CompositesWorld
Issue link: https://cw.epubxp.com/i/450689
FEBRUARY 2015 20 CompositesWorld WORK IN PROGRESS » Among the US National Aeronautics and Space Admin.'s (NASA, Washington, DC, US) many missions is Te Rotary Wing Project, established to increase helicopter speed, range and payload, while decreasing noise, vibration and engine emis- sions. Myriad projects are contributing to these overarching goals. Among them is one funded through a Small Business Innova- tion Research (SBIR) contract that is focused on designing better gears for helicopter drive systems. A team of composite materials specialists is pursuing that end. Headed by NASA Glenn (Cleve- land, OH, US) research engineers Dr. Gary Roberts and Dr. Robert Handschuh, the team includes members from the US Army Research Laboratory (Adelphi, MD, US) and braided rein- forcements supplier A&P; Technology Inc. (Cincinnati, OH, US). Why gears? A helicopter's drive system (engine, vertical driveshaft and geared transmission) makes up 10-15% of the aircraft's total empty weight and is a major contributor to helicopter noise due to vibration. Roberts explains that newer but all-metal multi-speed transmissions could be employed to improve helicopter speed and efciency but would add signifcant weight and wouldn't address the need for acoustic damping. He asserts, however, "Tere's potentially a very high payof in … reducing vibrations near the source, which is primarily at the gear mesh point." Why, then, composites? "Te reason we thought this was a good project," explains Mike Braley, A&P; Technology's VP of application By Sara Black / Technical Editor development, "was that if you replace some of the metal with composites, you could interrupt the vibration transmission pathway from the gear teeth to the center shaft, which will help reduce overall noise, and at the same time reduce the gear's weight without sacrifcing good load performance." Roberts and his team, therefore, decided to inves- tigate how composites incorporated into transmis- sion gears could reduce not only weight and vibra- tion but also, ultimately, maintenance and repairs, because they would have lower densities and higher damping coefcients than conventional steel gear alloys. During the project's frst phase, the feasi- bility of three metal/composite attachment concepts was investigated, small spur gear prototypes were built (using one of the concepts) and then tested. Te ongoing second phase involves investigation of larger, optimized gears and an all-composite web/ hub/shaft concept, along with vibration testing. A composite/metal hybrid What helicopter transmission gears lack in glamor they make up for in severe service (exposed to hot oil and high loads) and their designation as highly proprietary technology. Helicopter OEMs closely protect exact gear tooth profles, Roberts explains, adding, "Currently, gear teeth must be made with gear steel and special alloys, because of the very high loads generated when gears mesh together." For this reason, the project team looked, initially, at hybrid designs in which composites would be used only in the gear's fat inner web, with steel still constituting both the gear teeth and the inner hub. Fig. 1, above, shows one of the small "spur" gears built for the project. "Because the gears are rotating struc- tures, an important property require- ment for the feasibility study was that the constant-thickness composite web section had to exhibit the same stifness in all in-plane directions," Roberts explains. Tat led Roberts to choose A&P;'s trademarked QISO braided triaxial fabric, essentially a braid that is slit open to form a fat broadgood. "A 0/+60/-60 triaxial fabric has the quasi-isotropic property that we needed," says Roberts, "and with QISO, we didn't have to manipu- late multiple uni or woven plies." Te team specifed QISO "heavy" fabric, made with standard- modulus 12K T700S-50C carbon fber supplied by Toray Carbon New aerocomposites niche: Helicopter transmission gears? A NASA study shows that steel/composite hybrid gears save significant weight, and could mitigate vibration-related noise. What helicopter transmission gears lack in glamor they make up for in severe service .... FIG. 1: Braided Web for Gears A typical steel spur gear is shown on the left. A hybrid gear, with a composite material web, is shown on the right. The composite material is a QISO slit carbon fber braid from A&P; Technology, prepregged with an epoxy from TenCate Advanced Composites. Source | A&P; Technology