Vibration Testing

On 29-Sep-2004, Jim Weir wrote:

The question is whether to have a two-channel meter (which complexes up
both
the design AND the panel space), a single meter switchable between lateral
and
vertical) or a single meter with the two axes summed together.


Hmm... Interesting bit of instrumentation there. My take is that since
summing the two axes will involve some trivial electronics, why not add a
bit more trivial electronics and have a means to (at least momentarily)
individually look at either one axis or the other. I envision a 3-position
toggle switch which is spring-loaded to the center ("mixed") position.
Toggle to either "side" position will select one or the other axis for an
individual reading.

--
-Elliott Drucker

Jim Weir wrote...

The question is whether to have a two-channel meter
(which complexes up both the design AND the panel space), a
single meter switchable between lateral and vertical) or
a single meter with the two axes summed together.

You gotta decide what you're going to do with it and what
it's going to tell you in the cockpit. Are your procedures
going to be different if one axis reads high and the
other reads normal ("Holy cow, lateral vibes are off scale!
Good thing it's not vertical!")? Personally I'd think that
total vibration (RSS) would be sufficient to indicate a problem
that warranted more detailed troubleshooting on the ground.
Too much information in the cockpit can be just as bad as
not enough.

Dave 'standard gauge' Hyde

Well if you can produce a Power Density Spectrum, it might be useful for
engine condition monitoring. On my helicopter, I have a two channel device
with a photo sensor to give me some idea the crank angle (if I'm checking
engine balance) or rotor position if I'm doing a rotor balance. However, I
put a scope probe on the out put of the accelerometer and can't find an
intelligent signal. I'm told that my processor does a tracking filter based
on the RPM derived from the photo sensor output. The sensors that I'm using
are the piezoelectric type and the processor integrates their output to
derive an "inches per second" or ips reading. I find the vibration analyzer
both mandatory and frustrating. I will surely follow any KitPlanes article
you produce with your sensors. Good luck.

"Jim Weir" wrote in message
...
I've come across a marvelously cheap vibration sensor that I want to
convert
into an engine vibration instrument for a Kitplanes article. The
electronics
for me is relatively trivial...the mechanics of vibration are a little
harder to
fathom.

From a mechanical engineering point of view on a horizontally opposed
engine,
there are (as with most things) three axes of freedom -- fore and aft,
side to
side, and up and down (longitudinal, lateral, vertical).

The sensor I have reads two axes, and my hit is that fore-aft is the least
interesting vibration mode of the engine. The question is whether to have
a
two-channel meter (which complexes up both the design AND the panel
space), a
single meter switchable between lateral and vertical) or a single meter
with the
two axes summed together.

Comments and thoughts from the technonerds amongst us appreciated.

(It has nothing, repeat NOTHING to do with the fact that such a meter
might have
detected a crack in that cylinder WAY BEFORE it departed the engine on the
way
home from Oshkosh {;-) )


Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Gotta watch the temperatures around those sensors...

Up down left right is what you want to 'see'.

http://zone.ni.com/devzone/devzone.n...25684A004EB0F4


"Jim Weir" wrote in message ...
I've come across a marvelously cheap vibration sensor that I want to convert
into an engine vibration instrument for a Kitplanes article. The electronics
for me is relatively trivial...the mechanics of vibration are a little harder to
fathom.

From a mechanical engineering point of view on a horizontally opposed engine,
there are (as with most things) three axes of freedom -- fore and aft, side to
side, and up and down (longitudinal, lateral, vertical).

The sensor I have reads two axes, and my hit is that fore-aft is the least
interesting vibration mode of the engine. The question is whether to have a
two-channel meter (which complexes up both the design AND the panel space), a
single meter switchable between lateral and vertical) or a single meter with the
two axes summed together.

Comments and thoughts from the technonerds amongst us appreciated.

(It has nothing, repeat NOTHING to do with the fact that such a meter might have
detected a crack in that cylinder WAY BEFORE it departed the engine on the way
home from Oshkosh {;-) )


Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Jim Weir wrote in message . ..

From a mechanical engineering point of view on a horizontally opposed engine,
there are (as with most things) three axes of freedom -- fore and aft, side to
side, and up and down (longitudinal, lateral, vertical).

Can these things measure rotational vibration, or just linear? Seems
to me that the real shaking going on up front is rotational. Being
able to measure the vibration in the other 3 axis would be nice. I
vote for a combination of switchable/discrete channels and a summation
mode.
====================
Leon McAtee

I work for Bently, Nevada.
NOTE:
This is not a plug for Bently products!
Our market is large power generation stations/oil refineries, etc.
But the research done at Bently is paralled by none.
www.bently.com

I could go on for hours spewing information and quoting dissertations and
such, but I would recommend reading the ORBIT magazine from the bently
website.
There is always good information machinery diagnostics.

