This control effect can be got by designing the size of pokedwheel,using the self locking character of worm wheel and worm pole and connecting the pawl mechanism with overtravel ...

通有效设计拨轮的尺寸，利用蜗轮蜗杆的自锁性，将棘轮机构与行程开关联为一体，达到自控制的目的。