The chains 17 are located at {HEAD ~ a position} lower than the level of {TAIL ~ the moving surfaces} of the rollers 13 
The chains 17 are located at { HEAD ~ a position } lower than the level of { TAIL ~ the moving surfaces } of the rollers 13
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH