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