Design of composite materials for improved loss-of-lubrication survivability of hybrid rotorcraft gears

Hybrid gears featuring steel teeth mated to a composite body provide the potential for significant weight savings in aerospace applications such as rotorcraft and geared turbofan engines. For hybrid gears to be viable for use in these applications, they must not degrade mechanical performance or thermal characteristics, particularly under loss-of-lubrication operation. The heat generated by loss-of-lubrication operation may be especially problematic for the fiber-reinforced polymer composite materials used in hybrid steel-composite gear prototypes. Initial hybrid gear design and testing is described in the literature, but no prior studies have investigated optimization of the composite material for thermal performance. In the study presented herein, conductive fibers and high-temperature polymers are introduced to make a composite material better suited to high-temperature and loss-of-lubrication. Channel-flow resin transfer molding was used to fabricate composites with a variety of configurations, including hybrid reinforcement alternating highly-conductive pitch-based carbon fiber plies with typical high-strength polyacrylonitrile (PAN)-based carbon fiber plies. Performance was evaluated by thermal conductivity measurement and compression strength testing. Finite element modeling was performed to quantify strength and stiffness requirements for the composite gear web and to investigate the effects of using plies of different stiffness in a hybrid laminate. Results of this study show that the in-plane thermal conductivity of composite laminates can be greatly increased by using hybrid reinforcement, but the hybrid gear web design must account for the low compression strength of the hybrid reinforcement to ensure that the structural integrity of the composite gear web is not compromised.