A shape change for opening or closing the loop-spring is induced electrically by piezo-elements which are located on or are integrated in the spring. The piezo-elements may be located on segments of the surface of the loop-spring. The loop-spring may be evacuated, and the drive shaft or output shaft has a layer with piezoelements. The drive- or output-shaft has an ultra-hard covering layer on the piezo-layer. The shaft perimeter may be varied through operation of the piezo-layer. Thermally-conditioned changes of diameter of the shaft or hollow shaft may be compensated by controlling the piezoelements.
