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Old 05-12-2010, 06:06 PM   #1
BRENT in 10-uh-C
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Default Two-Blade failures are caused by ......

Over the years I have personally known quite a few folks who have joined the 'Failed Two-Blade Fan Fraternity' but has anyone ever done a study to see exactly why they failed?

Since I was a little kid, I have always been told that it was because they rusted from the inside but is this a "he said~she said" myth or fact?

Another theory I have been told is that the length of the blade is incorrect and the tips spin in a harmonics stage that fatigues the metal. While this may indeed be true, it is never the ends or tips that you see the crack. (I might add that it has been said that if " is removed from the tip of the fan blades, that they still cool just the same but the blade tip speed is slower and out of the harmonics area.)

So has anyone ever researched this enough to make factual statements why they fail? Your theory is, ...and 'why'??
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Old 05-12-2010, 07:22 PM   #2
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Default Re: Two-Blade failures are caused by ......

"blade tip speed is slower and out of the harmonics area"
Brent may have something here.
Let me preface my remarks...I have no knowledge as to why the cast [not stamped?] two blade fans fracture at the hub.
Bare in mind the the fan and water pump impeller spin 1-1/2 times the engine rpm.
The engine is good for 2600 rpm so the fan spins 3900 rpm at times.
Good point blades don't fracture at the tip. I doubt if it's harmonics but more like stress.

What causes stress? I can't hear anything 'whining', snapping like a bull whip [that's hyper sonic compression of air], or whomping air like a propeller and no thumping. Hummmmm says the boy.
~Vibration? Out of balance and shaking. Should feel the vibes by touching the water pump shaft. Or listening to the whirl with a long handled screwdriver to the ear.

~Bending? More like...

~Flexing toward radiator while pulling air? Straightening out when slowed. Back and forth till she breaks. So rpm's climbing and backing back down kill engines...why not fans? Could cause the root of the blade to flex and start to fracture.
You know...I wonder what a guy could see with a variable flash zeon strobe light.
Freeze the moving blade.
Might be able to detect flex.
The fan works hard at moving air.
Resistance through air squares as speed doubles.
Your hand out the window at 25 mph vs at 60 mph. Speed flips the hand right up.

Air has weight. 1.087 lb per one cubic foot at 70 deg F. Less weight when hot, but the same one cubic foot volume.
One cubic foot is two cubic foot at 600 deg F. But weighs the same as one cubic foot at 70 deg F. [standard air]
Engine hp used to turn the fan moving hot air decreases.
If the fan was driven by an electric motor the cold start motor amp would drop as the air through the fan heated. New York Blower manual on the four standard Fan laws explain this.
I think I read that the fan moves 220 cubic foot a minute at X? rpm.
That's about 240 lb of air a minute.
If anyone wants to play with the 'draft-through-the-radiator' divide the 220 cuft by the sq foot opening of the radiator. Just for poops and grins. That air heated to about 160 deg F gives the approximate btu cooling so 220 cfm x 1.087 weight of air x [160 deg F -70 deg F air; rise in deg F] = 21,522 btu per hour. [btu's are always in hours] Fun?

~Pulling like centrifugal forces. I don't see it. The blade will fling but that's after it fractures. That's the result. Not the cause.

~After 80 years, I think the hub/blade root just get worn out. The problem is the new replacement fans are sometimes of bad metallurgy, materials/process, and craftsmanship.

~I have heard that a cast fan that has been cleaned and vapor blasted with glass beads then degassed at a high temperature, then cooled and powder coated gloss black and cured at a lower temperature will show the crack developing. HIGH GLOSS thermo-set epoxy polyester cured powder paint is brittle and will show cracks if the blade flex's at the hub. An early warning? I don't know. Speed kills. A four blade fan doesn't move more air 'cause the blades have less pitch I hear but they make noise as Bruce says. Weigh more too.

When I wash my BMW 320iS and it's ideling away, the electric cooling fan kick's on and the car moves foward about 1/3 of an inch. Is that cool or what?

That's my 2 cents.

skip.

Last edited by skip; 05-13-2010 at 10:50 AM.
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Old 05-12-2010, 08:55 PM   #3
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Default Re: Two-Blade failures are caused by ......

I wonder how many Ford fans threw a blade "back in the day" ?

