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