PSI - Issue 80

Seiji Mitsubayashi et al. / Procedia Structural Integrity 80 (2026) 431–442 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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are anticipated. However, as motor speeds increase, issues such as vibration and noise become more significant concerns, which drives the demand for high-efficiency, compact, and lightweight transmissions [Hinov, N. et al. (2021); Miler, D. et al. (2018)]. These requirements, in turn, raise concerns about the reduction of gear strength and increased noise. In this context, this study focuses on CFRP (carbon fiber reinforced plastic) gears, which offer high specific strength and have favorable properties such as low weight, excellent damping, and the ability to operate quietly even without lubrication [Liu, H. et al. (2020); Jena, H. (2019)]. Common fabrication methods for CFRP gears include injection molding, machining of laminated plates, and additive manufacturing via 3D printing [Zmindak, M. et al. (2022)]. Injection molding uses thermoplastic resins, which are generally weaker than thermosetting resins. Furthermore, only short fibers can be mixed into the resin pellets used in injection molding, which limits potential improvements in mechanical strength [Zurunge, A. et al. (2020)]. In contrast, machining of laminated plates allows for the use of long fibers by incorporating unidirectional or woven fabrics [ Černe, B. et al. (2022)]. However, due to varying fiber orientations from tooth to tooth, strength inconsistency becomes a concern. While 3D printing allows for targeted reinforcement of the tooth surface and can minimize variation in strength among teeth, it remains unsuitable for mass production. Therefore, it is necessary to develop gears that are not only strong but also have consistent mechanical properties. To address these issues, this study proposes a method in which the tooth surface is reinforced with carbon fiber fabric prior to molding with short fiber – reinforced thermosetting resin. The effectiveness of this approach was evaluated by analyzing the formability of the CFRP gears and examining the effects of fiber length, fiber content, and the number of fabric layers on the bending strength of the tooth root. 2. Illustrations 2.1. Materials In this study, the epoxy resin matrix was reinforced by mixing short carbon fibers. An amine-based epoxy resin (jER828, Mitsubishi Chemical Corporation, Tokyo) and a curing agent (jER Cure Lv11, Mitsubishi Chemical Corporation, Tokyo) were used as the resin system. Two types of short carbon fibers were selected for mixing: chopped fibers with an average length of 3 mm (CFUW-MC, Nippon Polymer Sangyo Co., Ltd., Osaka) and milled fibers with an average length of 0.3 mm (CFMP-MC, Nippon Polymer Sangyo Co., Ltd., Osaka). To ensure good dispersion in the resin, both fiber types were used in an unsized form. Additionally, plain-woven PAN-based carbon fiber clothes (CO6343, Toray Industries, Inc., Tokyo) were used to reinforce the tooth surface of the gear. Figure 1 shows a schematic diagram of the CFRP gear fabricated in this study. In this study, CFRP gears were fabricated by placing carbon fiber cloth along the tooth surface and casting a bulk molding compound (BMC) consisting of epoxy resin premixed with short carbon fibers into a mold, followed by curing. Aligning the cloth to the tooth surface allowed the carbon fibers to orient in the direction of the maximum principal stress at the tooth root, which is expected to improve strength. The specifications of the fabricated gears are listed in Table 1. In this study, gears with a module of 3, 20 teeth, and a width of 15 mm were produced. To evaluate the suitability of the molding method, multiple gears were fabricated under various conditions, as shown in Table 2. For enhancing mechanical strength, chopped or milled carbon fibers were mixed into the base epoxy resin. During specimen preparation, the base resin and short fibers were placed into a homogenizer container and stirred using a homogenizer to uniformly disperse the fibers. The mass of carbon fiber required for compounding was calculated using Equation (1): = 1− × (1) where is the mass of added carbon fibers [g], is the fiber mass fraction [-], and is the total mass [g] of epoxy resin and curing agent. 2.2. Method of molding CFRP gears