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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.
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.
In the world of industrial, mobile and aerospace equipment, hydraulic power systems are very popular. They enjoy a high power-to-weight ratio in addition to being able to be stalled, operated intermittently and even reversed. They can also accelerate fast and are quick to respond. Another attractive feature of the fluid power system is that they can be very long lasting in addition to offer reliable operation rates.
Hydraulic systems are able to work as they contain incompressible liquid. In many situations, it’s much preferred to use hydraulics to move machinery. For one, fluid systems do not produce the same amount of wear as a dryer method would. It also does not require so many moving parts as a different type of system would.
The pressure of fluid in the hydraulic system is controlled by the valve. They also handle the flow rate and which way the flow is going. The funny thing about hydraulic valves is that they can change name depending on how they are being used and according to which system that are part of. Used in combination with cylinders and hydraulic pumps to control the flow of liquid, hydraulic valves are powerful.
The classification of hydraulic valves is determined by how much pressure they can handle. It is also related to the flow and how many directional control valves there are in them. They may also be classified on their looks and extra features such as needle valves, spools and poppets.
Apart from their ability at moving very heavy objects, one of the reasons that hydraulic systems are so popular is because they can operate at very low noise levels. In the manufacturing industry, a low noise level is sought after, in particular at less than 70dB. The hydraulic system and pump is able to accomplish this.
The hydraulic control valve is a clever piece of kit. Browse hydraulic valves here.
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.
Hydraulics has been around for a very long time. But are you aware of how far it has actually come? You wouldn’t be alone if you responded with no. It is a very technical subject that can be quite difficult to understand, but in this article we want to tell you the story of hydraulics! We want to share with you who discovered hydraulics, what it was originally used for and how hydraulic power got to where it is today.
So why don’t we start at the beginning! Where does the word hydraulic come from?
The word hydraulic originates from the Greek word ‘Hydros’ which means water. Why water? Well, this is because water was the first liquid to be used in the hydraulic system. Today, hydraulics includes the physical behaviour of all liquids, not just water.
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.
In this article we want to explain the ins and outs of hydraulic powerpacks. A vital piece of equipment that is used with so many machines we see every day.
In a nutshell, hydraulic powerpacks are self contained units that are used instead of a built in power supply for hydraulic machinery. Hydraulic power uses fluid to transmit power from one location to another in order to run a machine. It really is as simple as that.
So what do they look like?
In order to recognise and better understand hydraulic powerpacks, it is a good idea to get to know the key components. Hydraulic powerpacks come in many different shapes and sizes, some are very large and stationary whereas others are much smaller and more compact. In fact, some hydraulic powerpacks are so compact that they can easily be transported in a small van or even an estate car.
The only real way to identify hydraulic powerpacks is through its main components. No matter the size of the unit, all power packs will have the following; a hydraulic reservoir, regulators, a pump, motor, pressure supply lines and relief lines.
What do these components do?
It may be obvious to some but in this post we wanted to explain every component as simply as possible. So here goes.
First up is the hydraulic reservoir which quite simply holds the fluid. Reservoirs will come in different sizes.
Then we have the regulators. Regulators are vital as they control and maintain the amount of pressure that the hydraulic powerpack delivers.
Thirdly we have the pressure supply lines and relief lines. The supply line simply supplies fluid under pressure to the pump and the relief lines relieve pressure between the pump and the valves. The relief lines also control the direction of flow through the system.
Finally we have the pump and a motor. We will begin with the simpler component of the two, the motor. The motor is simply there to power the pump. Easy as that. Now the pump generally performs two actions. Firstly, it operates as a vacuum at the pump inlet and through atmospheric pressure forces fluid from the reservoir into the inlet line and then to the pump. It then delivers the fluid to the pump outlet and pumps it into the hydraulic system. We did warn you that the second part would be slightly more confusing.
So what is the function of hydraulic powerpacks?
Hydraulic powerpacks deliver power through a control valve which in turn runs the machine it is connected to. Hydraulic powerpacks come with a variety of valve connections. This means that you can power a variety of machines by using the appropriate valves.
Hydraulic powerpacks are relied upon by a range of different machines that use hydraulic power to do its work. If a machine is required to carry out heavy or systematic lifting then its likely it would need help from a hydraulic powerpack.
To make it easier for you to understand, we have included a list of trades that regularly rely on our powerpacks. On a building site you will see machines like bulldozers and excavators, which both need hydraulic powerpacks. But, it is not just on building sites that you will find these types of machines. Fishermen and mechanics both need hydraulic powerpacks too. If we did not have them then how would fishermen lift their nets or how would mechanics lift our cars?
When picking a hydraulic powerpack there are a variety of pumps and options to pick from and it is important to pick the right pack to meet your machines needs. It is also important to consider a pack that will help maximise productivity and minimise cost.
Many people will overlook the necessity of hydraulic powerpacks, but they really are vital to ensuring our society runs efficiently.
Do you need to maintain hydraulic powerpacks?
Yes you do and this is hugely important! Hydraulic powerpacks require regular maintenance to ensure they are working properly and safely and to help extend their life. Maintaining hydraulic powerpacks is relatively simple and includes checking the tubing, this can be for any noticeable problems such as dents or cracks. It is also vital to regularly change the hydraulic fluid and look at the reservoir to check for any corrosion or rust.
What hydraulic powerpacks do we provide?
Generally we provide four different types of hydraulic powerpacks. You can pick from a standard powerpack, a mini powerpack, a micro powerpack or a bespoke powerpack.
The standard hydraulic powerpack uses a standard range of modular components and is ideal for the most demanding industrial applications. The mini powerpack is ideal for applications requiring up to 5.5kW. The micro hydraulic powerpacks were originally produced for mobility applications, so are great for when space is limited. Finally, if none of these seem to fit your needs then we offer bespoke hydraulic powerpacks ensuring your application gets the hydraulic powerpack it requires.
Finally, who is the genius behind hydraulic powerpacks?
The man behind hydraulics was Laissez Pascal. A French mathematician, physicist and religious philosopher who lived in the mid seventeenth century. Pascal made observations about fluid and pressure which led to Pascal’s law. Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container. Hydraulic powerpacks have been designed based on Pascal's law of physics, drawing their power from ratios of area and pressure.
So, interested in our Power Packs? Come on over to the main website and see what we can do for your Hydraulic Power Pack Needs .
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