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marksl

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Hi Guys,

Looking for help with three kiln fans in a brick factory which seem to have constant issues with fan shaft looseness. I have noticed that in all three fans there is only a small difference between the Motor and Fan shaft speed (Ratios: 1.05, 0.94 & 1.048). Could this be caused by a beat frequency. The only thing common in all three is looseness at the fan bearing locations.

Thanks in anticipation

Noknroll

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Reply with quote  #2 
Close ratios like that are not uncommon, I've got some 1:1 motor/pump arrangements. 
These close ratios could create a beat frequency.
Sometimes I think they may be designed this way so the belt is a sacrificial component if a bearing seizure lock up were to happen (just my way of thinking) I cant see any other reason for not going direct drive
Are the fan shafts actually loose/worn in housings or just what you think might be happening. Got any spectrums showing beat or looseness to post?
marksl

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Hi Noknroll,

Here one of spectrums from a fan DE
Looseness.JPG 

RustyCas

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If the "beat" is strong enough to 'count' then the difference in shaft speeds is the beats/minute, that is 5 beats/minute = 5 rpm difference. For multiple fans in close proximity, count the bpm and the accurately measure all shaft speeds. This tells you which fan has the problem.

The use of belt drives is often just because the arrangement is much more compact than direct drive. The length will typically be only 60% of that required for a direct drive. Also cheaper to build (much less steel and fabrication), and cheaper to ship.

Looseness in a bearing can produce a beat of its own, but often quasi-random.

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Hi Rusty.

Could you please explain a bit your last statement

"Looseness in a bearing can produce a beat of its own, but often quasi-random"

Trying to understand how looseness can produce a beat.

Thank you.
RustyCas

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I hear it all the time on large motors with worn bearings (excessive internal clearance). I say the motor has the "wah wahs" because it goes waaaaah, waaaah, waaaah in a rhythmic pattern (a beat). Usually doesn't show up as discrete frequency but the waveform shows it well.
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marksl

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What does a beat frequency look like in the spectrum?
Noknroll

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2 peaks close together, very common is 2 x LF and 2 x running speed on a 2 pole motor or slip across a fluid coupling
In fact 1x m and 1x f marked in your spectrum are what you are looking for but typically they might be same amplitude.
Waveform is also very distinctive.
marksl

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Quote:
Originally Posted by Noknroll
Waveform is also very distinctive.


What should I be looking for in the waveform?
Noknroll

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Egg shaped patterns side by side
Sinski

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Reply with quote  #11 
Quote:
Originally Posted by RustyCas
I hear it all the time on large motors with worn bearings (excessive internal clearance). I say the motor has the "wah wahs" because it goes waaaaah, waaaah, waaaah in a rhythmic pattern (a beat). Usually doesn't show up as discrete frequency but the waveform shows it well.


I can't say I have ever come across that wah wah noise being caused by bearing wear or excessive clearance etc. Everytime I hear this vibration in a motor it is normally caused by pole passing frequencies or as Nok says with 2 pole motors having the 100Hz very close to 2x.
OLi

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Reply with quote  #12 
Or one phase voltage is dropped or rather have bad connection, or one winding is bad. Or you have a horiz mixer where the tank bottom is full of gunk and it say whaaaa every time a blade from 3 blade mixer push thru the gunk, also after the motor was swapped 3 times.....
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dnk

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Reply with quote  #13 
If you have this in all three fans might want to check that you the proper fit bearing for the temperture the fan is seeing. Would think on a kiln you would have at least a C3 fit. If you have a higher fit might contribute to looseness.
Ralph Stewart

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Reply with quote  #14 
marksl's quote:
Quote:
Hi Guys,

Looking for help with three kiln fans in a brick factory which seem to have constant issues with fan shaft looseness. ( does this mean the bearings are being changed and the "looseness" is still there?) I have noticed that in all three fans there is only a small difference between the Motor and Fan shaft speed (Ratios: 1.05, 0.94 & 1.048). Could this be caused by a beat frequency. The only thing common in all three is looseness at the fan bearing locations.

Thanks in anticipation


I see where, in the spectrum posted, that your fan position marker, "1xF, 2xF, Etc. is a higher frequency that is your motor position marker, 1xM, 2xM, etc.. Does this mean that the 2978 rpm note in the upper right corner of the spectrum is the fan speed? Is this the data spectrum related to the 1.05x position you stated in the original posting?

Thanks and Have a Great Day,
Ralph

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