A driving linkage is coupled between the barriers and {TAIL ~ the parallelogram linkage}, such as {HEAD ~ a simple bell crank linkage} 
A driving linkage is coupled between the barriers and { TAIL ~ the parallelogram linkage } , such as { HEAD ~ a simple bell crank linkage }
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD