Gas Turbines - Lubrication — Page 289, Lesson 377

I want to walk you through the reasons we lubricate a gas turbine engine, and then the two basic systems we use to do it. Let's start with why we need oil at all, because it's far more than just reducing friction. The first and most obvious reason is reducing friction between moving parts. But here's the thing — no matter how scrupulously clean we keep the engine, a small amount of dirt or impurities will always find its way inside. That dirt is dangerous. It has to be removed before it can damage the bearings or block the small oil passageways. So the oil itself becomes a cleaning agent. It carries that dirt along to the oil filter, where the dirt is strained out and stays trapped until the filter is replaced. Next, corrosion. The majority of the bearings in the engine are made from steel, and steel will oxidize — that's rust — if it isn't protected. A liberal coating of oil prevents that oxidation, so the lubricant minimizes corrosion inside the engine. Then there's cooling. The bearings, particularly those around the hot end of the engine, must be cooled if they're going to withstand the constant stresses imposed on them. The most likely medium for that cooling is the lubricant itself — the same oil that cleans, reduces friction, and fights corrosion. And finally, one task that's easy to overlook: the oil acts as a hydraulic fluid. In many turboprop engines, the control of the propeller blade pitch is achieved by passing some of the engine lubricating oil into the pitch change mechanism. So the oil isn't just protecting the engine — in a turboprop it's actually doing work, moving the blades. Now, how do we get that oil around the engine? Most gas turbine engines use a self-contained recirculatory lubrication system. That means the oil is distributed around the engine and then returned to the oil tank by pumps. It's a closed loop — the same oil keeps circulating. There are two basic recirculatory systems. The first is the pressure relief valve system. The second is the full flow system. I want you to hold those two names, because they describe how the oil pressure is controlled and how the oil flows through the system. There's a schematic layout of the basic system in Figure 19.1, which shows the relative location of the major components — the tank, the pumps, the filter, and how they connect. That's the picture you'll want in your head as we go deeper into how each of these two systems actually works.