Mechanical valves are used with a vacuum cup in systems having a central vacuum pump and an array of vacuum cups to pick up a family of workpieces that vary by known values of width or length.
Mechanical valves are used to seal off cups that are not directly over a workpiece to limit leakage into the vacuum system since these cups are not sealing.
If the workpiece edge positions vary randomly, a mechanical valve could be opened by the workpiece but with a portion of the vacuum cup overhanging the edge causing leaking which would defeat the purpose of using mechanical valves.
Mechanical valves are closed until the valve stem contacts a workpiece to open the valve and admit vacuum to the vacuum cup to allow gripping the workpiece.
Since mechanical valves are mechanically operated by contact with a workpiece, there is a possibility for the valve stem to leave a mark if there is any relative movement.
It is good practice to avoid using mechanical valves for delicate or highly polished surfaces and to make sure that vacuum cup movement is perpendicular to the workpiece surface.