a pair of shafts provided at one end with the paired horizontal rollers and at {TAIL ~ the other end} with {HEAD ~ mutually engageable gear wheels}, 
a pair of shafts provided at one end with the paired horizontal rollers and at { TAIL ~ the other end } with { HEAD ~ mutually engageable gear wheels } ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH