I tried to balance a vacuum blower with terrible results. Is there something I am missing on this type of fan(like a hydraulic force) that would look like imbalanc?
12 blade
3600 rpm
overhung rotor
open discharge
Used to pull a vacuum from the forming table on a paper machine.
All my data is pure 1x. Using csi fast bal I made the 1x worse. After remove all my balance weights the fan was back to normal(around 4 mils). When we ran the fan latter the vib went went down to a decent reading (.2 inches per sec. velocity). Is there a way to manipulate the suction somehow? Open the suction completely?

I suspect this machine may be similar in construction to gland exhaust fans used with turbines. We've had lots of problems with these over the years trying to get them to run smoothly. If they are similar, the fan housing is little more than sheet metal, so the housing distorts easily, throwing everything out of wack. I'd make sure the mounting surface is flat and the mounting bolts are tightened evenly, and make sure the ductwork is lined up so as not to distort the housing. We've chased balance around without success on some of these until the distortion issues were corrected.

Jon
Spintelligent Labs

Well no wonder if it don't know if it's sucking or blowin' ......... Here in WV huffin' and puffin'

Jon is probably giving good advise. 1X is not always a function of imbalance solely.

Me: I think it's Friday. It's time for Glen something or other.

quote:

Originally posted by Jon Chandler:
... the fan housing is little more than sheet metal, so the housing distorts easily, throwing everything out of wack.

Jon,

Are you saying that predominantly 1x was caused by internal bearing misalignemnt? If so, which analysis technique have you used in order to detect this condition in the vibration data?

Thanks,
David

There are other factors involved when trying to balance a fan and you already hinted at one of them. All fans, whether centrifugal or blade type, move air. With a centrifugal fans it is preferable (if possible)to mask off the discharge or inlet so that there is no airflow through the fan. You are then left with a straight forward mechanical situation. With a blade type fan, you can't effectively mask it so you are always dealing with a compound situation of mechanical and air flow. If one of the blades is slightly "different" to the others (not quite the same angle?), you will see a similar effect to imbalance as the airflow past this blade will be slightly different causing a pulsing effect at 1X.
Have you checked the alignment (angles etc)of all the blades. Maybe one is slightly bent???
Just a thought.

I do not do allot of balancing but isn't it important where the weight should be hung for an overhung fan? Loading of the fan can play a big part too but if weight is hung in correct plane but wrong place (inboard to outboard of fan) would that cause big problems?

Hi Ken
We have 10 of these vacuum blowers they get unbalance every 5 or 5 months. tricky balance too. the trick is to open the front and pull out the rotor and presure wash it. This usually brings it back to the original balance spec. If not then I balance. Hope this helps Don

David,

I wish I had some great analysis technique on this one but it kind of boils down to dumb luck. We've had cases where the readings are stable and repeatable...until somebody bumps into the fan or uses the ducting as a hand-hold. The units I'm familiar with are really flimsy, so any slop in the mounting can result in changes and distortion from just touching the thing.

Jon
Spintelligent Labs