From: Wolfgang <Wolfgang@MiCom.net>
Subject: [LML] FW: [LML] gear retract problems
To: lml@lancaironline.net
Date: Thursday, April 8, 2010, 7:13 PM

Randy Snar in his just posted comments has hit the nail on the head. The spool in the hydraulic pump does not always stay at the end point of it's travel and when it doesn't, and since there is no accumulator in the system, the fluid volume is trapped and heat will cause thermal expansion opening the pressure switches. If you don't experience a large temperature rise and the spool in your pump stays where it's supposed to, you won't see the problem. So as Randy stated, if both switches trip, nothing works except opening a fitting to relieve the pressure. Installing a "dump" valve that vents both sides to the reservoir instead of the standard of just equalizing between both sides will fix the problem but still requires pilot intervention after the re-plumbing. But how many people are ready to re-plumb their hydraulics to implement that fix ?

 

There is nothing in the pump except o-ring friction to keep the spool in it's needed end position. Vibration can move it trapping the fluid volumes. Activating the pump will force the spool back into it's end position relieving the offending pressure and all is well with the world. Problem is the pressure switch is open preventing the pump from activating. Forcing the pump to activate when both switches are open will do the trick. My electric module does exactly that . . . . automatically . . . . without pilot intervention. It connects to both pressure switches and forces the pump to activate in the direction selected by the gear switch if both pressure switches are open.

 

Wolfgang