The automotive propeller shaft transmits power from the transmission to the differential gear box in rear wheel driving vehicles. The torque capacity of propeller shaft for RVs (Recreation Vehicles) should be larger than 2,700 Nm and the fundamental bending natural frequency should be higher than 6,500 rpm to avoid whirling vibration. Since the fundamental bending natural frequency of a shaft is proportional to $\frac{1}{l^{2}}\\sqrt{frac{EI}{ρ}}$ (l: shaft length, EI: bending frequency stiffness, ρ : density per unit length of shaft), the steel propeller shaft is usually manufactured in two-piece to meet the requirement of 6,500 rpm. A two-piece steel propeller shaft consists of three universal joints, a center supporting bearing and a bracket, which increase the total weight of car and decrease the fuel efficiency. Since the carbon/epoxy composite material has more than 4 times higher specific stiffness (E/ρ) than steel or aluminum materials, it is possible to manufacture the composite propeller shaft in one-piece without whirling vibration over 6,500 rpm.
In this study, a new manufacturing method for one-piece aluminum/composite propeller shafts was devised, in which carbon/epoxy composites were cocured inside the aluminum tube rather than outside wrapping to prevent the composite layer from being damaged by external impact and absorption of moisture. The optimal sequence of composite layer was determined considering the thermal residual stresses of interface layer calculated by finite element analysis. The interference fitting joint between the aluminum/composite tube and the steel yokes was employed to improve reliability and to reduce manufacturing cost, rather than other joining methods such as adhesive, mechanical and welding joints. The appropriate protrusion shape of inner surface of yoke was suggested considering the torque capacity of interference fitting joint from finite element analysis. From the experimental test and finite element analysis, it has been found that the manufactured one-piece aluminum/composite propeller shaft has 75 % mass reduction, 160 % increase in torque capacity compared with the conventional two-piece steel propeller shaft. It also has 7,890 rpm natural frequency which is 1.2 times higher compared with the design specification of 6,500 rpm.
