Driving elements maintain oscillating tangential vibrations of {HEAD ~ the masses} about {TAIL ~ the z axis} 
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Driving elements maintain oscillating tangential vibrations of { HEAD ~ the masses } about { TAIL ~ the z axis }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL