Each sliding frame is provided with a pair of rack gears with teeth facing each other and cam surfaces facing each other in parallel with {HEAD ~ a pitch line} of {TAIL ~ the rack gears} 
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Each sliding frame is provided with a pair of rack gears with teeth facing each other and cam surfaces facing each other in parallel with { HEAD ~ a pitch line } of { TAIL ~ the rack gears }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL