In Anatomy and Types of Hydraulic Valve Part I, we went over two of the five basic kinds of hydraulic valve: relief valves and reducing valves. Today, we’ll take a brief look at sequence, counterbalancing, and unloading valves.
Whenever you have a hydraulic circuit wherein two actuators that are in sequence along a single circuit must move in succession (rather than all moving simultaneously), a sequence — or, more accurately, a sequence of sequence valves — is the tool for the job. Each sequence valve looks much like a relief valve (see last entry), except instead of draining into a reservoir, when the sequence valve cracks, the hydraulic fluid flows down the circuit and continues working — and, a sequence valve shuts it’s own input if the valve is over-pressured. Sequence valves do also have a drain placed above the point at which the circuit continues, so if they are forced open by over-pressure, excess fluid will drain out until the valve can open again and resume normal function.
To illustrate: you have a pipe along which fluid is being pumped. The first juncture splits two ways: to an actuator and to a sequence valve. The sequence valve is set to crack at a pressure above the amount needed for the actuator to do it’s work, so the actuator works first, then as pressure builds, the sequence valve cracks. Next in line is another actuator/valve juncture, and once again the sequence valve won’t crack until the actuator has done it’s work, and so forth, ensuring that each actuator goes off in sequence.
A counterbalancing valve looks and acts much like a sequence valve, but it has no drain to allow excess fluid to be removed from the system if it is over-pressured. Instead, a line feeds fluid from the over-pressure escape back into the valve’s output line, creating a feedback effect that limits the pressure that an actuator is able to force back into the system. This is vitally important, for example, when an actuator is working under a heavy load — the weight of the load could potentially cause extraordinary strain on the hydraulic circuit, but a counterbalancing valve alleviates that danger.
Unloading valves are basically user-operated doorways that open into a tank and allow hydraulic fluid to flow out of the system into the tank. They’re used in several circumstances, but the most common is in a system where you have two hydraulic pumps and an actuator that is designed to move in two ‘modes’ — under low fluid volume, both pumps activate and move the actuator in a fast (high flow rate) but weak (low pressure) fashion. Or, under high volume, a single pump moves the actuator in a slow but powerful fashion. An unloading valve is the key to switching between those two modes.