Gas Turbines - Gearboxes and Accessory Drives — Page 338, Lesson 426

Let's look at the problem of driving the accessories off a gas turbine engine, and the first real headache we hit: the compressor shaft moves axially as the engine heats up and cools down. Here's the situation. We take the drive off the compressor shaft through bevel gears — those are the gears with teeth cut at an angle, meshing at a right angle to turn the drive through 90 degrees down to the gearbox. The trouble is, the compressor shaft expands and contracts during the working cycle, so it moves along its own axis. If that axial movement pushes the bevel gear teeth apart, the drive is interrupted. And we're not talking about a gentle interruption — this drive is transferring 400 to 500 horsepower. A momentary break in that kind of power flow would impart massive damage to the teeth of the bevel gears and probably destroy them. So we obviously can't allow that. But we also can't stop the shaft from moving axially — expansion and contraction are unavoidable. So we need a method of arranging the gears so they don't disconnect themselves when the shaft moves. Figure 22.2 shows two methods currently in use on modern engines. The first is the stub shaft drive. If the compressor shaft is splined — that means it has grooves cut in it parallel to its axis — then we can fit a stub shaft around it. This stub shaft has teeth cut internally that conform to the pattern of the grooves in the compressor shaft. So the stub shaft is splined onto the compressor shaft. The key point is this: the shaft can move axially inside the stub shaft, while the stub shaft itself is held firmly in the correct position by the location bearing. The drive continues uninterrupted because the splines keep transmitting torque regardless of where the shaft sits axially. The second method is the idler gear drive. Here we use an idler gear shaft, again held firmly in position by location bearings. One end of this idler gear shaft terminates in a wide toothed spur gear — a spur gear is a straight-toothed cylindrical gear. That wide gear can accommodate the axial movement of the compressor shaft and the spur gear carried on it. The other end of the idler shaft has a bevel gear fitted, which meshes with the radial drive shaft — that's the shaft that carries the drive radially outward to the gearbox. So the wide spur gear absorbs the axial motion, and the bevel gear at the other end keeps the right-angle drive intact. Now, there's also the matter of spreading the load. Driving all the accessories from one shaft puts a lot of stress on one drive. So some engines take a second radial shaft from the low pressure compressor shaft — which is rotating at a slower speed — and use it to drive a second external gearbox. This spreads the load of driving accessories across two drives. And there's a second advantage: it allows the accessories to be divided into two smaller groups, which overcomes the difficulty of limited space around the engine. That's illustrated in Figure 22.3. So to sum up the core idea: we need to take power off the compressor shaft, but the shaft moves axially with temperature changes. The stub shaft drive and the idler gear drive both solve that by letting the shaft slide while the gear position stays fixed. And where space and load demand it, we can take a second drive off the low pressure shaft to run a second gearbox.