Gas Turbines - Gearboxes and Accessory Drives — Page 338, Lesson 428

Let’s pick this up right where the gearbox story gets interesting. We’ve got the high speed external gearbox driven by the HP compressor shaft, and now I want to walk you through the second gearbox and a critical safety feature called the shear neck. First, the second gearbox. Since the first one is driven by the highest-speed shaft, the other one is logically called the low speed external gearbox. It’s driven by a shaft rotating at a lower speed. Now, here’s the practical problem: the engine is round, and the gearbox has to fit around it. That means the gearbox has to be shaped like a banana — curved to wrap around the engine casing. And to make servicing easier, it’s usually located on the underside of the engine. Now, let’s look at what’s actually mounted on that external gearbox. Among the accessories you’ll find the engine hand turn access — that’s a device intended to assist the inspection of the interior of the engine. You physically turn the engine by hand to inspect it. Also worthy of interest are the oil pumps. Notice they’re contained in an oil pump pack — that’s a single unit which contains one pressure pump, but in some engines as many as six scavenge pumps. So one pack, one pressure pump, up to six scavenge pumps. Now, look at the gear teeth themselves. The width of the gear teeth tells you something important about the load. The greatest load from driving the accessories is taken on the right side of the gearbox — those teeth are wide. The thinner teeth on the left side gear wheels show that their load is much smaller. This grouping of small and large gears enables an efficient distribution of the drives for the minimum weight. So you size each gear for its actual load, and that saves weight. Now, the shear neck — this is a crucial safety feature. Think about this: mechanical failure of an accessory could cause the failure of the whole gearbox, and with that, the loss of the engine. To prevent that, the mechanical equivalent of an electrical fuse is fitted to some of the accessory drives. Just like a fuse blows to protect the circuit, a weak section is machined into the drive shaft. That weak section is known as a shear neck. Here’s how it works. The shear neck is designed to fail at a load perhaps 25% in excess of the normal maximum for the particular component being driven by that shaft. So it’s not fragile — it’s rated to handle normal loads plus 25% margin. But in circumstances of excessive overload, the shear neck will break. That allows failure of the individual component while the rest of the gearbox and accessories continue as normal. So one accessory seizes, the shear neck snaps, and the rest of the system keeps running. But here’s the critical limitation: this feature is not utilized in the drives of primary engine accessory units, such as the oil pumps or HP fuel pumps. Why? Because any failure of these components would necessitate the immediate shutdown of the engine. You can’t let an oil pump fail and keep running — you’d lose lubrication and destroy the engine. So those drives don’t get a shear neck; they’re built to be robust because their failure is catastrophic. Now, let me tie this back to the start-up sequence I mentioned. Because on start-up the starter motor causes the HP compressor shaft to rotate first, accessories specific to the engine — like the oil pumps and fuel pumps — are grouped on the gearbox driven by that shaft. That’s the high speed external gearbox. So the critical engine-specific accessories are on the high speed gearbox, and the lower-load accessories are spread across the low speed gearbox. Let me also address the two questions at the end of this section, because they test exactly what we just covered. The first asks about the effect of modifying a gas turbine engine to include one further hydraulic pump. The answer is an increase in specific fuel consumption. Why? Because you’re adding a load — the engine has to burn more fuel to drive that extra pump. The second asks where the drive for fuel, oil and hydraulic pumps is normally taken from. The answer is the HP spool — the high pressure spool, which is the shaft rotating at the highest speed. That matches what we said: engine-specific accessories are grouped on the gearbox driven by the HP compressor shaft. So to summarize: two external gearboxes — high speed and low speed — the high speed one driven by the HP shaft and carrying the critical engine accessories, the low speed one shaped like a banana on the underside carrying lighter loads. And the shear neck is your mechanical fuse, protecting the gearbox from a single accessory failure, but deliberately not fitted to the primary units like oil and HP fuel pumps because their failure means immediate engine shutdown.