splitter control valve...

[J_J]

A spool is also related to spindle and or shaft.

 

[Redneck in training]

Why do the call it a "spool"?

Generally a spool is a storage device of some sort, like a spool of thread.

P.S. This discussion is very informative.

If you have a spool from hydraulic valve you could wind thread on it and store it there. It would be expensive spool though. It is the shape that gave it its name.