A Method for Reliability Improvement of Torsion Spring for Exhaust Valve 2013-01-0076

The exhaust valve plays a role of reducing the mechanical noise and vibration of vehicle by smooth discharging of the vehicle emissions after combustion process in exhaust system of engine. The torsion spring is one of the most core components in exhaust valve, which generates variable torque for control of opening and closing angle of exhaust valve. Its performance represents all over the performance of exhaust system of vehicle. As it were, the failure of torsion spring means the failure of all exhaust system. So, as well as performance, the reliability of the torsion spring is very important. To secure the reliability is same to secure the security and comfort of passengers including driver. This paper proposed two methods for improvement of torque and reliability characteristics of torsion spring. One is improvement of heat treatment condition for getting of more constant torque characteristic of torsion spring. The optimal heating cycle for heat treatment is newly introduced, which is time and temperature concept based on related references and author's empirical knowledge. The propriety of this method is verified through comparison of torque characteristics between before and after heat treatment using the new condition. Another is inducement of design equations for theoretical design and decision of the design parameters. The design equations composed by combination of the manufacturer's opinion, references, and empirical knowledge. The correctness of the design equations are shown by similarity of the torque values between theoretical design and experimental results of prototypes. In addition, we also developed some kinds of failure analysis documents as like FMMA(Failure Modes and Mechanisms Analysis), FMECA(Failure Modes Effects and Criticality Analysis), FTA(Fault Tree Analysis), and 2-Stage QFD(Quality Function Deployment) for reliability assessment of torsion spring. These offer important failure information like major failure cause and effect, life time, test items, and accelerated life factor of torsion spring.