InsaneHydraulics - Sergiy Sydorenko © 2009-2010 All Ridghts Reserved
Despite of the fact that the main purpose of a charge pump in a closed
loop is to keep the circuit filled and to provide servo pressure, there
are still some people who are, for some reason, convinced that the
higher the charge pressure, the better is the machine's performance. On
these pics you can see a charge pump from an old A4V (Hydromatik), the
pump had been in a perfect condition a week before the pics were taken.
The client claimed warranty and said the machine was running OK
and then stopped suddenly (gee, I wonder why?). However a closer
inspection revealed a COMPLETELY BLOCKED charge pressure valve, and it
was not damaged, it had been blocked on purpose, by screwing in the
regulating screw till it stopped. Sorry, guys, no warranty here. I can
only guess how high the charge pressure went...
These internal gear pumps are not hydrostatically
balanced, which means the pressure inside the pump pushes the
outer ring towards the housing and the central gear in the
opposite direction, creating lateral load on the bushings. There is no
need to balance these pumps as their working pressure is relatively
low. However if the pressure goes beyond certain level, the disbalance
will cause excessive wear both of the bushings and the housing
(especially the housing). On the pics you see that the outer gear-ring
"machined" through the housing as far as the o'ring cavity before going
"kaboom". This type of wear and the fact the charge pressure relief was
"blocked" left no doubts as for why the failure happened.
Don't try this at home!