The casing(1) of the stator has a reduced diameter between its end sections over the greater part of its length. The end sections are hollow cylindrical and have a length of approximately 1-3 cm. The smaller diameter changes into the larger diameter by a conically widening portion(4). In the end sections the wall thickness(W) of the rubber lining(2) encompassed by the casing is approximately double in size in comparison to the reduced diameter center section(6) of the stator.
