Strength Criteria for Designing Hybrid-Fiber Reinforced Plastic Gears

Abstract

In this study, we investigated the bending fracture characteristic of FRP gears as an important design criterion. Gears made from four kinds of laminated materials were tested: cotton reinforced phenolic resin, cotton-carbon (30%:70%), cotton-aramid (70%:30%, 40:60%), aramid, and glass fiber cloth. Cotton-cloth and cotton-aramid cloth are called hybrid-fiber cloth in this paper. Gears were reinforced by setting the fiber cloth in radial and circumferential directions and were manufactured by hobbing. This results in the same bending fracture strength for all the gear teeth. First, we investigated the tensile strength of the laminated materials because this is a fundamental design criterion for metallic gears. Second, we estimated the bending fracture strength of each gear’s teeth because this is a fundamental evaluation index in the design of gears. Theoretically, the bending fracture strength of a gear is in proportion to the tensile strength of the materials from which it is made. However, in our experiments, we found this was not the case. To discover the cause, we looked at the fracture position on each gear tooth. That of the cotton-carbon gear was more towards the tip of the tooth than the in other gears, and that of the glass fiber gear occurred on the compressive pressure side of the tooth root. We then investigated the shearing strength and compressive strength of the materials because these may have influenced the position of the fracture points. The shearing strength of the cotton-carbon laminated material was low compared with its tensile strength, as was the compressive strength of the glass fiber laminated material. We found that if the shearing strength is low compared with the tensile strength, the position of the fracture that occurs is towards the tip of the gear tooth. If the compressive strength is low compared with the tensile strength, a fracture on the compressive side occurs. Our results show that it is necessary to consider the tensile, compressive, and shearing strength of FRP materials when designing gears made from FRP.