“In reality some hydraulic components only work when installed in a particular location; hydraulic pumps, for example, can only pump when they are installed after and plumbed to a reservoir.” load-induced pressure. You see, a counterbalance valve, a sequence valve and a crossover relief valve are all just versions of the relief valve with small modifications, such as check valves or external drains, to make them more suitable in a different location. A system relief valve installed after a pump really just needs to be a spring and a ball or poppet, one port plumbed to the pressure side and the other to the tank side—and that’s about it. Because it drains straight to tank, it doesn’t need a dedicated drain line for internal leakage. Let’s move the relief valve to the cap port of a cylinder under compressive load. That relief valve is now a direct acting counterbalance valve, and will not open to allow the cylinder to lower until its pressure setting is achieved. Same valve, different location. Nevertheless, the counterbalance valve requires a reverse-flow check valve if you ever want to raise the cylinder again, and it would be a good idea to drain the spring chamber to tank so that trapped pressure doesn’t lock the valve closed. Let’s flip that relief valve around so that it flows towards the cap port of the cylinder. Now the valve won’t open—and the cylinder won’t lift—until upstream pressure reaches the setting of the relief valve. www.fluidpowerworld.com
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We now have a sequence valve. Because this valve will see pressure on both ports, it needs to have a vented spring chamber. In these circumstances without a drain, any pressure in the spring chamber is additive to spring pressure, and the valve will quickly lock shut. Also, if you want the cylinder to lower, you’ll need to add that check valve again. There is a way to use a relief valve unchanged from its original purpose. If you have a hydraulic motor, you sometimes need to dampen the inertia of the load being turned by the motor. If you simply slam a directional valve shut, pressure spikes in the outlet port of the motor as fluid has nowhere to go. Simply plumb the pressure port of the relief valve in a tee to the motor outlet line, and when the directional valve shuts, the fluid bypasses to the inlet side of the motor plumbing. This has the effect of absorbing that inertial energy, and if the motor is bi-rotational, you can add one plumbed backwards from the other port. Relief valves essentially change their function by their location, but there are circumstances where you want to use a component the same way in a different location. A pressure gauge or pressure transducer, for example, can be mounted in various locations to provide differing metrics. I’ll use the transducer as an example, because its precisions and accuracy make it useful in ways pressure gauges cannot be. Just like a relief valve, a pressure transducer can measure system pressure, letting you know the total load on your pump and prime mover. You can also measure pressure at a subcircuit of the machine, such as the clamping function of a press. A pressure-reducing valve keeps downstream pressure lower than system pressure, and mounting the transducer downstream shows what load pressure is, even if it’s different from the rest of the system. A pair of transducers can be placed on either side of a metering valve, such as a proportional directional valve, and in this combination, you can compare the two signals to observe actual pressure drop, which tells you real flow rate. If a desired pressure drop—and therefore flow rate—is 11 • 2017
FLUID POWER WORLD
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