Gas Turbines - Lubrication — Page 300, Lesson 384

I want to walk you through the full-flow type lubricating system, because this is the system you'll actually see on modern turbofans. We've already covered the pressure relief valve method, so now let's look at how the full-flow system differs and how the oil gets around the engine. First, the oil is delivered through two galleries. One gallery feeds the drive gearbox. The other gallery transfers oil to the bearings that support all of the compressor spools. So you have a dedicated supply path for the gearbox, and a separate path for the bearing support structure. Now, the bearings themselves are lubricated by oil jets. These jets are positioned very close to the bearings, and that's deliberate — it minimizes the possibility of the oil being deflected from its target by local turbulence. Think about the high-speed airflow and rotating parts inside the engine; if the jet were far away, the oil stream could get blown off course. So we place the jets right up against the bearings. Just prior to the oil jets, we fit thread type filters. These perform the function of a 'last chance' filter. The name tells you exactly what it does — it's the final opportunity to remove any debris which may have managed to pass through the main pressure filter. So even if something got through the main filter, this last chance filter catches it before it reaches the bearing. Now, just like in the pressure relief valve system, once the oil has completed its tasks, it's collected and passed back through scavenge pumps. But before the oil reaches the scavenge pump, it must pass over a chip detector and through a suction filter. The chip detector is a magnetic plug that catches metallic wear particles — if you get metal chips in the oil, that tells you a bearing is breaking down. The suction filter is on the inlet side of the scavenge pump, cleaning the oil before it enters the pump. The scavenge pumps then force the oil through to the oil cooling system. In the engine shown in Figure 19.3, there are two types of oil cooler: a fuel-cooled oil cooler and an air-cooled oil cooler. Normally, the fuel-cooled oil cooler is sufficient to cool the oil on its own. But in the event that it proves inadequate, a valve opens automatically and brings the air-cooled oil cooler into operation as well. So you have a primary cooler that usually handles the job, with a secondary cooler that cuts in automatically only when needed. Now, there's an important venting consideration. As we've seen previously, air pressure escaping from seals cannot be allowed to build up within the engine. So it's vented through the hollow shaft between the intermediate gearbox and the external gearbox, leaving the latter via the centrifugal breather. The centrifugal breather spins to separate oil from the air before the air is released, so you don't lose oil overboard. Let me now move to the oil tank. The oil tank is normally a separate unit mounted on the side of the engine, although it can be part of the engine intake, or even an integral part of the external gearbox. So there are three possible configurations, but the separate side-mounted unit is the most common. As a separate unit, it must incorporate provision for filling, both by gravity and, more normally, by pressure. So you can pour oil in from the top, but in practice you usually pump it in under pressure. There must also be some method of determining the contents of the tank, either by a sight glass or by a dipstick, sometimes by both. And there's a de-aerator tray, which allows removal of air bubbles from the oil as it flows back into the tank. That's important because aerated oil doesn't lubricate as effectively and can cause pump cavitation. Now let's talk about the oil pumps themselves. Gear type pumps are the normal method of delivering and retrieving oil in a re-circulatory system. They consist of a pair of intermeshing steel gears located within a close fitting aluminium housing. So you have steel gears inside an aluminium body. When the gears are rotated, oil is drawn into the pump, carried round by the teeth, and delivered at the outlet. That's the basic gear pump principle — the teeth trap oil between them and the housing, and carry it from the inlet to the outlet as the gears mesh. The oil pumps, both pressure and scavenge, are fitted on the accessory housing. So both the pump that delivers oil under pressure and the pumps that scavenge the oil back are mounted together on the accessory gearbox. So to tie it all together: the full-flow system delivers oil under pressure through two galleries, one to the gearbox and one to the bearings. The bearings get oil from close-positioned jets with last chance filters. The oil is scavenged back, passing over a chip detector and through a suction filter, then through the coolers — fuel-cooled normally, air-cooled as backup. The tank stores and de-aerates the oil, and gear type pumps handle both pressure and scavenge duties from the accessory housing. That's the complete circuit.