Let’s talk about valve leakage.
There are two main types of valve designs used in hydraulic systems.
In a spool design, a spool is positioned in its bore to connect the various ports in the valve. The most common type of spool valve we’re all familiar with is the directional control valve.
Because radial clearance is required for the spool to slide in its bore, this valve design in not leakless. To say this another way, even when a port in a spool valve is closed off – a small amount of leakage is possible and should be expected.
In a poppet design, the valve ‘poppet’ closes against a seat. This design is generally considered leakless. That is, if the valve is closed and the poppet and its seat are in good condition – there is no leakage across the valve’s ports.
BUT there’s an important exception to this rule you should be aware of. Slip-in cartridge valves, also called logic elements are a type of poppet valve commonly found in today’s hydraulic systems.
Even though a logic element can be configured for flow in two directions, it is only ‘leakless’ in one direction.