A "float center" spool has both cylinder ports vented to each other and to exhaust, with inlet pressure blocked, in center position. This relieves pressure block on both sides of the cylinder position, alowing piston rod to be moved manually, or to be freely pushed and pulled by a moving member of the machine to which the cylinder is attached. This "floating" action is occasionally used on both air and hydraulic systems.
David Baird
mrbaird@hotmail.com
Sr Controls Designer
EET degree.
Journeyman Electrician.