Flow. Another (perhaps obvious) application parameter needed is the system flow, which lets engineers size the valves properly. Flow varies depending on the type of actuator being used. Flow from cylinders can be multiples of the inlet flow. Flows may also be split so that smaller valves will suffice, or regenerative systems can amplify the flow.
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In one case, a customer was fitting a cover to a skip truck with extendable rotary arms. The customer complained that the spool-type flow dividers we supplied were not accurate enough. When the arms reached their resting point, one bottomed out before the other by a small amount. With ±10% accuracy for this kind of product, we explained that the valves were working as they should. The customer then told us that another supplier’s product had been used and was working fine. On closer examination, we discovered that the minimum flow rating for the other valve was much higher than the flow in the system. In essence, the valve was doing nothing. To demonstrate, I took the spools out of our valve and the cover functioned in the same way as on the other supplier’s. We explained that, without the valve, or with a non-functioning valve, the flimsy framework would twist if one side of the cover gets snagged.
Environment. Imagine an olive tree shaker. To harvest the olives, the machine grabs the tree’s trunk and shakes, making olives fall onto a tarpaulin encircling the tree. Pilot check valves are mounted on the shaker cylinders, which vibrates fiercely. Many valves, including most of ours, are rated to last at least 1 million cycles, which is typical for SiCVs. Depending on the application, valves can last much longer or break down quickly. In this case, it took only two months for our valve to fail. Not good enough. As a result, the customer switched to another supplier’s valves. It also failed within a month. They tried yet another supplier’s valves and got the same results. They then called our company for help.
After some study, we determined the valve went through 1.25 million cycles per month with unknown pressure spikes. We decided to go with an older valve design with a pilot piston pushing against a poppet check. The valve, typically used in industrial applications, had been tested to 10 million cycles.
The point of this incident is that before specifying valves and other components, engineers should understand where and how they will operate. Environmental factors and ambient temperatures might be beyond the safe working limits of the valve or its electrical components. The answers to questions such as these will help design teams seriously consider safety and service life impacts:
What is the valve’s duty cycle?
How often does the solenoid operate?
How frequently does the relief valve open?
Could the valve be exposed to water, salt or acid?
Is the environment dirty or dusty?
Will the valve be used in dangerous areas?
Is there a risk of fire or explosion?
Can the valve be mistreated? For example, could it be used as a footrest for a heavy boot?
Understand the Product
Once the design team thoroughly understands the application, it can begin to narrow down the contenders and evaluate potential valves. This entails understanding the function and operation of each type of valve, as well as how it works with other circuit valves and components.
Function. Engineers must understand how each valve operates if they are to choose the best one for their design. Engineers should not be limited by the name of a valve. If they understand its operation, they can use it in brand-new ways. For instance, sequence valves can be used as a logic element, a flow compensator or a pilot-to-close valve in applications such as regenerative systems, spreaders and tool take-off circuits.
Engineers who do not completely understand a valve’s function and only look at the circuit can wind up making errors. In one case, an engineer mistakenly used standard over-center valves on bucket cylinders, causing them to bend. The engineer had assumed that relief valves in series would protect the cylinder. They didn’t. The standard valve was replaced with a part-balanced valve in which the relief remains open despite back pressure.
Understanding a valve’s operation also allows design teams to account for interaction between valves; such interactions are often why a system is not performing up to snuff. In one case, a client had four relief valves fitted in parallel in a high-flow application. Each valve fighting for flow created a loud, unsettling noise. To silence the noise, we changed the valves’ settings so that there was 50-psi between each.
Performance. Before specifying a valve, designers must understand what is expected throughout the machine’s life, the safety requirements,and the OEM standards that must be met. Important data points include the valve’s leakage, hysteresis, accuracy and response time. Some of these can change over a valve’s service life. For example, a valve’s leakage may change depending on the viscosity, oil cleanliness, erosion and whether aggressive fluids are used.
Installation and use. Sometimes the perfect valve has been specified for an application, but something goes wrong during installation or testing. Perhaps they weren’t torqued correctly or a technician did not know how to properly adjust them.
