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vogel

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Hi all,
I've been asked to analyze a machine which is vibrating at 1x. Levels change during steady operation so I'd like to see what is the phase doing. Unfortunately the machine doesn't have a phase reference sensor and chances that it can be stopped to install it while I'm on-site are low. I've recommended to install a tacho/keyphasor at the next opportunity.
However, if I were requested to measure before the tacho/kph is installed, what would be my chances to get some good data? Have you ever used a strobe lamp with TTL output as a phase reference? Is accuracy good enough?
I've heard a story about someone shooting reflective paint to the shaft but mot probable not something that I'll be allowed to do, and anyway I doubt that it yields good results.
Any other suggestions?
JuddJones

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Does your instrument have 2 channels to do cross channel phase?
John from PA

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Quote:
Originally Posted by vogel
I've heard a story about someone shooting reflective paint to the shaft but mot probable not something that I'll be allowed to do, and anyway I doubt that it yields good results.
Any other suggestions?


Not a story, Don Bently did this many years ago with a lipstick and sling shot.  In fact when I was introducing him at a national Vibration Institute Conference (where he received an award) I quizzed him about that exact story.  He said it was true and he actually was very embarrassed because when they shut the machine down to install a correction weight, they found reflective tape on the shaft.  Don was very much against strobe lights, mainly for safety reasons.

On to your problem, is this a casing measurement or proximity probe?  On proximity probes, sometimes you can take a scratch and turn it into a suitable (and temporary) reference marker using signal amplification. 
Shurafa

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Stroboscopes were/are used widely on machines without a permanent key phasor. One of their most common applications is field balancing which obviously depends on n 1X phase, which is what you want to see.

Having said that, portables create possible confusions and pitfalls because they are meant to be used in a variety of ways. One of the common difficulties one would face with strobes is the exact and constant match with speed. If there is a small (but tangible) difference between the shaft's speed and flashing frequency, you'll see the shaft moving at a very slow speed and as a result, the phase will not be stable.
 
Regards- Ali M. Al-Shurafa


OLi

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Reply with quote  #5 
If you have access to key you could with care use a inductive tacho or a eddy probe to get 1xRPM pulse.
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Curran919

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Quote:
Originally Posted by John from PA

Don was very much against strobe lights, mainly for safety reasons.


Can you elaborate on that?
OLi

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If you strobe a shaft it looks like it is not rotating and you and others are tempted to touch it and whatever you touch it with will discover it is very much rotating and that is dangerous.?
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MarkL

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Or bad for epileptics? But I’d imagine the stationary shaft illusion
Walt Strong

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Recently I was conducting a machine condition survey of a 450-hp natural gas compressor (wet-screw type). I took my Monarch ROS optical sensor and hand-held it to scan along the exposed shaft and found a reliable 1xSS speed/phase signal, so I temporarily mounted it with a magnetic base for measurements. I was fortunate that someone else had put a reflective marker on the shaft! It is always worth checking for a reflective target, even it other plant personnel don't think one is present.

Walt
vogel

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Reply with quote  #10 
Thanks all for your very valuable input.

Big J,
I can measure cross-channel phase, but I don't know how it would help here.

John,
the machine has no prox probes, installing them externally to the bearing with the machine running may be doable but not a risk that I want to take, and the plant operator wouldn't let me do it anyway. However, this is a technique that could have been useful on  another job that I had some time ago, balancing a steam turbine (which had prox probes but no kph), so I'm interested. I assume that you need the scratch to be have higher ampplitude than the vibration signal, correct? How would the signal amplification work?

Shurafa,
I've never balanced with a strobe ... I guess that you need reflective tape? or how do you keep phase consitency between runs? 
That's exactly my doubt, how "feasible" is to get a exact match between shaft speed and flashing frequency so that phase error is negligible?

Oli,
I don't know if I'll have access to a key, actually I'm quite sure that I won't have access. Even if I had it, it's a risky operation, not something I would try without a long range probe.

Walt,
that's exactly what I'd like to do, just to be sure, did you measure with the help of the reflective tape? I don't want to bet on the fact that someone put reflective tape there. If there is then good luck, but I'd like to look for an alternative in case there isn't.
Walt Strong

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Reply with quote  #11 
"I assume that you need the scratch to be have higher ampplitude than the vibration signal, correct?" I doubt you can get a good signal with just a scratch, but perhaps John says its possible.

"I guess that you need reflective tape? or how do you keep phase consitency between runs? 
That's exactly my doubt, how "feasible" is to get a exact match between shaft speed and flashing frequency so that phase error is negligible?" No need for reflective tape. Typically you look for one key or keyway or grease/rust mark and use strobe control to align "target" at the same place (say TDC or horizontal at split line). Some strobes have phase adjust that makes this easy. The strobe must have an AC output (TTL) signal to analyzer, and then you are good to go. Sometime you can find a target at the NDE of motor rather than in the coupling area. Check on the Monarch tachometers that have the needed features: 
https://monarchinstrument.com/collections/nova-strobe-portable-stroboscopes

Emerson-CSI has the SpeedVue that can get accurate speed, but not phase without a reflective marker.

The Monarch smart laser sensor can sometimes get speed-phase signal from a shaft key or keyway without reflective target, but you have to fiddle with it and get "lucky". 
https://www.tequipment.net/Monarch6180-022.html?Source=googleshopping&/?utm_content=monarch%20instrument%20adjustibel%20laser%20sensor&utm_term=&utm_campaign=Shopping%20Campaign(BSC)&utm_source=Bing_Yahoo&utm_medium=cpc

Walt
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Reply with quote  #12 
Quote:
Originally Posted by Walt Strong
"I assume that you need the scratch to be have higher ampplitude than the vibration signal, correct?" I doubt you can get a good signal with just a scratch, but perhaps John says its possible.

"I guess that you need reflective tape? or how do you keep phase consitency between runs? 
That's exactly my doubt, how "feasible" is to get a exact match between shaft speed and flashing frequency so that phase error is negligible?" No need for reflective tape. Typically you look for one key or keyway or grease/rust mark and use strobe control to align "target" at the same place (say TDC or horizontal at split line). Some strobes have phase adjust that makes this easy. The strobe must have an AC output (TTL) signal to analyzer, and then you are good to go. Sometime you can find a target at the NDE of motor rather than in the coupling area. Check on the Monarch tachometers that have the needed features: 
https://monarchinstrument.com/collections/nova-strobe-portable-stroboscopes

Emerson-CSI has the SpeedVue that can get accurate speed, but not phase without a reflective marker.

The Monarch smart laser sensor can sometimes get speed-phase signal from a shaft key or keyway without reflective target, but you have to fiddle with it and get "lucky". 
https://www.tequipment.net/Monarch6180-022.html?Source=googleshopping&/?utm_content=monarch%20instrument%20adjustibel%20laser%20sensor&utm_term=&utm_campaign=Shopping%20Campaign(BSC)&utm_source=Bing_Yahoo&utm_medium=cpc
yes the monarch strobe will trigger on light and dark. Paper machine felt line is a treat.  Yes it is fiddly but once you get it it is a treat. Back in the late 90's.   mmm those were the days.  Now we have Speedvue. A piece of modern magic. I have 2 just in case 1 fails. Being CSI unlikely. 
Walt
Shurafa

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Reply with quote  #13 
Vogel,

Walt did a great job replying in his last post to your question about the need for a reflective type.

I would add that a strobescope has an advantage compared to systems using a reflective type in giving you readings without the need to shutdown the equipment. So you save a shutdown and startup to collect the same data (for 1x).

Another note to consider is that the optical (laser) systems have a better phase accuracy and stability compared to the strobescope because the opticals receive an automatic feedback signal coming from the rotating shaft (reflective type). In the case of strobescope, the user manually give the feedback signal through adjusting the frequency of flashlight.

Regards- Ali M. Al-Shurafa
vogel

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Reply with quote  #14 
Thank you all, very useful ... this forum is such a great learning tool.
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