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Vibration Testing



 
 
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  #1  
Old September 29th 04, 07:15 PM
Jim Weir
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Default Vibration Testing

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
  #2  
Old September 29th 04, 07:23 PM
john smith
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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.
At that time they were a subsidiary of H H Robinson Company, a
construction company based in Pittsburgh PA.
We built our own crystal-transducers to work with the monitoring equipment.
I never worked in the analysis end of the business, so I cannot tell you
where the frequencies indicating "good" and "bad" lie. Each device is
different. A problem with each component was indicated by a different
frequency.
The only aviation use I am aware of was to dynamically balance props.

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.

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


  #3  
Old September 29th 04, 07:31 PM
Ron Natalie
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"Jim Weir" wrote in message ...


Comments and thoughts from the technonerds amongst us appreciated.

I suppose it depends what you're going to do with the data.
If it's just an idiot sensor that something is going wrong with the engine, probably a sum
of the total vibration would be best.
If you're trying to test something, two meters would be better than one (either summed or
switched). Frankly I kindof detest switched meters. I replaced that kludge in the navion
with individual fuel gauges.

Seems like the hifi apps should have some stereo vu-meters available, I know you used to be
able to get a stereo bar graph led display at RadioSnack a few years back.

  #4  
Old September 29th 04, 09:31 PM
Ben Jackson
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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.

--
Ben Jackson

http://www.ben.com/
  #5  
Old September 29th 04, 10:31 PM
Jim Weir
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I don't think serial ports need dynamic prop balancing.

You guys sure know how to complex up a bonehead simple idea, don'cha?

{;-)


Jim



(Ben Jackson)
shared these priceless pearls of wisdom:

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.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com
  #6  
Old September 29th 04, 10:36 PM
G.R. Patterson III
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Jim Weir wrote:

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.


My take is that summing the two axes would be perfectly acceptable. I agree that
fore-and-aft is unlikely to produce any interesting results. In fact, I'd bet that
any mechanical problem would show up as an increase in lateral vibration. Vertical
vibration would tend to occur only just before things come apart, IMO.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.
  #7  
Old September 29th 04, 10:50 PM
UltraJohn
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Jim if the sensors are that cheap why not just use two mounted 90 degrees
apart. If its analog output you could just sum the two together to get
~total vibration or convert to digital feed a cheap microprocessor chip
and display all three. Logging all three channels would let you post
process the data all you want and would not be all that difficult.
John





Jim Weir wrote:

I've come across a marvelously cheap vibration sensor that I want to
convert


  #8  
Old September 29th 04, 11:01 PM
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Default


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
  #9  
Old September 30th 04, 01:17 AM
Dave Hyde
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Default

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



  #10  
Old September 30th 04, 02:09 AM
Stuart Fields
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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



 




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