure a standard overcentre valve would struggle, as the standard spring chamber is vented to the valve port through the poppet, this increases relief pressure and systems which use a closed centre directional valve would run into difficulties.
Valves are now available that help to combat this problem as the relief sections of these valves are not affected by back pressure and they are identical in every other way to a standard valve.
Finally, some overcentre valves come complete with an atmospheric venting feature, which can be a beneficial feature until they are used in a corrosive type atmosphere which could cause running problems, so it is always important to check system plans and positioning when deciding on the type of valve to go for.
Hydraproducts have a comprehensive selection of valves as part of their new Components Division which can be viewed here.
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 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.
In a Hydraulic System, you are most likely aware that the main system pressure is maintained by the system relief valve or even another type of pressure setting device.
The purpose of pressure reducing values is to keep the secondary pressures correct in branches of hydraulic systems.
Most pressure reducing valves are open and 2 way, this allows the pressure to flow freely until they reach further downstream where there is a set pressure. They then shift to throttle the flow in the branch.
Forces from pressure downstream are what actuates pressure reducing valves. This is what will deliver the correct working pressure by enabling a pressure drop to occur in the main spool of the valve. The way that a press-reducing valve works is that it is not a device that is either on or off. In contrast, it delivers a continual adjustment to the pressure. Keep in mind that these types of valves are the most conducive to suffering from contamination when it comes to malfunctioning.
Pressure-reducing valves can go wrong in a number of ways. Again, pressure gauges will need to be installed in order to understand what’s going wrong with one. Once this has been done, you can look for:
· A low pressure at outlet port. If this drops below what it should be, the first action to take is to check the pilot head spool and seat. Check for wear and tear which may be affecting the drain flow. Too much drain flow through this area of the valve will result in reduced pressure and therefore affect performance.
· If you find that the valve will not retain a reduced pressure setting, and the pressure is exceeding it, then check whether the pilot drain line is blocked or affected by contaminants. This will increase pressure which will result in flow to the branch circuit. It’s also possible that the main spool is stuck open due to contaminants blocking it. Again, there could be scoring of either the main spool or bore.
· If you find that you cannot adjust the value to the low pressure setting, even after turning the adjustment knob, then check whether there is wear of the spool or bore. There may even be a broken spring in the pilot head, which will mean not enough force between spool to seat in the control head.
· If there is not enough pressure at the output port, check whether the main spool is stuck in the closed position. This will result in no pressure fluid being unable to flow to the branch. Contaminants could be to blame.
Come on over to our main site for detailed engineering knowledge and info on Hydraulic Systems, we're at www.hydraproducts.co.uk.
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.
A recent blog looked at the choice of hydraulic valves offered by Hydraproducts, all of which are CETOP valves (meaning they are interchangeable with the valves used in most hydraulic equipment), but focused mainly on solenoid valves and their function. Today we will look at relief valves and the importance of their function, as well as why those working with hydraulic equipment need to understand how they work and what they are used for.
The pressure relief valve is present in hydraulic equipment to serve the basic yet vital function of limiting and relieving pressure in the system when it is too high – without this function pressure would build up and cause irreparable damage to the equipment, leading to costly replacements and also the potential for serious injury to anyone in the vicinity of the equipment, should the pressure blows. Unfortunately, this knowledge is as much as many operatives have unless they are trained or have a background in hydraulic engineering. Every time the machinery experiences a pressure issue, especially the loss of pressure, the instinctive action to take is to adjust the pressure relief valve and although this may temporarily address the issue, it is not actually fixing the root cause of the pressure issues.
When the pressure valve is tampered with by several people over the course of a week or so, and each person thinks they are “fixing” the pressure issue by adjusting the relief valve, it is easy for the valve to be restricted to a dangerous level without anyone realising, until the pressure issues continue and eventually an engineer is called in. By this time there could have been substantial damage to the equipment that cannot be seen until the machinery is opened up, not to mention the risk of explosion. Any loss of pressure in hydraulic machinery should be reported to the person responsible for the maintenance of hydraulic equipment, not just “fixed” by an operator adjusting the relief valve.
Armed with the knowledge that the pressure relief valves have probably been adjusted by someone with no training, the hydraulic engineer can check the settings and also the adjustability of the relief valve to see whether this has happened. The relief valve should then be checked to ensure it is still in working order, and replaced if it is not. Once the pressure relief valve has been looked at, it will become apparent whether this is masking a bigger problem elsewhere in the hydraulic machinery that needs addressing; most likely this will be a leak or seal issue that has been causing the drop in pressure that led to the pressure relief valve being tampered with. On returning the hydraulic machinery to working function it is then important to take the further preventative measure of educating operators about the pressure relief valve and why it should not be frequently adjusted in order to address pressure loss issues.
There are a number of purposes for having hydraulic fluid inside a hydraulic system. Of course, its main purpose is to transfer force from the hydraulic power unit to an actuator. In addition it has to:
Without any of these functions, the entire hydraulic system would not work as well as it should. This leads us to the question of ‘what would prevent any of these functions from fully operating and how can we prevent that from occurring? The biggest threat to hydraulic fluid being compromised is from particles in the fluid.
Although particles may not affect the power of the machinery, the other functions can be compromised by having particles in the fluid. These particles can impact the surface tension of the fluid and encourage microscopic leaks that can become problematic. They can get caught between surfaces that would usually be lubricated. This friction can cause damage and it can also result in an increased temperature of the fluid, which can then go onto cause further damage.
The most effective way to combat particles in hydraulic fluid is to use good quality hydraulic filters that are changed regularly. By keeping the hydraulic filter optimally operational, it’s possible to minimize contamination by the particles, and keep down problems that can occur from it.
What Won’t the Hydraulic Filter Do?
Although hydraulic filters are good, they aren’t perfect. For example, they won’t stop water from getting into the system. If water does get in, it can cause all manner of issues.
However, it’s still wise to equip your system with the best hydraulic filters that you can – the result will be less maintenance and an increased lifespan for your machinery.
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