Vibration Testing

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

Forgot to add that
- the detection boxes were single channel
- the transducers were moved to each axis and the data collected
- the detection boxes had a built in strobe circuit to independently
measure RPM

Ben Jackson wrote:
In article ,
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.


If you really wanted to try to diagnose problems with it you should try
to couple it to RPM. Then you could measure vibration in terms of the
order relative to the moving parts. If you could include the phase of the
crankshaft you could probably spit out enough information to do a dynamic
prop balance on a serial port.

Alternatively you might be able to infer RPM by doing an FFT on the raw
data. That would be a neat party trick.

As far as mounting it seems like getting it as far forward as possible
(where you should see the largest magnitudes) would be good. And if you
only get 2 axes then I'd go with your idea and ignore push/pull and keep
side/side and up/down.

john smith writes:

Thirty years ago I worked for a company by the name of IRD Mechanalysis.
Their business was vibration detection, monitoring and analysis for
preventive maintenance of heavy machinery.

I remember them. We used it on pumps. Here "pump" mean 17-stage
centrifugal with a 800 HP motor that runs on 3-phase 4160vac.

As I recall, it took a fair amount of training to interpret
the results.


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

john smith wrote:

Forgot to add that
- the detection boxes were single channel
- the transducers were moved to each axis and the data collected
- the detection boxes had a built in strobe circuit to independently
measure RPM

Ben Jackson wrote:
In article ,
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.


If you really wanted to try to diagnose problems with it you should try
to couple it to RPM. Then you could measure vibration in terms of the
order relative to the moving parts. If you could include the phase of the
crankshaft you could probably spit out enough information to do a dynamic
prop balance on a serial port.

Alternatively you might be able to infer RPM by doing an FFT on the raw
data. That would be a neat party trick.

As far as mounting it seems like getting it as far forward as possible
(where you should see the largest magnitudes) would be good. And if you
only get 2 axes then I'd go with your idea and ignore push/pull and keep
side/side and up/down.

Weeel--kinda depends on whether you got a heavy metal prop, or a piece'o tree
mounted in the prop flange---the gyroscopic inertia of that 40# hunka 'luminum
kinda resists vibrating--I think the assend of the engine would kinda describe a
jitterbug motion around the nose of the crank.--If the prop is balanced, then a
"new" vibration introduced into the system would probably be detected easier
with a rear mounted pickup...'Course a wood prop & a front cylinder goin bad
might change the scenario somewhat. Y'all run a test & let me know!!

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)."

Ernest, what the heck does "... fairly steady state and random,"
mean??

....
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.

Or maybe better than a squelch threshold might be a LED bar-graph
showing relative magnitude..??

Sid Knox

Velocity N199RS
Starduster N666SK
KR2 N24TC
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