Spool Overlap
Overlap expresses how much wider the blocking land on the spool is than the port window in the body. With positive overlap the spool more than covers the ports at neutral and a deadband forms at center; with zero overlap the edges coincide exactly, and with negative overlap a small passage remains at neutral.
All three options have a cost:
- Positive overlap: leakage at neutral is lowest and the load holds better. In return, no flow comes out until the spool moves several hundred microns; typically 15-25% of the control signal is wasted.
- Zero overlap: the deadband is smallest and control starts crisply. Manufacturing tolerance is very tight and cost rises.
- Negative overlap: there is continuous leakage at neutral; the load does not hold, but control gives the smoothest transition.
The vast majority of mobile equipment valves use positive overlap, because holding the load at neutral takes priority for safety. The direct consequence is that nothing happens in the first few millimeters when you push the joystick.
In electronic lever control this gap can be skipped by defining a minimum current or a minimum position threshold. The remote control takes the lever straight to the point where flow begins; the operator feels the whole lever as usable.
Where this term is used: How Are Manual Hydraulic Levers Controlled Remotely? A Comparison of 6 Methods