Basic Hydraulics — Page 59, Lesson 87

Let’s start with the two big families of hydraulic pumps, because everything else in this section hangs off that distinction. We have engine driven pumps — we call them EDP — and electrically driven pumps. Both can be classified into two types. The first is the constant delivery, or fixed volume, type pump. The second is the constant pressure, or variable volume, type pump. Let me take those one at a time, because they work on completely different principles. The constant delivery pump supplies fluid at a constant rate. That means it pushes the same volume of fluid every revolution, no matter what the system actually needs. Because of that, it needs an automatic cut-out or a relief valve to return the fluid to the reservoir when the jacks — that’s the actuators, the hydraulic cylinders — have reached the end of their travel. And when the system is not operating, it needs an idling circuit, so the pump can keep running without building up destructive pressure. This pump is usually a single or double stage gear pump, giving a large flow at a small pressure, typically up to 2000 psi. So think of it as a high-flow, low-pressure machine. There’s a figure of a spur gear type oil pump for you — that’s the classic gear pump construction. Now the second type, the constant pressure, or variable volume, pump. This one supplies fluid at a variable volume and controls its own pressure. This is the type typically fitted in modern aircraft, whose systems operate at 3000 to 4000 psi. So higher pressure than the gear pump. Let me walk you through its construction, because it’s a clever mechanism. The cylinder block and the drive shaft are coaxial — they share the same axis — and they rotate together, carrying the pistons with them. The pistons slide up and down in the cylinder block. Each piston is attached to a shoe, and those shoes rotate against a stationary yoke. Here’s the key: the angle between the yoke and the cylinder block can be varied. When you change that angle, you increase or decrease the piston stroke — how far the pistons travel — and that increases or decreases the pump output. So by tilting the yoke, you change the volume of fluid the pump delivers. Now let’s look at how this pump actually operates, because it’s a feedback loop. When pressure in the system is low — as would be the case right after you select a service, like moving a flight control — spring pressure on the control piston turns the yoke to its maximum angle. The pistons are then at full stroke, delivering maximum output to the system. So the pump gives everything it’s got when demand is high. When the actuator has completed its stroke, pressure builds up. That rising pressure acts on the control piston, which moves the yoke to the minimum stroke position. In that position, a small flow through the pump is maintained. Why keep any flow at all? Three reasons: to lubricate the working parts, to overcome internal leakage, and to dissipate heat. That lubricating oil drains back to the reservoir through the case drain. So the case drain is the return path for the oil that leaks past the working parts and carries heat away. You can monitor the condition of the pump by a filter and an overheat detector in the case drain. So if the pump is wearing out, or running hot, that’s where you’d see it. On some pumps there’s a solenoid-operated depressurizing valve — also called an off load valve. Its job is to block delivery to the system and to off load the pump. When it operates, system pressure is maintained — the system stays pressurized — but the pump output falls to about 50 to 200 psi, allowing oil to circulate, lubricating and cooling the pump. The solenoid is energized when the pump is off loaded. So that’s a way to keep the pump spinning without working hard. Now, there’s an important note here for transport aircraft. A constant delivery pump delivers the same amount of fluid without regard to the flow required by the system. The unused fluid is returned to the reservoir via a relief valve. That wastes energy — you’re pumping fluid you don’t need, just to dump it back. A variable volume, or constant pressure, pump is better suited to the needs of a transport aircraft, because it can alter the outlet flow as more services are operated. It will increase flow to maintain working pressure. So regardless of the number of actuators or motors being operated, the system will function properly. That’s the whole point — the pump matches its output to demand, instead of wasting energy dumping excess fluid. So to tie it together: gear pumps give you constant flow at low pressure, up to 2000 psi, and need relief valves and idling circuits. Variable volume pumps give you constant pressure at 3000 to 4000 psi, and adjust their own output by tilting the yoke. That’s why modern transport aircraft use them.