Page 59 - PNEUMATIC SYSTEM INSTRUMENT
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With the exhaust-centre condition, the double-acting cylinder is free
to float with the valve in its neutral state, because both ports of the
cylinder are opened to exhaust. The cylinder rod can be moved
manually (subject to internal friction and external loading).
The blocked-centre condition blocks all working ports and often is
called closed centre. Depending on the circuit design and cylinder
loading conditions, this centre condition may provide a holding action
on the device to which the valve is connected. With suitable controls
— and considering the compressibility of air — this type of valve can
stop a cylinder at intermediate points along its stroke as it travels in
either direction.
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