I have run into trouble when balancing exhaust fans and I'm trying to understand how to find the "heavy spot" on the fan.
I'm using the SKF Microlog AX-S.

Example:
Initial run reads 0.8 in/s @ 250 degrees (similar on both bearings)

Can I read that as the heavy spot is 250 degrees counter-clockwise from the accelerometer? Or from the strobe light reference point?

Also, would I be better to read acceleration than velocity? When taking a horizontal reading wouldn't the max velocity on the bearing be when the "heavy spot" is halfway between 3 o'clock and 9 o'clock? Where the acceleration reading would be max when the heavy spot is right at the accelerometer?

Hope this makes sense... To summarize, how do I find the heavy spot?
Original Post
Here is a quote from Walt Strong from an old post which nicely summarizes this:

"At speeds well below critical speed, the peak vibration is almost in-phase with the heavy spot (close to 0-degrees. Approaching critical, the peak vibration lags behind the heavy spot. At shaft speed equal to critical speed, the peak vibration lags behind the heavy spot position by 90-degrees. A speeds well above critical speed, the peak vibration lags behind the heavy spot by 180-degrees, or you could say the heavy spot is a 1/2 rotation ahead of peak vibration."

The "Find" button on the toolbar above is your friend. Just search for "Heavy Spot" and you will see a number of discussions on this subject. Also "Trial Weight" would be a helpful search.
quote:
"At speeds well below critical speed, the peak vibration is almost in-phase with the heavy spot (close to 0-degrees. Approaching critical, the peak vibration lags behind the heavy spot. At shaft speed equal to critical speed, the peak vibration lags behind the heavy spot position by 90-degrees. A speeds well above critical speed, the peak vibration lags behind the heavy spot by 180-degrees, or you could say the heavy spot is a 1/2 rotation ahead of peak vibration."

Just to clarify for op - to the extent the assumptions for above are true (system acts like sdof responding to unbalance with pure 1x), it applies to displacement.

If vibration is in velocity, we need to adjust the relationship to reflect that fact that V leads D by 90. i.e.
Far below resonance V leads heavy spot by 90
Near resonance V in phase with heavy spot
Far above resonance, V lags heavy spot by 90.

For acceleration, we need a further 90 degrees adjustment to account for the fact that A leads V by 90:
Far below resonance A leads heavy spot by 180 (out of phase)
Near resonance A leads heavy spot by 90
Far above resonance, A in phase with heavy spot.
Thanks for the quick responses!

There are a lot of discussions on the ‘heavy spot’. I’ll start reading.

As far as critical speed, I've never concerned myself with it on these fans. I follow the balancing procedure on the microlog and it usually works in the first or second try. Maybe if I understand this mode shape thing more I could avoid putting the weight in the wrong spot??...

I was hoping to learn how to find the heavy spot to bring down the vibes to a reasonable level before starting the balance procedure. The microlog doesn’t seem to like over 1.5 in/s… It would save me a few start/stops trying to add weight to bring down the vibes.

So in order to find the heavy spot I have to know the critical speed then compare shaft speed to critical speed. Then I know the peak in relation to the “heavy spot”...
And since I’m taking velocity readings I will always compensate -90 degrees in the phase reading...

Thanks for the help!
Starting with over 1 ips pk I'd expect that the fan might show a "static" heavy spot when stopped by rolling to a preferred orientation with the nomnial heavy spot down at 6 o'clock. My trial weight would go opposite at 12 o'clock.

If the fan is mounted on well designed and installed isolators it should be operating "above" the first 6 rigid body modes.
RPM, if you can describe your fan, I can take you through my process. Horsepower, drive type (belt or direct coupled), fan type (center or overhung), bearings, approximate fan rotor weight, turning speed, bearing pedestals (metal A-frame or concrete), etc.... you get the picture. If you can give us that, we can estimate where you are relative to resonance, and what your lag will likely be.
When people give very limited information it is difficult to guess what to fill in. Many fans are mounted on springs. If not well designed, the fan can operate too close to a natural frequency. Poor foundations or bearing supports that are flimsy can result in natural frequencies near running speed, too.

Out of phase mode shapes end to end or on overhung fans can be present (springs or not). One must consider any things even on a fan. If a consultant (outhouse or in house) is involved, most of these get ruled out, and a balance job can proceed smoothly.
rpmaskluk want to find heavy spot, he only get the phase number. so i think he need to find high spot first by the locations of sensors and the means of phase number, then he can think about the lag angle that changing with the location of running speed by the natural frequency, and the difference of displacement and velocity.
i believe last post by electripete have addressed enough the OP concern. the relationship from a phase reading to a trigger as well as the relationship between the high spot and heavy spot during 3 different scenarios and the velocity-displacement difference already are all clear.

          Example:
          Initial run reads 0.8 in/s @ 250 degrees (similar on both bearings)

Hello sir RPMASKLUK, I want to ask between the sensor and tach how many degrees, as well as the rotation of the CW or CCW?
and the sensor used acc to vel, or acc to acc?

wahyu zain posted:

          Example:
          Initial run reads 0.8 in/s @ 250 degrees (similar on both bearings)

Hello sir RPMASKLUK, I want to ask between the sensor and tach how many degrees, as well as the rotation of the CW or CCW?
and the sensor used acc to vel, or acc to acc?

RPMASKLUK hasn't visited this forum since June of 2015.

Add Reply

Likes (1)
wahyu zain
Post

HACKATHON in association with IMC-2019

×
×
×
×