nicks in a fan

MARTIN AIDE · 09-28-2005, 03:58 PM

Hi guys,do nicks on fan blades cause problems?if so how much?thanks Marty

bigchillcar · bigchillcar's Garage

can you post pics of the nicks? you may be able to sand them down if they aren't too bad. as long as you're not rubbing the housing or have blades practically hanging on by a thread, you're probably safe. if it fails..you'll know..

ryan

Rot 911 · 09-28-2005, 05:15 PM

I've seen some fan blades with some pretty good size (1" x 1/8") slices out of a couple of blades (including a 72E of mine) that never had any additional problems. I wouldn't worry about it.

kqw · 09-28-2005, 06:58 PM

The question should be; How do you get nicks in the fan blades...All I can think of is that the fan was dropped and damaged.....

kstylianos · 09-28-2005, 09:25 PM

Sticking a tool between the blades to hold the fan while trying to remove the nut......don't ask.

Hetmann · 09-29-2005, 11:55 AM

You can make a little jig to rebalance the fan by drilling out the detents on the back side. Or you can look for a used fan with no nicks (good luck).

Jeff Alton · 09-29-2005, 01:06 PM

Or just leave it alone........

If it actually breaks a blade, look for a new one.

Cheers

mtelliott · 09-29-2005, 02:00 PM

Finding a used fan at a good price: forgetaboutit.

You can find a machinist that will smooth out the nicks and then rebalance.

What do the nicks cause: reduced airflow and vibration.
What does this lead to: overheating and greater wear on the alternator. How much is anyone's guess.

Michael

David · David's Garage

I just bought a fan with a nick in it. I'm also curious if I should worry about it so lets do some calculating.

The ISO balance tolerance for a fan is G6.3

The allowable imbalance in oz-in = (6.3 x 6.015 x W/2)/N

Where W is the fan weight in lbs and N is the maximum continuous speed.
I don't know the weight, but lets assume 1 lb.

Max engine speed would be about 7000 rpm so max fan speed would be 7000/1.6 or 4375.

Allowable imbalance = (6.3 x 6.015 x 1/2)/4375 = .00433 oz-in

For a 240mm fan the fan tip radius is 4.72 inches. Lets assume the nick is 1" from the tip so the max allowable imbalance is .00433 oz-in/3.72 in or .00116 oz.

Magnesium weighs .0637 oz/cu in. So we're allowed a nick with a volume of .00116 oz/.0637 oz/cu in or .0183 cu in or a cube with sides .263" long. This is a pretty large nick before it's out of balance tolerance.

I put the math here so someone can check my work (I've been known to make a mistake on occasion). As far as rebalancing the fan, it must be done with the center hub installed and it needs to be done on a balance machine that reads down to this tolerance, not static balanced.

As far as a crack growing from the nick, just make sure there are no sharp edges. I'd be concerned if the nick is near the blade root where the stress is highest.

nixter · 09-29-2005, 02:10 PM

You lost me at "fan"

nik

randywebb · 09-29-2005, 02:40 PM

I'll check your work if you convert it all to SI....

Jeff Alton · 09-29-2005, 02:53 PM

Does anyone know what standard the factory balanced the fan too?

Cheers,

kstylianos · 09-29-2005, 02:54 PM

nixon fan:

MARTIN AIDE · 09-29-2005, 02:56 PM

WoW,thats great David,LOL,so did you use the fan?the problem is as you all know,they are expensive and I am moving from a 5 blade to this one so given the air loss from the knicks it still should be better than what I have,right?thanks Marty

mtelliott · 09-29-2005, 03:32 PM

Where does the ISO balance tolerance number (G6.3) come from? The reason I ask is if that was from a fan sitting on an alternator shaft, then engineering approach to the solution is excellent.

The fact that you took an mathematical approach to answering the question though is excellent. The only true way to determine if there is an issue or not. I'm humbled. I think I might have wasted $60 on a fan rebalance.

Michael

David · David's Garage

Randy, the ISO standard is SI. I converted it to standard with the 6.015.

Some sources:

http://www.easa.com/mo/TN-pdfs/TN32-400.pdf#search='iso%20balance%20tolerance'

http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf#search='iso%20balance%20standards'

The next lower tolerance is G2.5 (gas & steam turbines, harddrives) and then G1.0 (tape recorders, small armatures) and then G0.4 (precision grinders, gyroscopes). At work we usually balance to standards lower than required because it doesn't take that much longer and it's a lot more expensive to balance in the field than in a shop. If this was a race car constantly running near redline then I'd worry about it. I'll probably file a little off the opposite blades on my fan just for good measure. Unfortunately I'm still a couple month away from getting my car running.

Hopefully the long downtime will be worth it!!!