At the core of the GT50 gearbox lies a set of incredibly complex and precisely manufactured gears. The primary function of this gearbox is to take the power turbine outlet speed of 36,350 RPM and reduce it to approximately 6,000 RPM for the main and tail rotors of the helicopter. Achieving this requires gears that are manufactured to an exacting standard.

Our gears are made to an ISO Class 5 standard, which is at the high end of accuracy. We meticulously control three key parameters:

• Profile Slope Deviation: Ensuring the trend of the tooth is perfect, with a tolerance of just ±5 microns.
• Total Profile Deviation: Controlling the overall shape of the tooth within 8 microns.
• Tooth Form: Maintaining the precise form of the tooth to within 6 microns.

To verify these microscopic tolerances, we use an ultra-high accuracy Coordinate Measuring Machine (CMM). This machine traces the gear flanks and generates detailed profile plots, allowing our machinists to make precise adjustments and ensure every gear tooth interacts perfectly. This level of precision is crucial not only for the longevity and 5,000-hour service life of the gears but also for ensuring they run smoothly and quietly—a key goal for the upcoming HX50.

The journey to create these gears involves a host of sophisticated processes, including:

• Machining blanks from the highest quality aerospace gear steels
• Developing processes to keep the core of the gear soft and tough to resist fatigue
• Hardening the gear teeth to withstand extreme wear
• Shot peening between the teeth to enhance fatigue integrity in high-stress areas
• Advanced plating and carburising processes.

The Casing

The gearbox casing presented its own unique set of challenges. It needed to be lightweight enough for flight but stiff enough to hold the gears in their precise alignment. We developed specialised processes to cast and machine lightweight alloy casings that could achieve this delicate balance.

Once the basic shape is cast, we machine all the critical surfaces for flanges, seals, and bearing locations. The thin-walled nature of these components means they are prone to warping during machining. To overcome this, we've developed a whole suite of specialist techniques, including unique machining strategies, custom jigs and fixtures, and carefully controlled stress-relieving and heat treatment processes. The result is a casing that ensures a perfect fit, proper sealing, and exact bearing alignment every time.

In-House Bearings

A significant part of this project has been our decision to design and manufacture our own bearings in-house. This was a strategic move driven by the high cost and large number of bearings required for the GT50. We've invested heavily in perfecting the finish on the inner and outer races, as surface quality is critical for preventing harmonic resonance at high speeds.

We are aiming for roundness tolerances in the order of 2-5 microns. One of the most critical components is the high-speed bearing at the top of the engine, which handles 500 horsepower at over 36,000 RPM. This bearing—from the races to the cage—is entirely designed and manufactured by us. We are now consistently hitting the required tolerances and surface finishes, and we're excited to see it perform under test conditions.

Ready for the Test Bench

The final assembly of the GT50 gearbox is a sight to behold. The core transmission, with its input and output stages, is encased and ready. For our development models, the entire assembly is outfitted with a comprehensive suite of instrumentation, including thermocouples, chip detectors, and pressure sensors. This will allow us to measure every aspect of the gearbox's performance during high-speed testing.