Valves are the fluid power designer’s calling card. Every hydraulic system has a pump supplying hydraulic energy to any combination of cylinders and motors, but the myriad possible combinations of valve technology offer a number so large only to be eclipsed by chessboard possibilities. This article provides a guide to the valve technologies available, allowing you to create your own unique approach to controlling hydraulics.
In general, valves are categorized into three primary groups of control: direction, flow, and pressure (and hybrids of these). Directional valves control the cylinder’s direction of linear motion, allowing both extension and retraction, while also offering bi-rotational control of hydraulic motors. Flow control valves are charged with limiting or redirecting the volume of hydraulic fluid through any given subcircuit, thereby modifying the speed of actuators. And of course, any valve responsible for limiting pressure also limits force, since each factor is a different description of the same thing.
I can think of creative ways to control an actuator without a flow or pressure valve, but unless you’re running a unidirectional motor continuously and with no variation, nearly every circuit must use one of the many possible directional valves. From poppet valves that simply open and close to permit or block flow, to complex spool valves used in mobile hydraulic systems, the range of possible tools for directional control is surprisingly broad.
Figure 1. Cartridge valves are extremely economical when purchased with manifolds in high quantities. Much of the initial cost of an integrated circuit is the engineering work required to place the 3D cartridge models into an envelope before running a test piece on the CNC mill, as pictured here.
Screw-in cartridge valves
The simplest, least expensive and perhaps most versatile valve is the screw in cartridge type. Possibly the only valve with a bottom price barely higher than a nickel-plated run tee, these valves utilize one or more industry-standard ported cavities to house their slim, segmented body. Using mostly poppet or spool valve technology, these valves screw into a complex, machined metal cavity that intersects with ports.
Although cartridges are often installed into inline ported bodies, such as the 2-Port body of a relief valve, plumbing individual components together using fittings, hoses, and tubes can be quite messy and convoluted. Cartridge valves beg for placement within a customized combination block where the entire valve and control circuit is neatly installed into a milled billet of aluminum or steel. Such a block may combine only two valves in a convenient and compact package, or, when offered with dozens of valve companions, it can constitute a highly complex circuit with multiple sophisticated functions.
Compared to separately plumbed valves, industrial stack valves, or even high-end mobile modular valves, cartridge valves are extremely economical when purchased with manifolds in high quantities. Much of the initial cost of an integrated circuit is the engineering work (Figure 1) required to place the 3D cartridge models into an envelope before running a test piece on the CNC mill. This engineering and machine setup work is expensive, so if you need one manifold only, it’s best to consider either stack valves or separately plumbed valves.
Some of the large manufacturers of cartridge valves offer hundreds of designs in various sizes and flow rates. Although you may need to wait eight weeks or more for less common valves, there is simply no better way to customize your circuit with unique combinations of valves found nowhere else. Pressure compensators, load-shuttle valves, thermal relief valves, hot oil shuttle valves, palm button 2/2 valves, handpump cartridges, bidirectional relief valves, torque dividers, anti-cavitation relief valves, flow dividers, pressure compensated proportional flow controls, orifice discs, and, of course, priority on-demand pressure compensators with dynamic load sense.
A mouthful indeed, but that list showcases the plethora of valves easily integrated into any circuit to create functions and combinations simply not possible by other means. The downside to cartridge valves is their often limited flow rates, especially with solenoid valves. These will typically require pilot valves to access big flows, but many valve manufacturers offer cartridge valves that perform similar to slip-in logic elements, offering yet another advantage.
Slip-in logic elements
The slip-in logic element is another tool on the designer’s long list of hydraulic valves. Similar to cartridge valves, except held in a manifold with a cover plate, these are among the highest-flowing valves on the market. Although precisely manufactured, they are simple. Available in various configurations of 2/2 poppet valves, which, when combined with pilot valves, can operate as directional, pressure, and flow valves.
Figure 2. Slip-in cartridge valves, also known as logic elements. Pictured here are all the same functional valve, show ing the ISO 1219 symbol, while the others are the classic DIN style of drawing.
Just as with cartridge valve circuits, these are highly customized, but not for the faint of heart. Be sure to study their use and application, as their circuits are designed with a sense of creativity and with respect to pressure differential and digital flow paths. Almost always spring offset, they are happy to open fully or be restricted from doing so by varying the pressure at one side of the poppet. At first glance, they appear less useful than a spool valve, but once you delve into area ratios, pilot valves, and massive flow potential, some designers prefer to use only slip-in cartridge valves.
Also known as logic elements, they are both simple and complex simultaneously. Consider that to recreate a 4-way, 3-position solenoid-operated spool valve, you’ll require no less than eight separate valves to achieve the same result: four logic elements and four pilot valves. To wrap your head around this, refer to Figure 2, which illustrates how ingenious these valves are. Don’t worry, these are all the same functional valve; I only show the first valve using the ISO 1219 symbol, while the others are the classic DIN style of drawing.
Spend some time learning this schematic. You’ll see that the pilot relief controls the slip-in valve in much the same manner as other pilot-operated valves, insofar as it can infinitely vary pressure by bleeding flow. The same tactic can be applied to each of the logic elements. Visualize the replacement of the 2/2 pilot valves with proportional valves. By metering the pilot side of the logic element, you also incrementally open the valve, metering flow, and with the very same circuit, you can meter in, meter out or even add some cylinder regeneration into the mix. Take the time to learn about slip-in cartridge valves — I promise you won’t regret it.
Lever valves
Let’s step back for a moment to discuss an entirely different path a designer can take in controlling their hydraulic machine: the lever valve. They come in various forms, ranging from monoblock valves, basic sectional valves and then more advanced electrohydraulic proportional valves used in more sophisticated machinery.
The single-spool monoblock valve with integrated relief function is the simplest and least expensive path into machine control, requiring only a pump source and an actuator to complete the hydraulic circuit. Bin-tippers, log splitters, and hydraulic winches are typical examples of economical machinery best suited by the monoblock valve. Do you have more functions? Great, step into a 2, 3 or 8-spool valve with a dedicated lever for each actuator.
Although simple in both design and execution, you can still upgrade and modify to suit your application. The spools are changed easily to suit motor, open, closed or other spools to meet your needs, while handle kits are offered with spring center or detents. Unique to these types of valves is the kick-off function, which is a detent that pops back to center when pressure rises past the kick-off setting, perfect for log splitters. Also unique to mobile lever valves is the power beyond function, which enables a valve to feed a downstream secondary valve through an adapted tank port.
Lever valve upgrades offer improved versatility, including hydraulic or pneumatic pilot operation, electric solenoid actuation, proportional coils, work port relief, and load control valves. Some of the big boys make high-end sectional proportional valves with load sensing, flow sharing, CAN network compatibility, and the capacity to add or remove sections as required by the application. In many cases, the “lever” is an extra-cost feature for such electronically sophisticated valves, highlighting just how vast the difference is between these automation-ready machines and a monoblock valve for a cattle squeeze.
Stackable industrial valves
Perhaps the only valve type with a wider range of sizes and purposes is the stackable industrial valve. Sometimes referred to as ISO or CETOP valves, with familiar NFPA nomenclature such as D03 or D05, these are a staple in factories worldwide. These modular valves (Figure 3) are bolted to subplates or manifolds with the directional valve mounted atop a stack of accessory valves. Relief valves, flow controls, reducing valves, PO checks and many other options fit between the manifold and DCV, each with the same port pattern running in series-parallel through each valve and fixed by tie-rods or socket-head cap screws.
Figure 3. Stackable industrial valves, sometimes referred to as ISO or CETOP valves, with familiar NFPA nomenclature such as D03 or D05, feature a modular design and are bolted to subplates or manifolds with the directional valve mounted atop a stack of accessory valves.
D03 valves are relatively economical, depending on the brand; however, prices can increase rapidly for larger sizes. Here’s a quick size rundown if you’re unfamiliar, just keep in mind that D02 valves are rarely used any longer: NG4=D02, NG6=D03, NG10=D05, NG16=D07, NG25=D08 and NG32=D10. The flow range of these valves starts low with D03 valves, depending on spool type, but expect the D10 to flow hundreds of gallons per minute. Also, imagine how bulky a stack of D10 valves is ‚— the directional valve alone is eleven inches tall, including the pilot valve.
Where the stackable industrial valve really shines is in the control side. Besides every form of network connection ever made, these valves are the undisputed champs of electroproportional control. They’re offered as modest and simple, low-flow D03 versions requiring inexpensive valve controllers, all the way to D10 monsters with on-board electronics and LDT position monitoring of both the main stage and pilot spool. The flow and pressure accuracy of these modern high-response proportional valves is topped only by the venerable servo valve.
The servo directional valve is the ultimate high-accuracy proportional flow, as its internal, mechanical feedback is inherent to its construction. Instead of the traditional valve coil, these use torque motors to pivot a flapper to throttle fluid into the pilot chamber on either side of the valve spool, directing it to shift. If flow forces push the spool out of its desired position, a feedback wire attached to both the flapper and spool will automatically adjust pilot flow to bring the spool back into position.
From tiny size 6 cartridge check valves to gigantic pilot-operated proportional valves, the variety of valve choices available to the hydraulic designer is tremendous. The unfortunate reality, despite possible creative control solutions, is that designers stick to what they know. Regardless, I encourage you to step outside the box and explore one of these other valve types mentioned above.
Filed Under: Components, Components, Valves & Manifolds
