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QUESTION:I'm trying to repair a hydraulic system on a machine tool (EDM). The
servovalve is not operating at all. 12vDC is being supplied to the coils
(inductors) yet the flapper doesn't move. There are 2 pairs of coils (about
3/8" dia) with an iron core on each side of the flapper stem. I tested each
coil for continuity with an ohmmeter at the solder point leading in and out
of each coil and each indicates open. Can all 4 coils go bad, or am I
missing something?
The rest of the valve appears normal. The spool moves freely.
The valve is an Koehring Pegasus mod 131a. A new valve (MOOG) costs $6000. I
searched for Pegasus but didn't come up with anything. Lots of good general
info on http://www.servovalve.com/ but no specifics on this valve. I could
send this one out for rebuilding, but maybe I can fix it myself.
ANSWER: I'm not familiar with this type of valve, but it sounds as though it's a
continuously variable, modulated or balanced voltage coil sort-of device.
A caution about checking coils with ohm meters.. some meters, particularly
analog types can have enough current in their ohms measuring circuits to
open sensitive, fine wire coils.
So you might now have more than one fault.. the valve driver circuit(s), and
the open coils.
I dunno about this manufacturer, but many solenoid valve manufacturers
provide replacement coils for simple solenoid valves.. maybe not for this
type of valve though.
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