I would also recommend looking at some of the applications for Bently's
recip monitors (most of the monitoring is for rotating machines, i.e.
generators).
There has been a great deal of research in recip recently.
You can read some of the product literature, too.
In particular, read about the 3500/70M, the 3500/72M, and the 3500/77M.
These are recip impulse/velocity monitors, rod position monitors, and
cylinder pressure monitors.

It's probably way more information than your looking for, but it may come in
useful for an article.
It's not aviation specific, but more about machinery theory.
Bently also offers a machinery diagnostics class, which is pretty good.

Adam

There is also 4th and 5th degrees of freedom, rotation and torque.

Anything is measurable if you have the right equipment and techniques.
15 years ago or so, I met a couple of guys who marketed their services of
predictive vibration analysis for very large diesels in the oil patch and
marine fields.
They could tell you when a valve guide was beginning to wear.
They smiled big when asked about their fees.

Predictive vibration analysis is well known by turbine aircraft operators.
I decided long time ago that its not economically viable for aircraft
piston engines. I think that the other techniques of predictive
maintenance do just as well. Exception maybe is the front alternator 520.

Really, who is goin' to pay for it?

The first requirement would be to have a well established baseline for a
specific engine.
And if one is taking measurments that have nothing to do with cylinder head
separation, one would not have to account for the percentages of difference
of combustion forces of all the cylinders and the percentage difference in
the other factors, like valve spring pressure, that influence the horizontal
yawing motion a flat opposed aircraft engine undergoes during normal
operations.

Another angle to consider could be......how to describe this? .....is to
ask what musical note the cylinder rings at.
Everything rings at its own frequency and a crack will change it.

Good luck
Kent Felkins
Tulsa Oklahoma





15 years ago or so I talked to some guys who
"Jim Weir" wrote in message
...
I've come across a marvelously cheap vibration sensor that I want to
convert
into an engine vibration instrument for a Kitplanes article. The
electronics
for me is relatively trivial...the mechanics of vibration are a little
harder to
fathom.

From a mechanical engineering point of view on a horizontally opposed
engine,
there are (as with most things) three axes of freedom -- fore and aft,
side to
side, and up and down (longitudinal, lateral, vertical).

The sensor I have reads two axes, and my hit is that fore-aft is the least
interesting vibration mode of the engine. The question is whether to have
a
two-channel meter (which complexes up both the design AND the panel
space), a
single meter switchable between lateral and vertical) or a single meter
with the
two axes summed together.

Comments and thoughts from the technonerds amongst us appreciated.

(It has nothing, repeat NOTHING to do with the fact that such a meter
might have
detected a crack in that cylinder WAY BEFORE it departed the engine on the
way
home from Oshkosh {;-) )


Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Jim Weir wrote in message . ..
I've come across a marvelously cheap vibration sensor that I want to convert
into an engine vibration instrument for a Kitplanes article. The electronics
for me is relatively trivial...the mechanics of vibration are a little harder to
fathom.

snip

Comments and thoughts from the technonerds amongst us appreciated.

sig snip

How 'bout comments from an ignorant wrench-bender?

Have allegedly repeatedly measured 1st order vibration (referenced to
propeller rpm) of both recip and turboprop aircraft engines using a
single axis sensor mounted perpendicular to the crankshaft. The
indicated amplitude of the vibration was not appreciably affected by
the "clocking" of the sensor axis.

In short, mounted either near the propeller end (front), or the on the
accessory case (rear) it did not matter whether the axis of the sensor
was parallel to the cylinders (call it side-to-side), perpendicular to
the cylinders (call it up and down), or at any clock angle in between.

If this information is not useful, just remember what you paid for it,
and where it came from.

TC

Jim Weir wrote:
I've come across a marvelously cheap vibration sensor that I want to convert
into an engine vibration instrument for a Kitplanes article. The electronics
for me is relatively trivial...the mechanics of vibration are a little harder to
fathom.


Jim,

I was reading an article on the NASA LARC website which discussed
flutter detection techniques. Would it be possible to tag the ailerons
with a couple of these babies, then add a "pull-power-and-pull-up" buzzer?

--
http://www.ernest.isa-geek.org/
"This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Ernest Christley wrote:
Jim Weir wrote:

I've come across a marvelously cheap vibration sensor that I want to
convert
into an engine vibration instrument for a Kitplanes article. The
electronics
for me is relatively trivial...the mechanics of vibration are a little
harder to
fathom.


Jim,

I was reading an article on the NASA LARC website which discussed
flutter detection techniques. Would it be possible to tag the ailerons
with a couple of these babies, then add a "pull-power-and-pull-up" buzzer?


I'm sorry. I didn't give quite enough information as to why the
vibration sensor would be beneficial.

The study was a review of techniques used to generate flutter. It seems
to be a well known fact amoung the engineering types that it is fairly
easy to detect the onset of flutter. The vibration is fairly steady
state and random, and just before flutter sets in the vibration begins
to diverge logritmically (I know that ain't spelt rite).

A circuit with a sort of squelch mode would be very useful, especially
during initial flight testing.

--
http://www.ernest.isa-geek.org/
"This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."