Basic Hydraulics — Page 52, Lesson 78

Let’s pick up right where we left off — we were talking about efficiency losses in a hydraulic system, and I want to finish that thought before we move into seals. I said that large bore pipes and frictionless pistons would let you use nearly 100% of the force, but they’d cost you dearly in weight and money. So in practice, you accept some loss. The friction between pistons and cylinders, and between piston rods and bearings, can’t be completely eliminated — good design and workmanship can only lessen it. Then there’s the friction between the pipe walls and the fluid itself, and that depends on four things: the velocity of the fluid in the pipes, the length, bore and internal finish of the pipes, the number of bends, and the viscosity of the oil. Those four factors govern how much friction you get, and therefore how much resistance. And because you also need glands, seals and backing rings to stop leakage, the most practical way to counteract this loss in efficiency is to use the correct fluid. Now, seals. They perform a very important function — preventing leakage of fluid. There are two broad families. Static seals, which include gaskets and packing, are used in many locations, and they effect a seal by being squeezed between two surfaces. Dynamic seals, on the other hand, are fitted between sliding surfaces, and they come in many different shapes depending on their use and on the fluid pressures involved. Here’s the key distinction: “U” and “V” ring seals are effective in one direction only. But “O” rings and square section seals are often used where pressure is applied in either direction. Dynamic seals need lubrication to stay effective, so wetting of the bearing surface, or a slight seepage from the seals, is normally acceptable. And where you have high pressures, an “O” ring is normally fitted with a stiff backing ring. That backing ring retains the shape of the seal and prevents it from being squeezed between the two moving surfaces. Finally, seals are made in a variety of materials, depending on the type of fluid they’re going to be used with. So the picture is: static seals get squeezed between two fixed surfaces, dynamic seals live between sliding parts and need lubrication, U and V rings only seal one way, O rings and square sections handle pressure from either direction, and high pressure calls for a backing ring to keep the O ring from extruding. That’s the seal story.