Valve Spool
The spool is the cylindrical element that slides axially inside the valve body, opening and closing the port paths. Its wide lands block the paths and its grooves give passage to flow. Radial clearance to the body is typically 5-15 microns; the relationship between travel and opened orifice area is the source of the flow characteristic.
The farther the spool slides, the larger the opened orifice and the more flow passes. This relationship is not linear: edge geometry, the shape of the opened windows, and pressure differential all come into play. In proportional valves these edges are deliberately shaped to obtain the desired flow-travel curve.
- Leakage: a small internal leakage occurs continuously through the micron-level clearance; that is why most spool valves cannot hold a load with a seal.
- Sticking: dirty oil or side loads push the spool into the friction region; dither current exists precisely to break this.
- Centering: springs pull the spool to neutral; a broken spring causes the lever not to return to neutral.
How many degrees the manual lever moves corresponds directly to spool travel. In electronic lever control the purpose of calibration is to map the joystick's full range onto the lever's usable mechanical range; if this mapping is not done correctly, you either cannot use the spool's full travel or you force the lever against its limit.
Where this term is used: How Are Manual Hydraulic Levers Controlled Remotely? A Comparison of 6 Methods