The Bendix designed shaft is used on H-500 series as well as the Apache and the Agusta 129. Each of these shafts has a nodal point about which the shaft will whip. At each nodal point the manufacturer installs a stainless ring circling the shaft. This ring rides inside of a damper assembly, which is also installed at the nodal point. If the shaft for any reason starts to whip about its’ rotational axis the stainless ring will contact the inside of the damper causing it to displace. The displacement provides a damping effect canceling out the shaft displacement. The damper is installed between two plates and is restrained in position by springs that are preloaded to a specific setting thus providing friction and restraining damper movement. If the springs are set too loose the shaft will be unrestrained and will whip resulting in fatigue of the shaft and or the Bendix couplings. If the friction is set too tight the shaft will be unable to whip and will be exposed to high stress levels.
At least that’s what I remember from about twenty years ago.
