CompositesWorld
Issue link: https://cw.epubxp.com/i/450689
FEBRUARY 2015 22 CompositesWorld WORK IN PROGRESS MI, US), is working with A&P; on a topology optimization model that will help guide design refnements, but notes that the hexagonal interlock attachment scheme with a constant (not yet optimized) web thickness, was used for simplicity's sake, in the initial hybrid spur gear evaluation. "Te laminate disks were overdesigned for structural loads in the initial tests since the primary purpose of these tests was to identify potential problems with material and attachment endur- ance in a gearbox environment," Roberts explains. "It will be interesting to use the model to optimize web thickness profle and attachment geometry for even greater weight savings in future hybrid gear designs." Te A&P; team, led by application engineer Nate Jessie, fabri- cated the attachment test elements and the demonstration spur gears in a press-molding process, after frst laying up 12 plies of QISO prepreg. Jessie explains that each ply was rotated 60° during the layup, despite the QISO's already balanced weave, to ensure that the laminate would be "fully isotropic." Te laminates were ovencured, then waterjet trimmed to net shape. Te gears were fabricated in the sequence shown in Fig. 3 (p. 21), with a second heated press cycle to cure the flm adhesive. "Tere are three 12-ply QISO laminates in each gear; two outer and one inner, assembled with adhesive flm," explains Jessie. To see if they could stand up to the heat, vibration and torque loads, the press-cured, bonded spur gears were subjected to endurance tests in a gearbox operated at 10,000 rpm and nearly 500 in-lb of torque. After 1 billion cycles, there was no detectable gear damage nor composite delamination (see Fig. 4, p. 21). In separate static tests, larger versions of the gears (without gear teeth) were used to evaluate each of the three attachment concepts under similar conditions and to provide data for valida- tion of structural optimization analysis methods, says Roberts. Te larger structures were mounted on a test fxture to which acceler- ometers were attached, so that acoustic emission "events" (sounds that indicate breakage of the adhesive bonds) could be moni- tored during load application. Te static results showed that, at room temperature, bond failure of the gear web occurred at about 50,000 in-lb of applied torque for the frst concept, at about 55,000 in-lb for the hexagonal interlock design, and at 68,000 in-lb for the lobed concept. Te hexagonal and lobed interlock designs not only performed better than the adhesive-only design but also provided ongoing load-carrying ability after bond failure. Te lobed interlock was the last to disbond, failing at about the 5-minute mark. Although the results of modal analysis and free vibration tests were inconclusive, Roberts says, "We believe that composites will give us the ability to tailor the natural vibration frequency, for better damping." Toward that end, the team will alter the laminate thickness, adjust its stifness by using diferent carbon fber types, and might add visco-elastic material. During the project's second phase, the team will fabricate and test scaled-up steel/ epoxy composite hybrid "bull" gears (see Fig. 5, p. 21), and bismaleimide (BMI) and polyimide (PI) matrices will be investigated. "Some gear applications require continued operation for a specifc time period if loss of lubri- cation should occur," Roberts reports, so the gear laminate might need a high-temperature-capable resin "to survive the thermal tran- sient after loss of lubrication, when we go into actual feld trials." A&P; will deliver several bull gears for testing in NASA's High Speed Helical Gear Test Rig. During additional structural tests of the attachment approaches, high-speed, high-resolution digital image correlation methods will be used to evaluate local defor- mation near attachments during operation. After this testing is completed, says Roberts, the team will be better able to assess the possibility of commercializing hybrid gears in rotorcraft. Positive results, more work So far, composite material and a steel/composite adhesive bond have proved capable of handling temperatures, loads and oil exposure within a gearbox. Static testing shows the mechan- ical interlock features provide sufcient strength for torsional loading. Hybrid spur gears have shown nearly 30% weight savings compared to all-steel gears, without design optimization. And A&P; is investigating the use of composite material in the gear hub and connecting shaft to form an integrated web/hub/shaft in compos- ites with steel gear teeth. Roberts anticipates that this design could yield 50% weight savings (Fig. 6, above). Te braided preform and resin infusion process are under development. "An initial design and prototype will be delivered at the end of the contract," he predicts. "A&P; is working out some very intricate fber architecture that results from the shape of the preform, and how that architec- ture can handle the required loads." But Roberts contends, "We are just beginning to explore the possibilities." Te helicopter gearbox is a challenging environment. "As a result, fight qualifcation for rotorcraft will be very expensive and take a long time. But," he adds signifcantly, "I believe there could be other commercial/industrial applications for gears of this type in the nearer term." ABOUT THE AUTHOR Sara Black is CW's technical editor and has served on the CW staf for 17 years. sara@compositesworld.com Read this article online | short.compositesworld.com/HeliGears FIG 6: All-composite Gear Concept? This rendering shows a conceptual all-composite gear under investigation that potentially could cut gear weight by 50% vs. a comparable steel gear. Source | A&P; Technology