Basic Hydraulics — Page 73, Lesson 98

Let's start with flow control. The components I'm about to walk you through all do one job — they control the flow of fluid to the various services operated by the hydraulic system. So think of them as the traffic managers of the system. First, the non-return valve. This is the most common device used to control fluid flow. It permits full flow in one direction, but blocks flow in the opposite direction. If you've met diodes in electrical circuits, it works the same way — current flows one way, not the other. There's a simple ball-type version shown in Figure 2.20. Now, when a non-return valve is used as a separate component, the direction of flow is indicated by an arrow moulded on the casing, so you can't install it the wrong way round. This valve also goes by other names — the One Way Check valve or the Non-reversible valve. Same device, three names. Next, the One Way Restrictor Valve, also called a choke. It may look similar in construction to a non-return valve, but it behaves differently. A restrictor valve permits limited flow in one direction and full flow in the other direction. The restriction is usually of fixed size, as shown in Figure 2.21. So where does it get used? It limits the speed of operation of an actuator in one direction only. For example, it might be used to slow down flap retraction or landing gear extension — the up line for both. So the actuator moves slowly in one direction, but fast in the other. Now we move to selector valves. These direct fluid to the appropriate side of a jack and connect the other side to return. Some are manually operated, but on large transport aircraft they're operated remotely, either mechanically or electrically. Selectors come in two main types — open centre or closed centre — and they may be rotary or linear in construction. Let's look at rotary selectors first. Different types are used depending on the type of actuator. A simple two port selector is used with a single acting actuator. A double acting actuator uses a four port selector. Here's how it works: the four port selector enables the actuator to be extended by directing fluid to one side of the actuator and opening the opposite side to return. Then you rotate the selector to redirect the fluid and retract the actuator. So one rotation extends, another retracts. Figure 2.22 shows a typical four port selector. Finally, the linear slide selector, also called the spool valve selector. This type is operated by a linear movement — a straight-line push or pull — and is typically operated mechanically by a rod or cable, though it may be operated electrically. Figure 2.23 shows it. So to tie it together: non-return valves let fluid flow one way only, restrictor valves limit flow in one direction to control actuator speed, and selector valves direct fluid to the right side of a jack — either by rotating or by sliding a spool. That's the flow control family.