how to change a centrifugal fan natural frequency without adding weights

 

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Best Regards, 

Alain Raad

Maintenance Coordinator & CMMS Developer

CIMENTERIE NATIONALE PLANT-LEBANON-Chekka
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Original Post

 

Do you have some pictures you can post?

 

What do you know about the " natural frequency" ?

 

Depending what is going on, it may be possible to significantly change the offended frequency by:

- Tightening fasteners (zero weight change)

- Adding a few ounces of Welding rod in strategic locations

- Welding a few pounds of reinforcement in strategic location

- Driving in some wedges in strategic locations

http://www.mcmaster.com/#5775a1/=zltyya

- MANY other possibilities, all best done with at least a modest level of understanding what is resonant, and why.

 

Then again, if the resonant frequency is torsional, a change in coupling stiffness may be required.

 

Then again, if it can be shown the resonance is blower wheel back plate bending or coning then a re-designed blower wheel is about the only solution.

 

Last edited by Dan Timberlake

Natural frequencies are determined by weight and stifness. So if you can not change those, you can not change natural frequencies.

Investigate if you can change bearing distance if resonance is on rotating part/shaft, then change speed, then plan B add damping or plan C parasitic damper. If no go of these buy a new fan with different design...... Normally you can add a substantial amount of weight in the thin plate welded foundation if problem is mounting. Olov

Originally Posted by OLI:

Investigate if you can change bearing distance if resonance is on rotating part/shaft, then change speed, then plan B add damping or plan C parasitic damper. If no go of these buy a new fan with different design...... Normally you can add a substantial amount of weight in the thin plate welded foundation if problem is mounting. Olov

i have a resonance on the operation running speed after changing the bearing distance. the air gate is on its maximum so i cant reduce the Motor speed to a safe zone and open the gate more, so i will affect the production, and i can t change the pulleys to speed up the impeller to another safe zone after resonance because i don't  know at wish speed the safe zone will start. i think i have no choice... i will add weights or i will build a small concrete wall next to the fan with 2 holes and i will weld 2 screw shafts on 2 strategic fan housing points thru the 2 holes and tight them from the othe side of the wall until i pass the resonance zone.

Quote:

"i will add weights or i will build a small concrete wall next to the fan with 2 holes and i will weld 2 screw shafts on 2 strategic fan housing points thru the 2 holes and tight them from the othe side of the wall until i pass the resonance zone".

 

What does the fan do? Is it in danger of exploding from the high vibration or what? Question based on building a "concrete wall".

Do you have any data or pictures you can post?

Hi Alainraad,

 

"I have a resonance on the operation running speed after changing the bearing distance."

 

So the high vibration is at 1X fan rotating rpm? 

 

1 - does your blower look anything like this?  (If you'd post a couple of pictures this would be A LOT EASIER)

https://d2t1xqejof9utc.cloudfr...99900be28/medium.JPG

 

2 - Is this what caused you to move the bearing location?

http://hitekbalancing.com/gall...in.php?g2_itemId=105

 

Did you change the bearing distance by sliding one bearing (probably the drive end bearing)  inward due to shaft damage from an inner race getting loose?  When that works OK, it works OK. But potentially it can cause problems a variety of ways that Bracing to the wall will not help  

 ( >>IF<< I understand your pictureless description of the planned modification correctly ).

 

2A - what shaft gripping method do the bearings use? 2 setscrews, eccentric collar, sqweeze lock, centrilock, tapered adapter.  Most of them are inadequate for reliable service in belt drive applications in my opinion. If your change in bearing distance increased the pulley/sheave overhang then the inner race will lose grip quicker than before.

 

3 - Before I'd embark on any modification I'd try to arm myself with at least a modest level of understanding what is resonant, and why.

******** The first step would be a bunch of vibration measurements ( including 1X amplitude and phase) of several locations from bearing housings down to the concrete floor (is this mounted on a slab on the ground, or a mezzanine, or something else altogether.)*************

 

For instance, when you moved the bearing (inward I assUME)  2 things happened. The support of the shaft changed, changing the effective shaft bending stiffness, and possibly affecting some bending resonance of the shaft/blower wheel assembly. However the stiffness of the pedestal (guess) top plate that supports both pillow block bearings ( guess) could simply be much more flexible under the new bearing location. >>If<< that is the case, then the top plate simply needs to be stiffened. A wall brace won't be of much use at all.

