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Hvac_vibe

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Reply with quote  #1 
Hello all. I am new to this particular field of work after 20 years in the hvac service field. I am ISO II certified but with no field experience. So I am learning on the fly very slowly. My question is how does motor slip speed affect a spectrum? If I input a speed of 3600 knowing that it's not gonna be exactly at that speed and have no way of verifying the exact speed, how does that affect my analysis? Hopefully that's an elementary question.......
Danny Harvey

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Reply with quote  #2 
You can probably correct the speed with the data collector or after data collection with the software.

Tell us what kind of equipment you are using and you can probably get something more specific.
Hvac_vibe

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Reply with quote  #3 
I'm using Commtest VB7 with Ascent. Honestly learning the software is my biggest hurdle. Any help there would be great also.
Beatnik

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Reply with quote  #4 
I like to use the gearmesh on our gearbox to find the real rotation speed of the motor (Gearmesh/number of teeth = speed).

A high resolution spectrum on the motor also works. There is always some imbalance so 1x RPM is almost always seen.

It's important to have the good speed, it's easy to mix up some frequencies, like 2x frequency line and BPFO.
Hvac_vibe

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Reply with quote  #5 
That's what I'm finding Beatnik. So, take a realtime, high reslotuion reading to identify 1x and then set my analyzer. That sounds too easy. I should have thought of that! Lol
MachDiag

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Reply with quote  #6 

You can always take a spectrum to find 1xRPM , most times this'll work just fine.  If you don't already have one, get a small, easy to carry LED strobe light, like Monarch's Pocket LED Strobe or one similar (you find used ones on eBay for cheap money sometimes).  On belt driven equipment, take a quick look at the drive and driven sheave sizes and you should have an rough idea where the speed should be, and then tune in to it with the strobe.  I use an old shell pouch & belt from trap shooting days to stash my strobe, along with other key items during data collection.

When looking over your collected data you'll be interested in knowing if those peaks are synchronous, or non-synchronous.  Is that peak a multiple of v-belt speed, a multiple of fan speed or motor speed, maybe it's a bearing defect frequency, or just what?  You really do    > need< to know the rotational speeds of the drive and driven components when performing vibration analysis on machinery.  Make it a routine to check 1xRPM in the collected data and adjust/set it to match actual speed, only then can you be sure your analysis can be accurate.

Hvac_vibe

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Reply with quote  #7 
Thanks for all the good info. Most of my stuff is hermetic so I can't necessarily get to a shaft all the times. I have a strobe and a tach light in my bag for such occasions. Good info everyone. Keep it coming.
ukvibes

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Reply with quote  #8 
You shouldn't be Cat II certified without at least 2 years field experience and the Cat II certification material covers exactly what you have asked. Where and how did you receive the Cat II certification?
In most situations you should find the 1X in your FFT. When you can't a laser tach I find is the easiest, Rusty has linked to a cheapish one elsewhere. High res/low frequency can show you other frequencies such as belt so it's handy to tach also.
Can you describe further 'motor slip' please. What exactly are you referring to? VSD motors or slip?
Aubrey

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Reply with quote  #9 
In Ascent, find your turning speed peak. Put the crosshair on it and hit Ctrl 1. You can set your speed for the entire machine at that point if you want.


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RustyCas

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Reply with quote  #10 
I suspect many folks don't bother with the exact speed for "fixed" speed machines. As you allude to, most fixed speed machines are actually variable, as the "slip" varies with load. As load increases, the slip (difference between synchronous speed and actual rotor speed) must increase to develop increased torque (most of you know this, but some may not).

So actual running speed, especially if trended, can tell you a lot. I enter the exact RPM for every machine or shaft I check (if I can get to it, or calculate it).

For example, one of the most common motor bearings you'll see is a 6313 with an outer race defect frequency of 3.09 x RPM. If you don't know the exact speed, or you use too few lines of resolution, you will think this is 3X and spend a great deal of time trying to figure out what it is.

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