InsaneHydraulics - Sergiy Sydorenko © 2009-2011 All Ridghts Reserved
Orifices are commonly used in hydraulic pump displacement controls to
meter servo-cylinder flow. In some cases, pump manufacturers replace the
orifices with very compact one-way restrictor valves, small enough to
fit the standard orifice places. Normally, these valves consist of a
small hardened pin, which has differently shaped cuts on each "nose",
and a body, that contains the pin and "provides the seats".
Alternatively, instead of placing the pin inside a single-piece body, it
can be mounted between two separated seats, that are tightened one
against the other in a threaded oil passage of the pump's body.
We all know that an obstructed orifice is one of the
most common reasons for a hydraulic pump control to malfunction.
However, one-way restrictors can bring about another failure
pattern, because apart form the natural clogging, these extremely
small flow-control valves are subject to mechanical wear, as the
small but very hard internal pin will hit the seats every time a
servo-cylinder movement is made. When the wear reaches "critical stage",
the pin can break the seats and "go with the flow", causing all
sorts of mishaps.
An example of such special orifices, that were mounted on a Rexroth A4VG closed loop pump, can be seen on the pictures to the left. One of the valves is lacking the pin
- over the many hours of service it smashed its way through the body of
the valve and finally fell into the servo-cylinder, where it managed to
mash-and-cut the plastic cap,
that was retaining the two piece lock ring of the servo-cylinder spring
plate. In the end the lock ring halves fell out of the shaft groove and
jammed the servo-cylinder.
In oil hydraulics small things can cause big damage, therefore no "small" things should be neglected during an overhaul of a hydraulic pump, like checking whether the pump control is using a simple orifice or a one-way restrictor valve, and evaluating its condition. Here
you can see that one of the valves is broken, and the other one is
about to break. Most likely during the previous pump overhaul this
damage could have been discovered and this failure could have been
Attention to detail, my good hydraulic friends, it's all about attention to detail....