In one unfortunate example, a customer managed to bend a 20-foot-long pallet stacking cylinder. The valves had a dampening device that could be adjusted to prevent the compensator spool from moving too far. The customer saw a bit of instability and adjusted one of the dampening screws, only to close it off completely. This resulted in a container being lowered on one cylinder while the other bent like a coat hanger. We worked with the customer to ensure everyone who went near the hydraulics on this machine knew how the valves functioned and what could go wrong.
Boiling down SiCV design and troubleshooting into knowing the application and product is perhaps a vast oversimplification. Understanding the valves is easier said than done. Eaton, for example, offers 12 types of control valves. There are 177 different functions; five sizes, in most cases; and an array of pressure settings, flow settings, manual overrides, adjusters, solenoid voltages and terminations, seals, and line body options. That provides more than 200,000 options from which to choose. With so many valves, it can be difficult to understand the nuances behind all of them.
So, what’s a designer to do?
There are many ways to learn. The best way, undoubtedly, is through experience. Beyond that, attend training. Look into virtual sessions and in-person classes (the latter, if pandemic conditions allow). These courses go into great depths on the function, operation and application of each valve. Also inquire as to what materials suppliers have so you can learn on your own time. Eaton, for example, offers several articles in its SiCV catalog that explain valves.
OEMs, for their part, generally know the endgame and want the most cost-effective valves. Partner with a supplier or integrator that can help. These firms understand the intricacies of valves and will work closely with you to explore and understand the application, then develop a solution, normally in the form of a hydraulic integrated circuit (HIC).
Gone are the days when only SiCVs were considered for these applications. Suppliers have access to a broad range of hydraulic components to ensure they will likely have the right one. They can have industrial valves that are simple to incorporate given their standard mounting patterns, as well as mobile valves which can be mounted to manifolds. Filters and accumulators can be added to the package, and the manifold can be attached to the pump or motor, depending on the customer’s preference.
The keys to a successful outcome are a close partnership at every stage of the design cycle, a full understanding of all requirements upfront and the flexibility to provide the performance needed while addressing client’s preferences.
Maurice Ashmore is global chief engineer of SiCV & HIC at Eaton Corp.
Hydraulic spool valve operation requires repairs and troubleshooting procedures not used for other types of devices. To avoid costly replacements that could eat into a company’s profits, floor managers must understand how these valves work and recognize signs that they need adjustments or other repairs. Just as the operation of every part of the machinery is essential to your company’s productivity, so too are spool valves used in hydraulic systems.
Hydraulic Repair Services
What Is a Hydraulic Spool Valve?
How Do Hydraulic Spool Valves Work?
How Do You Adjust a Hydraulic Spool Valve?
Tips for Troubleshooting Your Hydraulic Spool Valve
External Leaks
Internal Leaks
Spool Binding
Insufficient Pressure
Maintenance Tasks to Prevent Problems
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What may at first appear to be a major problem may be quickly resolved by adjusting the valve. Without knowing how the valves work, though, floor managers may stop production for too long, causing major setbacks. Training those in charge of operations on how machinery works and basic troubleshooting can improve your company’s efficiency and save money.
Hydraulic systems use fluid to increase the mechanical ability of the system. Without the fluid, mechanical forces would only be able to provide a limited amount of pressure. Adding fluid into the system increases the available pressure, allowing hydraulic systems to have more power than those that only rely on mechanics.
Part of the system is the hydraulic spool valve, which is a type of directional control valve. This valve balances out the pressure and flow of the hydraulic fluid in the system by shifting the fluid to turn a switch on or off. The general design of a spool valve is difficult to see because a cylinder encases the mechanism. This valve bridges the gap between the pump and tank combination and the rest of the hydraulic system.
To create a spool mechanism, start with the valve inside the cylinder, which typically has a solenoid at one end and a spring on the other. The spool inside the cylinder directs the movement of the hydraulic fluid to create pressure where needed. Applying power to the spool valve’s solenoid changes the direction of the hydraulic fluid, thus moving the fluid pressure. Changing the spool type gives the system more paths through which to direct the hydraulic pressure.
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Depending on the hydraulic system, more than a dozen spool valve options exist. Different types of spool valves may have solenoid, hydraulic, spring, manual or proportional operators — not just spring and solenoid. These options increase the number of spool valve types.
Several systems in a manufacturing facility may use spool valves in their hydraulic systems. Frequently, these valves operate pistons, cranes, power steering systems in vehicles and many other hydraulic systems. Modern hydraulic systems now include many upgrades not seen in older models. For example, today’s hydraulic systems may consist of smart interfaces with touch-screen controls, heat-resistant thermoplastic construction, straightforward hose selection, mobile tools for diagnostics and more.
Operation of the hydraulic system comes from maintaining the pressure of the fluid and ensuring all parts fit correctly to prevent leaks. Valves that require adjustment could impact the operation of the entire hydraulic system. To understand how problems with the spool valve could affect operations, operators must know how the system works.
The specifics of spool valve operation depend on the exact configuration of the valve. However, most have a basic design and function with some alterations depending on the operator and number of direction controls.
Generally, operation of a spool valve happens as the spool inside the mechanism slides into place to reroute hydraulic fluid. Because this type of valve controls the direction the hydraulic oil goes in, it is known as a directional control valve.
Inside the spool valve, the spool controls to which work ports the fluid goes. The number of inlet ports from the hydraulic tank and pump and the number of outlets to various work locations indicate the type of valve. How many paths the fluid can follow gives the number of positions for the valve. The ports and positions combined specify the spool valve type.
These combinations of ports and positions make spool valves versatile and extremely useful. The ability to open, close or leave an actuator in a neutral place allows for multiple combinations of possibilities when several spool valves work together in a hydraulic system.
Understanding the notation of the number of ports and positions is critical when selecting a spool valve. American nomenclature refers to the number of ports as ways. For example, instead of saying a valve is 3-port, 3-position, American manufacturers may call it a 3-way, 3-position valve.
Regardless of whether you purchase from international or American valve makers, the first number always refers to the number of ports and the second is how many positions the valve can create. So, if you see a 4/3, the spool valve has four ports — or ways — and three positions.
When working correctly, power to the solenoid or activation of the operator at one end of the valve will move the spool into the correct position to allow hydraulic fluid to flow or stop. Several problems can occur, however, and when they do, the system may not operate as designed. The foundation of spool valves is their fine movement, and if anything prevents the spool from cleanly moving, it can reduce the effectiveness of the spool valve.
Hydraulic spool valve troubleshooting starts with understanding how the valves work and what to look for if something goes wrong. You may not always be able to fix it. However, you can at least make an educated decision on whether you must install new parts. Knowing whether you can make an adjustment or repair versus replacing the part can save your company maintenance costs over time.
If the hydraulic fluid pressure is too low or high, you may need to adjust the spool valve, saving your company time and money. The valves will have a factory-set pressure and pressure range that you can safely set them to. Do not go outside this range to avoid damaging the valve and reducing its efficiency.
The flow rate will help determine pressure. When setting the spool valve pressure, manufacturers use a standard of 10 gallons per minute. For higher flow rates, lower the pressure of the valve to make up for the higher pressure from the increased fluid flow. Conversely, low flow creates low pressure — to compensate, increase the pressure.
To adjust the spool valve, loosen the valve-locking nut on the side. Removing the locking nut gives you access to the adjustment screw. Turn the screw clockwise to raise the pressure, and twist it counter-clockwise to lower the pressure. Use a pressure gauge to ensure you create the ideal pressure adjustment before replacing the locking nut and putting the spool valve back into place.
If you correctly adjusted the spool valve and still have problems, you may need to try troubleshooting the valves in your hydraulic system. Generally, only three main issues exist for these hydraulic components. However, just because there is a small number of problems does not mean troubleshooting is effortless.
Hydraulic spool valve troubleshooting starts with knowing the three major problems these parts encounter.
