There are two main types of valve designs used
in hydrauic systems. Spool-type and poppet-type.
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 todays
hydraulic systems.
Even though a logic element can be configured for flow in two
directions, it is only 'leakless' in one direction.
Even though a logic element can be configured for flow in two
directions, it is only 'leakless' in one direction.
To understand why, read pages 138-139 of
'Industrial Hydraulic Control':
http://www.
Yours for better hydraulics knowledge,
Brendan Casey
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