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2004 Edition, October 2004

Complete Document

Design of a High Ratio, Ultra Safe, High Contact Ratio, Double Helical Compound Planetary Transmission for Helicopter Applications

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Description / Abstract:

An ultra-safe, high ratio compound planetary transmission, for application as a helicopter main rotor drive, has been designed under the sponsorship of the National Rotorcraft Technology Center- Rotorcraft Industry Technology Association (NRTC/RITA). It is anticipated that this new planetary transmission offers improvements relative to the current state-of-the-art including, reduced weight, reduced transmitted noise and improved fail-safety. This paper discusses the analysis and design results for the subject planetary transmission. Fabrication and testing of the transmission will be conducted in subsequent phases of the project.

Typically, the final stage in helicopter main rotor drives consists of one or two simple planetary stages which themselves are composed of a sun gear,multiple planet gears (typically between 3 and 6) and an internal gear which is generally the fixed member of this epicyclic system. The main rotor transmission is the most critical and usually the heaviest assembly in the drive system for any rotary wing aircraft, be it a single or tandem rotor helicopter or a tilt rotor vehicle.

The new ultra-safe, high ratio planetary transmission design utilizes a compound planetary configurationwith a 17.5:1 reduction ratiowhich would replace a conventional two stage simple planetary transmission. The new design uses ultra-safe principles such as split-torque paths and high combined contact ratio gearing. Double helical gears in the planet/ ring meshes balance axial tooth forces so that axial bearing reactions are not required. The spur gear sun/planet meshes are staggered to achieve a compact spatial arrangement.

In order to provide major, simultaneous improvements in weight, noise, power density, and reliability, a paradigm shift must occur in the basic design of the gear system in this component. This program brings together a large number of individual gear system innovations which, taken together, are expected to yield a large improvement in noise and performance combined with improved reliability and fail safety.