{HEAD ~ a ring gear} of the reduction gear is screwed into an inner peripheral surface of {TAIL ~ the first housing} through the first housing opening, wherein 
{ HEAD ~ a ring gear } of the reduction gear is screwed into an inner peripheral surface of { TAIL ~ the first housing } through the first housing opening , wherein
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH