REFLECTOR: Gear dump valve

[Al Gietzen]

I believe there is a lot more pressure across the opening valve just as you
crack the valve.  After all, 1500 psi or so is used to raise the gear.  The
pressure switch cuts the pump off, but there is still 1500 psi there.  The
pump will run again when this pressure drops a few pounds.  Now, if you
slowly open the valve until the gear starts to drop, you are bleeding
1300-1500 psi fluid across the ball and seat.

 

The actual pressure required at the cylinder to raise the gear during
travel, or hold at some intermediate point, is about 150 psi (more at the
start to unlatch the linkage).  You can compute this based on the area of
the piston and the force required on the cables. The pressure out of the
pump during the travel is about 600 psi; almost all of which is spent on the
overcoming the pressure drop in the too-small ¼” lines and fittings.  

 

The “Up” pressure switch opens the circuit at a pressure of about 1050 psi;
not far below the internal bypass limit in the pump.  If you hear the pump
“squeal” a bit at the end of either stroke, you know the internal bypass
pressure is being exceeded.  If the gear is driven to the end of the stroke,
the peak pressure in the system at the end of the stroke can end up a bit
higher.  So the valve can see quite high pressure during the first tiny
amount of flow (the pressure will drop very quickly) from the end of the
stroke, but when opening it from intermediate points in the travel the
pressure is relatively low.  

 

The pressure at the cylinder during the down travel is very low as the gear
will drop by gravity.  The pressure out of the pump is 250 – 300 psi; again
spent forcing the flow through the small lines. The “down” pressure switch
opens at about 550 psi, but because cylinder is already at the end of the
stroke when the pressure reaches that point, the slight delay in the relays,
and the inertia of the pump carries it enough to drive the pressure up to
the bypass pressure, somewhere around 1100 as I recall.

 

This issue may be whether it is harder on the valve to release the pressure
than it is to let it set for long periods of time holding the pressure.

 

It was during all these measurements, and discussions with the makers of the
switches, that I learned that the screws at the top of the pressure switches
are not for adjusting the pressure at which the switch ‘opens’, but adjust
the bandwidth – the pressure difference between the ‘make’ and ‘break’
points. This can influence the turn-off point but more directly influences
the point at which the pump turns back on; which can determine whether the
pump “cycles”, or not. 

 

At least this is what I learned doing measurements on my system; which isn’t
necessarily the same as someone else’s.  And I’m sure this is more detail
than anyone wants to know.

 

Al

 

 

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