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