The driving devices act into the ring gap, and {HEAD ~ each} of the driving devices has {TAIL ~ a separate drive} 
has
The driving devices act into the ring gap , and { HEAD ~ each } of the driving devices has { TAIL ~ a separate drive }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL