建立了三线摆无阻尼自由扭转非线性振动方程，研究解的非线性特下，求出了周期和解的精确表达式。图示振幅，线长，盘径等参数对周期的影响，截取级数展开式给出近似解。
The differnetial equation for three string pendulum in torsional oscillation is derived.Its nonlinear characters are studied.Exact solutions for the torsional oscillation equation and period are given.Influences of parameters (amptitude,length of string and diameter of disk) on period are discussed.Truncation of series expansion for exact solution reduces to the approximate solution.