The first two problems relate to how hydraulic systems work. If you have a leakage of hydraulic fluid, the system will not build up enough pressure. How to fix these problems depends on the source of the leak.
If the valve body leaks hydraulic fluid, replacing the spool or other parts will not fix the problem. Install a new spool valve to remedy this issue. The new valve might require pressure adjustments for the system if the first cylinder required changes.
External leaks don’t always require complete replacement of the valve. If the leaks originate around the seals, replace them to fix the problem. Replacing the O-rings on cross spool valves often fixes leaks from these areas caused by old or worn seals. Solenoid spool valves lack O-rings. However, solenoid cartridge seals may solve the seepage.
Minor leaks may occur inside the valve without issue. However, excessive leaks can drop the pressure in the hydraulic system. Internal leaks happen when contaminants in the oil wear down the spools preventing them from moving as needed. Gaps in the spool and internal structure allow for leakage of hydraulic fluid.
Because the hydraulic fluid is at fault, replace the valve, clean out the system and replace the oil. If this task surpasses your abilities, hire a professional service to check the equipment for damage. Don’t attempt to disassemble the hydraulic system without knowledge of how to piece it back together. Turn over that task to a professional who can pull apart the system and restore it to its original condition.
Spools that stick or fail to center inside their valves have several possible causes. To get the valve working again, identify the cause of the binding.
A hydraulic system may experience other issues caused by the spool valve. These go beyond the problems with the spool itself and incorporate problems with the entire cylinder the spool valve is a part of, including other components of the hydraulic system.
A lack of pressure could occur for many reasons. The spool valve may need adjustment for the amount of fluid flowing through the system. Less than 10 gallons per minute may require adjusting the valve to increase the system pressure.
A lack of pressure may occur when the centering spring is defective or does not work well. Without the spool in the correct position, adequate fluid may not reach its intended destination. Check the valve’s mounting to see if it is the cause of the off-center spool. Otherwise, the spring could be the problem.
Hydraulic Repair Services
Bad hydraulic fluid that is too hot, too low or too dirty will damage the spool valves and other parts of the system leading to a multitude of problems beyond just the valve. To keep the system running at its peak, schedule regular hydraulic system maintenance.
Because the hydraulic fluid plays such a critical part in the system’s operation, maintenance tasks focus on keeping the fluid clean and the level adequate.
Change filters regularly. The screen is the first line of defense against debris in the hydraulic oil. A dirty filter makes the pump work harder to force the fluid through it. Additionally, the fluid is more likely to pick up dirt from an old filter and deposit it throughout the system. Choosing a preventative maintenance task rather than putting it off until you must make emergency repairs will prevent downtime and lower costs.
Contamination in the hydraulic oil may come in the form of water, dirt or air. All these contaminants will affect the system’s operation and potentially cause damage to the spool valves and other working components. Look at the hydraulic oil carefully. Cloudy fluid means there is water in the fluid. A foamy appearance indicates air in the fluid. A rancid odor hints at burned fluid. Also, the fluid should never have visible dirt in it to protect the longevity and operation of the equipment.
In the case of water or air, check the system for leaks. Burned fluid could indicate overheating. Dirt in the system could come from contamination from parts breaking down or an old filter. If these conditions persist after making corrections, get an expert repairperson to service the system. Ideally, have the entire system cleaned and checked for problems if the hydraulic fluid shows contamination.
Check the hydraulic oil levels regularly. Low levels need topping off. However, most hydraulic systems are closed, which should not require new fluid to be added. If the oil levels drop frequently, suspect a leak somewhere in the equipment that will need repairs.
We want to hear your stories about hydraulic spool valves and your experience with maintaining them and troubleshooting problems. Did you fix the issues in-house, or did your business have another company make the repairs? Has your company experienced problems with hydraulic spool valves? What were the solutions your business sought? Were they effective? Leave your answers and other comments below.
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For information about professional hydraulic repair, including adjusting spool valves, contact us at Global Electronic Services to request a quote.
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