 

regards,

 

Dan T

If you made the shaft shorter and it got worse, buy a new longer shaft or the opposite. Do a knock test or run up down to verify if you run on or above or lower than resonance and if you are spot on you can go either way. If you run above, make shaft longer to lower resonance or the other way around. If your problem is the frame, installing a floor and filling with sand or if you are confident concrete do the trick, leave space to reach the bearing mounting bolts. Olov

No where is all of this thread is there a qualification and quantification of what is in resonance.  How can you possibly expect to fix it without knowing what is in resonance.  Is it the fan shaft bending from the weight of the fan and sheave?  Is it a support stiffness issue with the entire fan assembly?  Is it a fan backplate bending mode?  Have you done any impacting of the fan to find out if it is indeed a bending mode and maybe not a rigid body mode?

What type of fan is it and what is it's speed?  These may help some of us visualize what problem you have.

Ron Brook

 

Hi Alainraad,

 

I don't think you have said yet  >>If<<  shaft wear was the problem that resulted in needing to shift the bearing location/bearing distance, or provided a picture, drawing or sketch of what was shifted.

 

I left out a part when I posted this -

" 2 - Is this what caused you to move the bearing location?

http://hitekbalancing.com/gall...in.php?g2_itemId=105  "

It shows that adding a very nicely made sleeve and using a modified bearing or modifying the bearing support can allow moving the bearing back to where there were no problems.

The pictures 24 thru 35 at the link provided show the principle  behind the repair.

 

regards,

 

Dan T

buenos días: mi pregunta es como esta usted seguro que es una resonancia lo que se presenta en su soplador.

- la resonancia se presentara a una velocidad determinada del equipo, fuera de ella desaparecerá.

- otra posibilidad es una deflexion del eje, en este caso la alta vibración se presentara a lo largo de todas las velocidades. digo esto por la vibracion axial que le aparece.

- posiblemente la deflexion se produzca por la distancia del apoyo a la polea que al aumentar flexione el eje, en este caso seria bueno quitar tensión a sus bandas así también variara la rigidez y posiblemente ayude en caso de tener una resonancia.

 

good morning: my question is as sure you that it is a resonance that occurs in your blower.
- The resonance is present at a given speed of the computer, outside disappear.
- Another possibility is a deflection of the shaft, in this case the high vibration is present throughout all speeds. I say this for the axial vibration that appears.
- Possibly the deflection occurs due to the distance from the support that increasing bend pulley shaft, in this case it would be good to remove tension bands and also vary their rigidity and possibly help if you have a resonance.

buenos días: mi pregunta es como esta usted seguro que es una resonancia lo que se presenta en su soplador.

- la resonancia se presentara a una velocidad determinada del equipo, fuera de ella desaparecerá.

- otra posibilidad es una deflexion del eje, en este caso la alta vibración se presentara a lo largo de todas las velocidades. digo esto por la vibracion axial que le aparece.

- posiblemente la deflexion se produzca por la distancia del apoyo a la polea que al aumentar flexione el eje, en este caso seria bueno quitar tensión a sus bandas así también variara la rigidez y posiblemente ayude en caso de tener una resonancia.

 

good morning: my question is as sure you that it is a resonance that occurs in your blower.
- The resonance at a certain speed of the equipment present outside that speed vibration disappear.
- Another possibility is a deflection of the shaft, in this case the high vibration is present throughout all speeds. I say this for the axial vibration that appears.
- Possibly the deflection occurs due to the distance from the support that increasing bend pulley shaft, in this case it would be good to remove tension bands and also vary their the stiffness and possibly help if you have a resonance.

Have you consider using tuned vibration absorber.

Tried once before and it did split the natural frequency.

My vertical motor reduced vibration from 18mm/s to 5 mm/s.

But the welded part of the abosrber did not last long..

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