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Integrated Hybrid Gear/Shaft Technology for Rotorcraft Drive Systems

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: The goal of this program is the development and application of innovative methods to design, build, and test a prototype composite integrated bevel gear/shaft design that is sized for a helicopter intermediate gearbox. This effort is expected to enhance the state-of-the-art and cover all aspects of hybrid gear development, including composite material selection, tooling, joining, and molding techniques, leading to the delivery of working prototypes. 

DESCRIPTION: Propulsion systems on military (and commercial) helicopters are a key contributor to the overall weight of the aircraft. In some instances it accounts for almost 30% of the aircraft’s empty weight. This percentage is expected to be even higher in the Army’s future vertical lift (FVL) aircraft that employ more advanced variable speed transmissions due to the need for improved operational flexibility. Unfortunately, transmissions with variable speed capabilities are heavier than the fixed ratio transmissions currently in use due to the weight from the additional components. To counter the added weight and help meet Capability Set #1 and #3 requirements in FVL, the Army is currently pursuing innovations in hybrid gear technology where the steel hub is replaced by a strong, lightweight composite material. Recent experiments have successfully demonstrated the technology on a representative hybrid bull gear that was able to transmit 5000 Hp in a simulated transmission environment while weighing 20% less than the full steel counterpart. This approach is expected to reduce operating costs while increasing performance in terms of greater speed range and payload. In an effort to reduce drive system weight even further, research is focusing on the concept of an integrated design by fabricating the gear hub and its adjoining steel shaft as one piece from a strong, lightweight composite material and then joining it with a steel gear. This approach builds on previous hybrid gear achievements and is the next logical step in reducing weight in rotorcraft drive systems. It also poses new challenges for the designer to identify the proper molding techniques and tooling to fabricate the composite material into an integrated, one-piece design. Both the Army and Navy are very interested in utilizing hybrid technology to reduce the overall weight of a rotorcraft’s main transmissions. However, demonstrating the viability of the technology in an actual main transmission would be expensive due to the high costs of running such a specialized test facility. For this research topic, the Army and Navy have selected a helicopter intermediate gearbox (IGB) as the technology demonstrator because of its relatively simple single gear pair reduction stage design that features two meshing bevel gears with integrated shafts supported by bearings. The bevel gear configuration adds complexity that should advance the state-of-the-art of hybrid gear technology. Whereas the hybrid bull gear was a double helical design with predominately torsional loads, the forces associated with bevel gears in the IGB are complex and include tangential, radial, and axial loads. This load environment requires advanced fabrication and joining techniques, especially at the composite material-bevel gear interface. The integrated assembly must meet the precise gear dimensional and performance specifications for aerospace applications and the tight dimensional tolerances on the shaft inner and outer diameter for balance requirements and the accommodation of tapered rolling element bearings. The goal of this program is the research, development, and application of innovative technology and methods to design, fabricate, and experimentally evaluate a prototype composite integrated hybrid bevel gear/shaft technology that is applied to a helicopter IGB. This effort is expected to enhance the state-of-the-art and cover all aspects of hybrid gear development, including composite material selection, tooling, composite-metal joining, and molding techniques, leading to the fabrication and delivery of working experimental prototypes. The success of this effort will elevate the TRL to 6 and accelerate the implementation of integrated hybrid gear/shaft systems into rotorcraft drive systems. 

PHASE I: Phase I should identify potential lightweight composite materials, tooling requirements, and molding processes that will enable the design and fabrication of an integrated hybrid bevel gear/shaft assembly that can withstand the speeds and loads present during endurance qualification tests for Army helicopter IGB’s. The qualification tests for IGB’s simulate normal cruise and more aggressive flight maneuver conditions. To simulate normal cruise the IGB transmits 524 shp with the input pinion operating at 4114 rpm and the output gear operating at 3318 rpm. Total time at this condition is 60 hrs. To simulate more aggressive flight maneuvers the gears are required to transmit 630 shp with the pinion operating at 4114 rpm and the output gear 3318 rpm. Total time at this condition is 30 hrs. Dimensional drawings for the pinion and gear assemblies will be provided by the government for design purposes. Each all-steel gear assembly (pinion and gear) currently weighs approximately 5.5 lbs. Major focus points of the effort should include maximizing strength while minimizing weight, identifying adequate joining techniques to connect the composite material to the steel bevel gear, and developing molding processes that can meet the strict dimensional tolerance requirements. The geometry of the hybrid gear/shaft assemblies should closely mimic the current all steel designs sufficiently to be drop-in replacements. Based on previous results it’s expected a weight savings of at least 20% over the current design should be achievable. The final report shall identify the composite material, tooling, and molding processes for fabrication of the hybrid gear/shaft assemblies along with structural analysis to demonstrate the viability of the composite design to successfully complete the qualification tests. 

PHASE II: Demonstrate the ability to fabricate and deliver at least two integrated hybrid gear/shaft prototypes based on the lessons learned in Phase I. The steel gear portion of the hybrid prototypes will be made available from all-steel input pinion and output gear assemblies from previously flown IGB’s. The prototypes should be able to transmit 524 shp with the pinion operating at 4114 rpm and the output gear at 3318 rpm for at least 60 hrs. The prototypes should also be able to withstand the aggressive flight maneuver load of transmitting 630 shp for 30 hrs with the input pinion operating at 4114 rpm and the output gear at 3318 rpm. Their geometry should closely mimic the current all steel designs sufficiently to be drop-in replacements. The prototypes will be delivered to the Army and undergo flight qualification testing in the Helicopter Drive System (HeDS) Facility at Naval Air Station, Patuxent River, Maryland. The flight qualification tests will be conducted by the Army and any test data will be shared. It’s expected successful completion of Phase II should boost the technology to TRL 6. 

PHASE III: Commercialization of the technology enabling integrated hybrid gear/shaft designs should find a large market in both the military and commercial sectors. Major helicopter manufactures will use the technology to reduce weight, decrease operating costs, improve operational flexibility, and extend mission range. Successful integration will also propel the technology into other areas requiring power transfer and distribution 

REFERENCES: 

1: Handschuh, R.F., LaBerge, K.E., DeLuca, S., Pelagalli, R., "Vibration and Operational Characteristics of a Composite-Steel (Hybrid) Gear," NASA TM 2014-216646

2:  ARL-TR-6973, 2014.

KEYWORDS: Gear, Hybrid Gear, Composite, Transmission, Drive System, Future Vertical Lift, Rotorcraft 

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