Basic Hydraulics — Page 52, Lesson 76

Let’s pick up the thread of the Bramah press, because that’s where we left the idea of force multiplication. I want to show you how the work equation ties it all together, and then we’ll move into the two big families of hydraulic systems — passive and active — and finally into the fluids themselves. So, the Bramah press. The key relationship is: work done by a machine = force × distance moved. Work is measured in joules, and it’s the product of the force you apply and the distance through which you apply it. Now, in the Bramah press we have two pistons — piston A and piston B. If I push piston A through a distance of 0.6 metres with a force of 1000 newtons, and the system is assumed to have no frictional losses, then the work done in the system must be constant. That means the work put in at piston A equals the work coming out at piston B. So we write: force × distance on piston A = force × distance on piston B. Plug in the numbers: 1000 × 0.6 = 2000 × the distance moved by piston B. That gives 600 = 2000 × distance B, so distance B = 0.3 metres. And you can check: 1000 × 0.6 = 600 joules, and 2000 × 0.3 = 600 joules. Work is conserved. Here’s the takeaway: for a given fluid pressure, the force produced can be varied by adjusting the piston area, and the resultant linear motion will vary in inverse proportion to the area. So if piston B has twice the area of piston A, it produces twice the force but moves only half the distance. That’s the gearing effect — force up, travel down. Now, this Bramah press arrangement is what we call a passive hydraulic system. Let me define that precisely: a passive hydraulic system is one in which there is no pump, and pressure is only produced when a force is applied to a piston. So you generate pressure only when you actually need to move something — you’re not maintaining pressure all the time. A classic real-world example is a light aircraft braking system. You have a master cylinder that generates pressure when you press the brake pedal, and a slave cylinder that does the work — moving a piston and applying the brakes. The master cylinder is your input, the slave cylinder is your output. Only one brake is shown in the figure, but that’s the idea. Now contrast that with an active hydraulic system. Here, a pump is required to deliver a flow of fluid into the system, and some form of restriction is required to obtain pressure. In hydraulic systems, that restriction is provided by movable pistons which travel backwards and forwards in cylinders — and these assemblies are known as hydraulic jacks or actuators. The reason you need that restriction is simple: a pump alone just moves fluid; it’s the resistance to that flow that builds pressure. And because different services — undercarriage, flaps, spoilers, nose wheel steering, power flying control units — require different amounts of power depending on their size and loading, you need a “gearing” effect. That’s achieved easily by varying the size of the actuator pistons, while the hydraulic pressure remains constant. Bigger piston, more force, less travel; smaller piston, less force, more travel. Now let’s talk about the fluid itself. The efficiency of a hydraulic system is governed by the resistance to motion encountered by the fluid. For all practical purposes, hydraulic fluids are considered incompressible — except at high pressures, specifically 27.6 MN/m² and above. That’s 276.7 bar, or 4300 pounds per square inch. Let me give you a concrete sense of that. If a container with a certain volume of liquid has a pressure of 34.6 MN/m² — that’s 346 bar — applied to it, the reduction in volume is small, compared to a similar container of air. Here’s the contrast: liquid is compressed by only 1% of its original volume, and 99% remains. Air, on the other hand, is compressed by 99% of its original volume, and only 1% — that’s 1/100 — remains. So the liquid barely changes volume; the air collapses. And one more important point: the pressure in both will be felt equally in all directions. That’s Pascal’s principle in action — pressure acts uniformly. Finally, in practice, a certain amount of force is expended in overcoming static resistance — that is, friction. And that friction occurs in three specific places: between pistons and cylinders, between piston rods and bearings, seals, or glands, and between the fluid and the pipe walls. That’s the resistance that eats into your efficiency, and it’s why we care about fluid properties and surface finishes. So to tie it together: passive systems use no pump and generate pressure on demand; active systems use a pump plus a restriction to build pressure continuously; and the fluid itself is treated as incompressible except at very high pressures, with friction losses at the pistons, rods, and pipe walls.