Synchronous belts are rubber-composite materials with rubber, helical cords and facing fabrics. The helical cord is the tension member of the belt and is made of glass fibers, aramid fibers, or steel wires. Recent trends require increasingly high stiffness for the rubber belts. The use of carbon fibers and hybrid cords with carbon fibers are considered to be an effective way to achieve high stiffness for helical cords. This paper presents the study is to improve the bending fatigue strength of hybrid cords, where the center strand is made of carbon fibers, and the outer strands are made of glass fibers. The optimum cord composition for good bending fatigue durability is discussed following experimentation, mechanical analysis using a simplified mechanical model and FEM analysis. The model reasonably explained the initiation of the fatigue failure initiation site in the hybrid cords. The optimum cord composition was proposed for the bending fatigue strength basing on the simplified mechanical model. This was verified by experimental data showing good fatigue life. The use of such helical cords can considerably extend the operating life of synchronous belts.

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