On Fri, 07 Jan 2005 13:19:36 GMT, "Lloyd E. Sponenburgh"
vaguely proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email

From my (limited) experience, ........

most of my machinery makes so much noise anyway, a little hydraulic
whine is nothing! G

.......you have covered the most obvious reasons for cavitation with
good flow. If the pump is not submerged, you may be getting cavitation
because it's sucking a bit of air. IMLE, this may get _worse_ with low
pressure, higher flow in open centre. Is there any sign of bubbling in
the reserve tank? It may also be simply because the pump has to suck
to a height and is starved. Is there any sort of non-return valve on
the outlet of the tank, before the pump, that may be causing a
restriction?

I have an open center bypass (safety) valve and an open center control
valve.
When the valves are centered, the pump circulates to the tank at essentially
zero pressure.

My pressure gauge is on the HP line, on the pump side of the control
valve -- so I can see the pressure the pump is delivering, rather than the
working pressure at the cylinder.

The system makes a "screech" when the relief valve opens under full
pressure. That I expect, and don't have a problem with the intermittent
noise. The noise I'm fussing over is continuous while the pump is running
at zero pressure -- just recirculating.

The pump doesn't appear to be cavitating. I've installed a much larger
return filter than necessary, and the intake screen to the pump is clean.
I'm running a low-vis oil comparable to the newer Ford tractor gearcase/lift
oils (about 5W-20 equivalent). Those oils run quieter than the older
straight-30 I used to use. Also, FWIW, the pump is NOT submerged in oil,
but a free-in-air design mounted on a motor-mount plate.

However, as in my original post... it's not the direct noise from the pump
and motor. I've suppressed that with earthworks and drywall. It's the
"ringing" of the HP line and press frame I'm trying to eliminate.

LLoyd