20 dependent on the direction of the rotary movement to move either into or out of the chain path 22 the inner housing 11 12 stretches only very little beyond the position of the sprocket 25 towards the chain magazine and the side wall 59 inner housing shells 11 12 stretches no further than shown in fig5 the outer housing 10 assumes the role of encapsulating the internal components of the actuator in particular the chain on the side of the sprocket where the chain magazine 60 is located the as shown in fig5 the back of the chain 20 is placed right at the boundary of the lower inner housing art 12 and the back of the chain 20 is guided into the chain magazine 60 by the inner wall of the outer housing fig9 is a cross sectional view in detail of a part of a prior art worm 41 u2032 and wormwheel 42 u2032 the worm 41 u2032 is formed of metal such as messing brass or carbon steel for machine structural use or any other steel the teeth of the worm 41 u2032 have a substantially trapezoidal shape as viewed in the direction perpendicular to the axis of the worm the teeth of the worm are formed by.