Therefore, when {HEAD ~ the cylinder} operates for {TAIL ~ the wheels} to move relative to the propped block, the head member can be moved up and down 
operates for
Therefore , when { HEAD ~ the cylinder } operates for { TAIL ~ the wheels } to move relative to the propped block , the head member can be moved up and down
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH