Basic Hydraulics — Page 67, Lesson 95

Let’s pick up with the double-acting balanced jack, because it sets up everything else we’re about to do with pressure control. A double-acting balanced jack is a balanced actuator where equal force can be applied to both sides of the piston. That means the hydraulic pressure can push the piston in either direction with the same amount of force, depending on which side you pressurise. It’s used in applications like nose wheel steering and flying control boost systems. Either one side, or both sides, of the piston rod may be connected to a mechanism — so the rod can push or pull a load from either end. Now, the hydraulic lock. When fluid gets trapped between the piston of the jack and a non-return valve, we say a hydraulic lock is formed. Because the fluid is incompressible and can’t flow through the system, the piston cannot move, even if a load is applied to it. So the piston is locked in its position. That’s the whole point — trapped, incompressible fluid acts like a solid block, holding the piston still. Next, hydraulic motors. These are a form of rotary actuator — instead of moving in a straight line like a jack, they rotate. They’re sometimes connected through gearing to operate a screw jack, or to drive generators or pumps. In some aircraft they’re used to drive a hydraulic pump unit, which lets you transfer power from one hydraulic system to another without transferring fluid between them. The construction of a hydraulic motor is generally similar to a variable volume multi-piston pump. And the speed of the motor depends on the flow rate of oil into it — more flow, faster rotation. Now we get into pressure control. Maximum system pressure is often controlled by adjusting the main engine-driven pump, but a number of other components are used to maintain or limit fluid pressures in different parts of the system. As a reference point, typical system pressure is 1500 psi for small aircraft and 3000 psi for large aircraft. Relief valves are used for three things: expansion, which is thermal relief; ultimate system protection, which is full flow relief; and mechanical overload protection, which is flap relief. All of them act as safety devices to relieve excess pressure back to the reservoir. Let me take each one. A flap relief valve is fitted to prevent excessive air loads from damaging the flaps or their attachments. It does this by allowing the flaps to blow back to the UP position if the air loads are excessive — for example, if the flaps are selected down at too high an airspeed. Thermal relief valves are usually fitted into lines that are isolated by non-return valves or selectors. They’re adjusted to blow off at a pressure slightly higher than normal system pressure — typically 10% above. In some systems, a full flow relief valve, or high pressure relief valve, is fitted downstream of the pump. Its job is to bypass the full pump output to the reservoir in the event of a failure of the cut-out valve, or a blockage elsewhere in the system. Finally, pressure maintaining valves — also called priority valves. This is basically a relief valve that maintains the pressure in a primary service at a value suitable for operation of that service, regardless of what the secondary services require. So it protects the critical service by giving it priority over the others. Let me show you the cut-out valve and the accumulators, because they tie into this. shows the automatic cut-out valve in its cut-in position, allowing delivery from the pump to pass. And shows the hydraulic accumulators.