In a message dated 11/21/2006 11:31:19 A.M. Central Standard Time,=20 Christopher.Zavatson@baesystems.com writes:

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The fluid circuit is really a closed loop. The only options for= =20 returning fluid to the reservoir are either through one of the pressure re= lief=20 valves or the low resistance back pressure circuit. Excess fluid nee= ds=20 to return to the tank every time the rod volume displaces fluid volume.&nb= sp;=20 When the rod volume is removed from the system, make up fluid is pulled in= =20 from the tank through a one way check valve. During retraction, the=20 spool valve opens a low resistance path from the low side to the reservoir= ,=20 eliminating a large pressure build up in the low circuit during retraction= . -=20 which also eliminate the possibility of pressure build up due to thermal=20 influence.

One side effect of the correct spool valve orientation and a leak fre= e=20 system is that you cannot manually push the gear up even with the dump val= ve=20 open unless the spool valve is moved into the retract position. Afte= r=20 opening the valve you still have a closed loop from high to low that is no= w=20 bypassing the pump. When you try to push the gear up you get hydraul= ic=20 lock. You are trying to add the rod volume to a closed system and th= e=20 only route back to the reservoir is through a relief valve. By=20 bumping the pump in the up direction, even with the dump valve open, = you=20 can push the spool valve into the retract position which opens up the=20 low resistance path to the reservoir for the low side circuit.&n= bsp;=20 If you are able to push the gear up at all without bumping the pump, I wou= ld=20 suspect the spool valve is backwards. Let us know what observe in th= e=20 low circuit during retraction. You can also try the direction-revers= al=20 test during retraction. The response should be instantaneous.

good luck

Arrrrrghh, I forgot about the rod volume. Luckily, that is a bene= fit=20 for emergency gear extension (assuming fluid from the reservoir is not also=20 blocked) since the reservoir could supply the extra fluid for the non-rod=20 side. Interestingly, this helps explain why the nose sometimes pushes=20= the=20 nose gear door out of the way on a gear down operation - the high immed= iate=20 demand for fluid on the gas actuated nose gear (along with the mains) may no= t=20 allow the nose door to open fast enough because of fluid deprivation to=20 the door actuator.