It is known that outside the actual working periods of a starter of this kind its shaft is subjected to considerable vibration communicated to it by the engine with which it is associated.
In order to avoid the fracture of the shaft in question two methods are employed. The first consists in using a very rigid shaft of which the natural vibration frequency is outside the frequencies of the range of utilisation. The other solution provides for the shaft an intermediate bearing fastened to the yoke of the starter.
When a starter is constructed in accordance with the first solution, only radial vibrations of the shaft are limited by the stiffening of the shaft. With the second solution the amplitude of deformation of the shaft is limited to a greater extent, but the drive pinion assembly then behaves like an independent vibrating mass, so that there is premature wear of the grooves or splines of the shaft and drive pinion assembly.