A projection 18 is provided in the chains 17 to push {TAIL ~ the rear end} of {HEAD ~ the base frame} 5 
A projection 18 is provided in the chains 17 to push { TAIL ~ the rear end } of { HEAD ~ the base frame } 5
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 OTH