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Electrohydraulic steering systems have been commonly used in passenger vehicles since the 1980s, as solely hydraulic steering systems started to fall out of favour when electrical components in cars became the norm. Electric windows and sunroofs replaced manual versions, and in high end cars even the seat adjustment mechanism moved from a manual operation to an electrically operated system. The presence of electrical circuits in passenger vehicles led designers and engineers to explore how electrohydraulic components could be used to improve vehicle efficiency and the driver experience for passenger cars and steering systems were an obvious place to start.
The same technology has been slower to penetrate the heavier commercial vehicle market but it is starting to gain ground now, thanks to the improved fuel efficiency offered by the electrohydraulic power steering offering compared to fully hydraulic systems – around 70% for hydraulic systems and far less for those that cleverly employ electrical integration. The rise in electrification of mechanical components in commercial and industrial vehicles and the same trend in the adoption of steer-by-wire technology, has also contributed to the increased adoption of electrohydraulic power steering. As more parts of the vehicle use electrical power and it becomes easier to adapt the steering systems to include electrohydraulic actuators they will be more and more widely used, improving the driver experience and fuel efficiency.
Electrohydraulic systems are becoming more popular in many areas of industry thanks to the prevalence of electrical power, and the recognition that hydraulic technology is beneficial in many applications. Although traditional hydraulics may be replaced in small pieces of equipment that are not required to exert a large force, there are many other places where hydraulic power cannot be matched, and designers and engineers are noticing this and encouraging the integration of the two power sources to maximise efficiency while saving energy. The cumulative effect of this will be an exciting time for hydraulic power industry.
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.
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.
Humans are creatures of habit, we like routine and familiarity as it makes us feel safe. Change is a hard pill to swallow; although some people deal with it better than others it can take a lot for someone to proactively look at a different way of doing things. Change is usually something that is imposed upon a person out of necessity, so if there is no perceived reason to change a situation then we generally carry on as normal.
This thought pattern may be one reason why hydraulic systems designers tend to opt for high pressure systems and the familiar components that can cope with being under extreme pressure. It is far easier to take an existing basic idea and tailor it for a new application without even considering alternative approaches, which is why low pressure hydraulic systems are something of a rarity in the world of fluid power.
The basic equation of force = pressure x area, lends itself to working with a smaller area and a higher pressure to exact the same amount of force that a large area under lower pressure would exert. This is attractive to designers as it means systems can have sleek, narrow cylinders and in many cases, this is needed to ensure the assembly fits in the space available. That is not always the case, however, so hydraulic designers should consider low pressure systems as a possibility for some applications.
Low pressure hydraulic systems can be a lot more cost effective than high pressure ones, as there is a reduce possibility of leaks, and if they do occur they will take less time to clean up and fix. The materials used to build the components can also be a lot cheaper, as they will not have to withstand the high pressures normally associated with hydraulic systems. Plastic components, flexible nylon tubing and even thin extruded aluminium cylinders all work perfectly well at pressures under 50 or 60 bar, and are a lot more economical than the high pressure alternatives.
Sometimes a low-pressure system is really the only possible solution to a problem, especially when designing complex systems with many lengths of tubing, serving several small cylinders off a central motor. This is when the materials that need to be used dictate the operating pressure of the system, rather than the operating pressure dictating the materials. If the pipes need to fit through pre-defined holes in the machinery casing, or need to wind around parts of the machinery then flexible nylon is the best option. This low-pressure approach allows designers to consider every angle from which to solve the design problem, and can result in some great innovations that otherwise might have gone undiscovered.
The only drawback to low pressure hydraulic systems is the need for a larger reservoir to hold the extra fluid that is needed to fill the system when it is in operation. Size and space can be a stumbling block for low pressure systems, as the larger cylinder diameter means more space is needed. Sometimes this can be cleverly engineered in, by placing the reservoir further away from the operational components of the hydraulic system and making good use of the cheap nylon tubing to run the fluid up to the moving parts.
There will always be cases where high pressure systems are a must, due to the application, the forces needed and the space available to house the hydraulic system, but at the same time there will always be systems where low pressure is more effective in terms of performance and cost, so considering both angles before diving in to a design is a worthwhile task that could lead to the next big thing in hydraulics.
But why do they fail? A strainer located in the suction port of a pipe is common as is a return to tank side of a circuit. The pressures throughout the circuit will vary and so will the conditions applied to the strainer and its housing.
This is the number one cause of failure irrespective of the location. Whilst a thorough examination of the system may be difficult after such a failure it will pin-point the source of the contamination and lead to a plan to avoid further hydraulic failures throughout the hydraulic system components.
Contamination is of course any particle that is not part of the intended hydraulic fluid and its additives.
Contaminants throughout the hydraulic equipment must be identified, but these would normally be categorized as :-
Solids – Oxidation, Metallic and polymer wear particles
Liquids – Water, Fuel, detergents etc
Gas – Air
Understanding the effect each of these contaminates has on the system components and materials will be essential before a system can be re-commissioned. For example water in a hydraulic fluid can freeze, causing filters to clog with ice and collapse with pressure applied by the hydraulic pumps.
Sources of contamination are many and varied, it can be a simple as un-clean oil being used to “top-up” or air bourn dirt drawn in through breather vents.
Contamination must be removed before a system is brought back in to service and the best way to do this is to select appropriate filtration. Circulation of the oil with an off-line system will quickly clean the oil within the hydraulic reservoir. Once an oil analysis confirms the oil has been cleaned to a good level, the filters can be serviced. Regular monitoring of the cleanliness will indicate the need for filter changes. If the filters are of the correct design and location they will need changing less regularly. This will then lead to a more efficient and longer lasting hydraulic machine.
Global demand is not easing up when it comes to farming vital resources that are found in subsea environments. In fact, industries have now begun to expand their efforts and are putting more energy and effort into devising machines that are capable of delivering what is required.
With over 60% of the surface of the earth covered by water, it’s no secret that there are many resources that are awaiting exploration and development. This new frontier has a number of industries involved including oil and gas, natural science, mining, energy generation and infrastructure.
Hydraulic systems are incredibly useful to remote operated vehicles that are used underwater. They offer high power density and a reliability not found in other systems. It’s possible to use hydraulics in such a way that the vehicle is very compact and therefore it can be deployed and recovered easier.
Highly technical and complex systems need to be serviced and maintained by subsea remote operated vehicles. For example, equipment needs to be lowered and lifted to the seabed, emplaced systems need to be monitored, such as communication cables and petroleum wellheads.
Although some subsea hydraulic equipment is designed specifically for the task, in some cases, the equipment has been manufactured to a quality that can handle the high pressures and the corrosive conditions of the depths of the sea anyway and needs just a little customisation to perform at a reliable level. A major factor that is considered is the depth of the water and how that could impact the hydraulic system.
Here are other considerations that go into developing hydraulic systems for subsea operations:
Machines that operate at 1000ft below sea level are required to operate in salt water, but the water-pressure is not significantly high. Another factor that has to be catered for is that sunlight can reach up to 800ft into the water and could promote the growth of sea life over the surface of the equipment such as the cylinders and the rods.
Beyond 1000ft to as deep as 6,000ft, pressure becomes a major factor. Increasingly 14.5psi for every 10m of depth, it will be as high as 7250psi at 5000m. It’s at these depths that work is performed by subsea robots such as AUVs (autonomous underwater vehicles) and ROVs (remote operated vehicles).
Subsea vehicles aren’t typically in use for long periods of time. They will be used to accomplish tasks in electromechanically and electrohydraulic subsystems. Although they can operate beyond 100m of depth they typically won’t be submerged for long periods of time. However, they need to be ready when required and any downtime must be kept to a minimum.
Special design features may be required for components exposed to water pressures this high. For example, structural modifications may be required or pressure compensation.
These depths would normally be found a long way from shore, therefore would be operated by either ships, platforms or floating platforms. Water that is from 6000ft to 35,800ft is rarely entered unless it’s by subsea vehicles from the military or research. The conditions are so extreme, that every piece of equipment, including hoisting and tethering will need to be engineered to handle the weight and dimensions of systems at this water depth. In addition the size of the waves are larger, as are the forces brought on by maritime currents.
Ambient hydrostatic pressure is exposed to the hydraulic fluid using a pressure compensation system, with a flexible seal to prevent hydraulic fluid and seawater from making contact.
The benefits of hydraulic drives are brought into their own in these types of machines. Not only are they powerful and compact, rugged but precise, they are able to deliver power and be flexible for a wide range of tasks.
Engineers continue to work on how they can make the best of what hydraulic systems offer when it comes to subsea conditions.
Welcome back to part 2 … we continue looking at IoT and uses in the home.
The IoT is not confined to commercial and industrial applications, however, and the concept of the smart home is becoming more mainstream as people try out smart home apps such as the ones that control your heating and lights. Although there is no requirement for using hydraulic components in these applications, combining the ability to control temperature and lighting with the ability to pre-emptively open your gate or garage door as you arrive home (perhaps with the use of GPS, so your connected home knows where you are and how long you will be) is something that will involve electrohydraulic components.
Some smart home systems know when a window is open and allow you to close it remotely, which is great if you have gone to bed and realise a door or window is unsecured. Electric actuators may be able to control the motion of most domestic windows and internal doors, but large windows, fire doors and external doors would require more power which can be offered by hydraulic actuators. These systems are also perfectly suited to being used as a security solution in large buildings that are guarded by a skeleton staff. Security cameras can be used to monitor all areas of the building, both inside and out and an intelligent network of hydraulic actuators can be used to close doors that have been left open without the security guards needing to leave their station. In a break in situation doors can be closed and locked remotely, trapping a burglar until the police arrive. Hydraulic components have a clear security advantage here as they cannot be tampered with by messing with electric circuits or by using magnets to interfere with electrical signals, and the hydraulic pressure locking a door shut can only be released by accessing a relief valve or by having access to the main control panel.
Micro and mini packs from Hydraproducts are perfect for automated door and window security applications, as they are small enough to be retrofitted in a building without causing a lot of disruption and offer a surprising amount of force for their size. Multiple units can be wired in to add extra security when needed, or they can be used to affect a further mechanical lock that is not prone to tampering or damage.
Hydraproducts also manufacture and design bespoke systems, so if you have the seeds of an idea regarding how electrohydraulic components can help you automate certain features of your home or business we are the people to call. With experience of working in hazardous industries such as subsea drilling and demanding locations like film sets where performance is key we are well versed in working within parameters set by our clients and always come up with the solution that suits the client and performs well. Give us a call today on 01452 523352 to see how we can help.
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