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Interlocking gears used in clocks give Army helicopters more power

October 28, 2013

The U.S. Army Research Laboratory (ARL) helped figure out 20 years ago a revolutionary way to increase transmission power in the Apache helicopter without increasing the transmission’s size or weight.

Split-torque face gear technology is now inside the Improved Drive System of the new Apache Block III helicopter that began delivery in October 2011.

With split-torque face gear technology, helicopters can now have more power without becoming heavier or bigger, said Lt. Col. David “Blake” Stringer, Ph.D., who is the Chief, Vehicle Technology Directorate Field Element Office in Cleveland. With increased power density, the helicopter’s drive system now has advanced from a horsepower of 2,828 to 3,400, with growth potential, and the helicopter can fly longer, at higher altitudes carrying almost 200 pounds more weapons with a fuel tank—thanks to, essentially, basic scientific research begun by ARL decades ago.

The current Army objective is to field 690 AH-64D Apache Block III helicopters over the next 15 to 20 years. The initial production phase calls for 74 transmissions plus initial spares.

“Face gear technology is really unique because it allows you to send a lot more power through the same geometric footprint than you could normally do with any kind of conventional or other gear configuration,” he said. “The biggest benefit when we started the program was weight reduction. The initial projections from the project were 40 percent weight reduction compared to the current baseline Apache transmission.”

Face gears are a specific type of intersecting-shaft gears that have been traditionally used in positioning mechanisms such as clocks, but the Army found an application to use them in high power transfer applications. Think of gears as a circular disc, Stringer suggested. “The disc consists of the outer circular edge and two ‘faces.’ In spur or helical gears, the teeth are on the outer edge of the gear disc. In bevel gears, the teeth are set at some angle between the outer edge and the face. With face gears, the teeth are cut on the ‘face’ of the gear disc, or perpendicular to the edge of the disc.”

The tests that the Block III and the PM have had with the face gears has been incredibly successful, a lot more successful I think than they had anticipated and so we’ll just keep going and see how it evolves and if it’s as successful on Block III as it continues through the acquisition process, it will probably proliferate through most of the helicopter fleet eventually.

Transmission failure is the second most dangerous in-flight emergency after fires, Stringer said, which is part of the reason that the U.S. Army began an effort in 1988 to improve drive system technology. They focused most of their efforts on reducing aircraft weight and noise. He said the Army also undertook a project to find a way to keep the airframe itself in service much longer, and to develop technology for a future attack helicopter.

Propulsion experts in ARL’s Vehicle Technology Directorate in Cleveland, Ohio teamed with industry, academic, and government partners on this effort, which is primarily studied and tested at the Glenn Research Center. The results led to the creation of the first use of face gears for high speed, high load rotorcraft applications.

“The Army and NASA relationship has been in place since 1970, right before this organization was part of the Army Research Lab. The facilities that have been here because of NASA funding and Army funding are not easily replicated anywhere else,” Stringer said from his office in Cleveland. “They’re unique. They’re always state of the art or better. And that Army-NASA relationship has enabled us – from both organizations – using people and resources from both organizations to really advance the state of the art, not just here in face gear technology but in engine technology, engine materials research technology and all other aspects of helicopter propulsion technology.

Other important contributors included Boeing, Northstar, the Army’s Aviation Applied Technology Directorate, the University of Illinois, NASA, and DARPA.

During initial research and testing, the team developed methods to control tooth contact, simulate meshing, and predict transmission error. They established tooth generation simulation tools and methods to define limiting inner and outer radii and performed initial experiments in-house to demonstrate the feasibility for the use of face gears in helicopter applications. Their studies projected that a split-torque, face-gear transmission gives a 40 percent decrease in weight compared to a conventional design for an advanced attack helicopter application. However, researchers say there’s benefit for just about any helicopter type.

Throughout the 1990s, VTD’s technical guidance helped design and conduct prototype testing. VTD also influenced the design of a face gear grinding machine, which was needed to manufacture high-precision face gears but was commercially unavailable. Gears used in helicopter power transfer applications are highly precisioned mechanical components. The gear tooth geometry is designed to a precise shape with very tight tolerances. This is needed to ensure smooth and proper meshing during operation as well as ensuring durability. The gears are made from high-strength, high-quality steel, undergoing many processes in fabrication to include carburizing and hardening. This is required to produce the high-strength mechanical properties but requires a final grinding step to achieve the accurate tooth profiles. Currently, there are only two machines in existence that are qualified to manufacture face gears for the Apache drive system. ARL tested gears produced on both machines to ensure that they met or exceeded required specifications.

He said face gears are being studied for use in other helicopter drive applications beyond Apache Block III.

ARL holds the institutional knowledge on face gear research and development, and on how to approach new, high-risk technologies, perform the basic and applied research tasks required for proving the concepts, and then transitioning those to the appropriate research, development and engineering center for development. This knowledge has enabled ARL to pursue other high-risk propulsion technologies in the areas of condition-based maintenance, advanced drives materials, and surface engineering techniques.

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