misunderstood valve specifications.

[Tatonka]

I'm examining a military specification for a servo valve assembly and there are a few requirements/specifications that don't make a whole lot of sense to me...

The spec states that the "valve bias pressure at hydraulic null shall be 5 to 50% of supply pressure." The valve is a hydraulic pilot vlave (where the pilots are supplied by a servovalve). The test procedure reads that the control ports should be sealed and the servo valve adjusted so that "the blocked control port differential pressure is zero" and "the common control port pressure...shall be 415 +/- 340 psig." Does this mean that the servo valve is actuated appropriately to equalize the control port pressures and this common pressure should be within these specifications? If so, what does this tell us about the valve's operation (why is this specification necessary)?

The specification also lists a required hydraulic stiffness of 8,000psid/cubic inch of fluid added @ 750 +/- 25 psi. I did some research and could not find any listed hydraulic stiffnesses with these units: they all had units of lbf/in. Are these equivalent units?

M Woelfersheim

 

[tannguyen]

psi/cubic inch is lbf/in.

Breaking the units down:

[lbf][in^2]
-----------
[in^3]

don't ask me what the "d" is...dynamic psi???

 

[Tatonka]

I tried working out the units, but it looks like this, doesn't it?

lbf
psi = ----
in^2

lbf 1 lbf
psi/in^3 = ---- x ---- = ----
in^2 in^3 in^5
Is this a specification on the elasticity of the system (tubing, etc)? If you add a cubic inch of fluid, the pressure must increase at least 8,000 psi?

 

[Tatonka]

sorry...didn't preview...

lbf
psi = ----
in^2

lbf 1 lbf
psi/in^3 = ---- x ---- = ----
in^2 in^3 in^5

 

[IRstuff]

Did you find the spec number?
TTFN

 

[Tatonka]

sorry it took me so long to get back to you...spec number is 10237-3102 for part 12283102 - Servomechanism, Elevation.

 

[IRstuff]

Sorry, I don't have access to that document.

psid is differential pressure, of course.

Sorry can't offer more help. I tried to find some related mil-specs, but there didn't seem to be any that I could find that were directly applicable.

TTFN

 

[Tatonka]

The only way I can access the spec is through the "Viewer" program on an Army terminal. It has images of each page and you have to print them individually...great use of time, eh? our tax dollars hard at work...

 

[tannguyen]

sorry I screwed up.

It's a really old (1978) spec that is inactive

 

[estimator79]

Ask the contracting officer for an explanation.

 

[Tatonka]

the company i work for does contract engineering work through GSA...we don't supply parts, so we don't have a contracting officer...i'll try to find one, though...