Logic elements are fantastic little creatures. Sometimes call DIN valves or slip-in cartridge valves, they are the most basic valve design capable of controlling direction, flow and pressure. Their simple poppet construction nearly guarantees reliability while flowing upwards of a thousand gallons per minute or more. Unlike most of the Symbology series, this one requires some primer to grasp fully.
Logic elements (Figure 1) are machined steel poppet valves traditionally ranging from 16 to 125 mm, as defined by the valves “B-port” diameter. The two-way poppet valve resides within the fixed sleeve or bush, installed into a large, machined and ported manifold block. Sitting atop every logic element is a cover not only to hold the spring-loaded cartridge valve in place but also to provide a method to control the valve’s operation. This directional, pressure or flow control valve atop the cover acts upon the logic element with pilot passage to produce one of the three primary hydraulic functions.
Moving along to the symbology used for a single logic element, as shown in Figure 2, I show two different versions of the same symbol. The first is the ISO 1219 symbol, which should be your first choice when designing logic element schematics. However, my personal preference lies with the DIN symbol of the same valve. I find the DIN symbol better expresses what is happening inside the valve.
The ISO symbol depicts a 2/2 directional valve’s traditional dual box, which I show normally-closed with a spring offset. Following the pilot lines from either A or B to the left, they both push against the operator side with different forces. Typically, the A-side area offers half the area to push against as the B-side. The A-side’s smaller cone resides at the bottom of the seat and blocks the A-port (sometimes called Port 1), while the annular B-side spans the diameter of the poppet minus the diameter of the A-side.
Opposing both the A- and B-side, the spring side area equals the combined area of A+B and benefits from the strength of the spring to help keep the poppet seated. What’s not shown in the basic symbols of Figure 2 is the cover, and its method of controlling the X pilot port, which I will discuss shortly.
Reading the previous two paragraphs while now inspecting the DIN symbol for the same valve likely provides more clarity. The symbol itself more accurately represents a poppet valve, where you can see the A-side and its smaller area relative to the B-side. So long as pilot pressure is minimal across the X-side area, any pressure at A or B will lift the poppet off its seat. The B-side requires half the pressure as A-side to directly open the valve, allowing flow to occur.
Moving along to Figure 3, you can see I’ve constructed what appears to be an overly complex circuit controlling just one cylinder. Valve 1 on the left is the ISO version of the symbol, while the remaining three use the DIN symbology to make things confusing. However, as mentioned, I prefer the DIN symbol, which indicates the actual valve construction, providing a more intuitive sense of operation.
Logic elements are useless without pilot control, so each valve gets its marching orders from the pilot valve above. Tracing the orange pilot line from where it tees off from the main pump line, you’ll find it travels up and around to each of the pilot valves. The pilot valves are discreet functions that either transmit full pilot pressure into the poppet’s spring side or open the same chamber to the tank. When the spring chamber is open to the tank, the poppet is at the mercy of any pressure from the A and B ports. The poppet opens any time A and B pressure exceeds the force of the spring.
I’ve drawn the circuit showing the cylinder retracting under pressure, and each of the four valves reflects what must occur when four pairs of valves are used to mimic a single 4/3 valve. Valves 1 and 2 handle the cylinder’s cap port via their B-ports, while valves 3 and 4 take care of the rod side action. Below the logic elements, the A-port paths from valves 1 and 3 connect to the primary pump pressure, while the A-ports of 2 and 4 run parallel with the tank line.
Pilot pressure keeps valves 1 and 4 locked in place, while the pilot valves 2a and 3a have opened the spring chamber of their respective main stage valves directly to tank. The pressure acting on valve 3’s A-side easily overcomes the spring force, opening the poppet where the extra force on the B-area keeps the valve wide open. However, the pressure at the B-area of valve 4 cannot overcome the larger X-side surface area above, so valve 4 remains locked.
Flow continues through valve 3, subsequently flowing into the rod port of the cylinder. Force against the rod side means force is also created on the piston side, which in turn flows down into the B-side chamber of valve 2. The backpressure created during retraction forces open the B-area poppet, providing a path to tank. Essentially, just opening valves 2 and 3 can retract the cylinder. Conversely, opening only logic elements 1 and 4 extend the cylinder.
Besides the advantage of high flow, there are clever tricks possible with logic elements. Using this same circuit, for example, opening valves 3 and 4 unloads the pump, which is essential for open circuit systems. Also, simply opening valve 1 puts the cylinder into regeneration mode, sending rod side fluid to pass through valve 3 to join pump flow in speeding up the cylinder.
Logic elements, as mentioned earlier, also operate in these circuits as pressure and flow valves. I’ve shown a relief valve example and displayed a group of symbols missing from the directional valves. The enclosure surrounding the relief valve represents the cover, as shown sandwiched between the pilot valves and the manifold in Figure 1. This cover houses a pilot relief valve and two orifices.
The pilot source is the X-line teed in from the pressure line, itself in parallel with the pump’s pressure line and the A-Port of the slip-in relief valve. The first orifice on the supply side limits pilot flow into the pilot valve, preventing it from being over-saturated at any given time. The second orifice dampens the main-stage relief valve’s action, preventing some pressure spikes that may quickly open and close the pilot valve from doing the same to the main stage.
This relief valve symbol is the basis for many other valve functions using logic elements; counterbalance valves, sequence valves, flow controls and even proportional valves. All logic elements use pilot control of the spring chamber to mimic a hydraulic system’s traditional valves. Imagine, for example, a proportional pressure reducing valve instead of the relief valve pilot. Electrohydraulically controlling the spring chamber in effect controls the flow rate possible through ports A and B of the poppet.
This circuit example is elementary, and real-life examples of logic element circuits reach dizzying proportions, with myriad possible combinations of valves, pilots and controls. If you get the chance to study logic element schematics, I recommend you do so to appreciate the ingenuity and creativity possible.
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Filed Under: Fluid Power Basics, Valves