Theoretical Design and Performance Improvement of the Torsion Bar in Constant Force Retractors

Abstract

Constant force retractors are a new type of seat belt retractor which, during a crash, can generate nearly constant belt restraint forces on occupants required by advanced restraint systems. There are several types of designs to provide the constant belt force by retractors. One of these is the torsion bar. In this design type, a metal bar serves to take the belt load in the torsional mode then to yield at a preset load level and keep the belt load at that level for a certain amount of belt pay-out. This paper first presents the theoretical design of the torsion bar. It applies material plasticity to derive all the equations for the design of torsion bars with circular cross sections. In addition to design equations, this research reveals the mechanism of a transition process from elasticity to plasticity in the torque rotation property that reduces performance quality of the constant force retractors. Concepts and theory of two alternative designs to reduce the transition process and improve performance are also presented. Most design equations have been verified by test data.