The fluid circuit is really a closed loop. The only options for
returning fluid to the reservoir are either through one of the pressure
relief valves or the low resistance back pressure circuit. Excess
fluid needs to return to the tank every time the rod volume displaces fluid
volume. When the rod volume is removed from the system, make up fluid
is pulled in from the tank through a one way check valve. During
retraction, the spool valve opens a low resistance path from the low side to
the reservoir, eliminating a large pressure build up in the low circuit
during retraction. - which also eliminate the possibility of pressure build
up due to thermal influence.
One side effect of the correct spool valve orientation and a leak free
system is that you cannot manually push the gear up even with the dump valve
open unless the spool valve is moved into the retract position. After
opening the valve you still have a closed loop from high to low that is now
bypassing the pump. When you try to push the gear up you get hydraulic
lock. You are trying to add the rod volume to a closed system and the
only route back to the reservoir is through a relief valve. By
bumping the pump in the up direction, even with the dump valve
open, you can push the spool valve into the retract position which
opens up the low resistance path to the reservoir for the low side
circuit. If you are able to push the gear up at all without bumping
the pump, I would suspect the spool valve is backwards. Let us know
what observe in the low circuit during retraction. You can also try
the direction-reversal test during retraction. The response should be
instantaneous.
good luck
Chris,
It would seem that there is no one
answer for this problem.
On my 235, with the plane on
jacks;
I can open the dump valve and easily push
any of the gear up. Spool valve is backward?
When I do the direction reversal test the
gear reverses instantly. Spool valve is OK?
When I put the gear up and then open the
dump valve, all 3 drop within 2-3 seconds.
Years ago, I experienced a few instances
of the gear not coming down when the gear switch was put down. This
situation was always relieved by briefly opening the dump valve. It
stopped happening, so I did not change anything.
Am I just lucky, or what?