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Axial vibration unbalanceable Fan

Axial vibration unbalanceable Fan

(OP)
Hi Guys,
i have a very big issues in vibration on a belt drive Fan.

It is combustion air fan with 300°C inside pipe. On the base there are 6 rubber dumpers.
The speed of the motor is 1498 RPM and speed of the fan is 3018 RPM.

During vibration analysis i found 1XFAN speed in every place with high amplitude (max found is 16 mm/s rms in horizontal direction).

i tried to balance the fan with no result. Effectively i change vibration only in location referred for balance work.

while i change vibration from 16 to 0,2 mm/s in horizontal direction, the same change from 4 to 13 mm/s in vertical direction.

Furthermore dropping off vibration in ID3 results in increasing vibration in ID4.

I made also this test to well understand where vibration coming from:

  • Change the air flux for inlet and outlet pipe but no changing happens
  • block dampers on base to avoid some resonant phenomenon with no results
  • control about pulley eccentricity and bent shaft and everityng were ok
  • Inspected visually the fan internally to prevent axial movement
Sincerly i finish my cards and i'm thinking if modal or ODS analisys could give me correct indication to solve the problem.

Do you have any suggestion about it?
Anyone use or used software like ME'SCOPE can give me idea if could be the right way?

Thanks

RE: Axial vibration unbalanceable Fan

Quote:

Furthermore dropping off vibration in ID3 results in increasing vibration in ID4.
Can you clarify what are ID3 and ID4?

=====================================
(2B)+(2B)' ?

RE: Axial vibration unbalanceable Fan

(OP)
Sure electricpete,

ID3= pulley side bearing
ID4= fan side bearing

RE: Axial vibration unbalanceable Fan

If it were just unsuccessful balancing where you can't get the magnitude down at a given point,, then I'd be inclined to expect excess clearance somwhere.

But you say that correction attempts cause the vibration to change to other locations (from H to V, from one bearing to another). It suggests a change in the operating deflection shape.

One thought is maybe you are using a 1-plane solution where you need 2. (Are you using a 2-plane balance solution? If not what is L/D of fan wheel?).

Another thought is that maybe the rotor is flexible. 3000rpm is pretty high speed for a fan. Can you give a rough idea of the geometry of the fan? (example shaft diameter, distance between bearings, impeller dimiensions). Also I didn't quite understand the word "axial" in the title since you mentioned only radial vibrations. I assume a belt driven between-bearings machine is a squirrel cage fan (radial flow, not axial flow)… is it squirrel cage type?

=====================================
(2B)+(2B)' ?

RE: Axial vibration unbalanceable Fan

Got pictures of the installation?

Was the vibration OK before some repairs, like bearing replacement?

What were original 1 X amplitude and phase? And the results of the balance 'shots" ?

I'd be measuring all up and down the structure, not just the bearing housings, to understand what is happening.

A common reason for "balancing" to change vibration direction is the influence of drive sheave runout.
I like to mount a dial indicator on the driver to measure on the driven, in the plane of belt pull.
If I rotate the fan by hand, and the indicator says the motor and fan move towards each other by a few thousandths, I'd expect the running vibration to be no less

RE: Axial vibration unbalanceable Fan

Can we do static and dynamic balancing in a impeller deposited with huge scales without cleaning it?

RE: Axial vibration unbalanceable Fan

Check if fan is running in the STALL area of the fan curve. If so, it will vibrate a lot. In mining, we listen to fan noise to tell if it's in stall mode. If it is, reduce impeller blade
angle to decrease demanded airflow or reduce airflow resistance of system. From experience, a fan that ran in the stall area of the fan curve will have reduced performance afterwards.

Ingenieur Minier. QuTbec, Canada.

RE: Axial vibration unbalanceable Fan

(OP)
Hi Guys,
I want to clarify some details about your correct dubts.

First of all the fan is 1 meters in diameter and 5-6 cm in lenght so it is very disk form.
The bearing installed are a C3 execution (6317-C3 in both sides). Before attemping the balancing i performed the semi complete overhaul of the fan.
-change the bearing
- change the bearing housing
- Check for fan skew
- Check integrity of dampers
- check for fan clearance and fan cleaning
- check runout of both pulley in lot of places but radially i found maximum 6/100 mm

The vibrations found on this fan still there for more or less 2 years in while lot of maintenance have made.
Furthermore i have another exactly the same fan 10 meters apart where i haven't found any type of problems during balancing.
Regarding the axial vibration in the title is because for this type of fan is uncommon to found (in my experience) so big amplitude in axial directions that you cannot eliminate with balancing considering highest amplitude is in horizzontal directions ( and change during balancing as mentioned before)

Because the same fan have the suction in the same pipe but in different location i need to check about if it running in a STALL mode.

Thanks a lot


RE: Axial vibration unbalanceable Fan

Certainly confirm the fan is operating on it's curve.

  • Is this an overhung fan?
  • Were your balancing efforts single plane or two plane?
A few balance shots based just on the axial vibration is sometimes useful

  • Got pictures of the installation?
"control about pulley eccentricity and bent shaft and everityng were ok".
  • Please explain more the test method and test results for eccenticity and runout.
A common reason for "balancing" to change vibration direction is the influence of driven sheave runout.
I like to mount a dial indicator on the driver to measure on the driven, in the plane of belt pull.
If I rotate the fan by hand, and the indicator says the motor and fan move towards each other by a few thousandths of an inch, I'd expect the running vibration to be no less. AND that balancing will/can not compensate for sheave runout.

  • 16 mm/sec rms at 3018 rpm is only about 5.5 MILS / 0.2 mm pk to peak / TIR

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