I think Skip has something about the blades flexing fore & aft as the engine speeds-up and slows down, and eventually the metal fatigues and cracks near the base of the blade.

I also have to wonder if running Model A's at generally higher speeds for prolonged periods of time than was possible "back in the day", on dirt roads, is partly responsible for increased fan-failure ?

Do these fans fail putting around town at 30 MPH, or out on the road at higher speeds ?
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Old 05-12-2010, 09:26 PM   #4
Tom Wesenberg
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Default Re: Two-Blade failures are caused by ......

My 28 Phaeton has a small dent from someone throwing a fan blade years ago. I'm running an original fan, but like to keep my speed down to 45 or less. I wasn't paying attention to speed and found myself doing 57 on the freeway once, but that was the only time I drove that fast.

I also think the slight fan flex over time is what causes the fan to crack where it leaves the hub.

Last edited by Tom Wesenberg; 05-12-2010 at 09:28 PM. Reason: forgot a statement
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Old 05-12-2010, 09:33 PM   #5
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Default Re: Two-Blade failures are caused by ......

I think all of the above, but the fatigue factor is the ultimate determinant. The blades of the few broken ones I have seen were full of dust/dirt because of the opening at the hub and that traps moisture. When you look at some intact ones, you can sometimes see them bulging in places out on the edges due to this build-up and the expansion caused by the growth of rust. The extra weight in the blades accelerates the fatigue.
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Old 05-13-2010, 08:46 AM   #6
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Default Re: Two-Blade failures are caused by ......

Our club likes long drives and we've had a couple fan failures. Surprisingly, several of 'em cracked and held together long enough to be detected and replaced. Ticking seems to be the tell-tale.
A good diagnosis is to give your fan blades a little tug when you check the oil. Any flexing would be good reason to condem the piece and (finally) buy an aluminum fan.
You may want to avoid the four-bladed fan. They are really noisy. Probably a lot of 'em hanging on the pegboard because of their damnable whirring.
ANY FAILURE OF A genuine FORD REPLACEMENT FAN TO REPORT?
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Old 05-13-2010, 04:00 PM   #7
Larry Seemann
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Default Re: Two-Blade failures are caused by ......

Flexing is the culprit. But, what really causes the flexing that leads to failure is the fact that the blades pass in front of the engine block as they rotate. In other words as the blades go around and pull air they are under stress, but when the blade passes in front of the block the air is blocked and the stress on the blade is momentarily but violently relieved. So, the blade "snaps back" each time it passes the block.
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Old 05-13-2010, 04:51 PM   #8
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Default Re: Two-Blade failures are caused by ......

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ANY FAILURE OF A genuine FORD REPLACEMENT FAN TO REPORT?
WHAT IS A genuine FORD REPLACEMENT FAN?
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Old 05-13-2010, 05:28 PM   #9
Bill Goddard
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Default Re: Two-Blade failures are caused by ......

What if we made a two blade sheet steel fan by cutting off two opposite blades of the four and rebalancing it. Would it move enough air and be quieter? Could we repitch the blade to move enough air?
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Old 05-13-2010, 05:58 PM   #10
Barry B./ Ma.
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Default Re: Two-Blade failures are caused by ......

I had exactly that on my 29 tudor, looked like a four blade design but had only two and the fan was purchased that way. Cooled well and had no sign of fatigue when I sold the car.
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Old 05-13-2010, 06:37 PM   #11
BRENT in 10-uh-C
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Default Re: Two-Blade failures are caused by ......

Bill, the truth is many engines do not need a fan whatsoever. Once you are moving above 10-15 mph, the frontal area pushing the wind does the cooling without a fan.

While in retrospect, this was not one of my more stellar moments, I did drive the car pictured below on the mandatory tour at the MARC Nat'l Meet in Dearborn a few years ago with a hopped-up banger engine and NO cooling fan. We were all in one big parade driving through the streets of the city from traffic light to traffic light, and it never heated up until I was just about through entrance line where they were giving out the ribbons as we entered the field at the airport. It steamed but never boiled, so it kinda proves that oftentimes you really don't need a cooling fan. One thing I do believe is too many fan blades are more detrimental as they rob horsepower and set up a wall impeding airflow through the radiator at higher speeds.


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Old 05-13-2010, 06:51 PM   #12
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Default Re: Two-Blade failures are caused by ......

Quote..."Bill, the truth is many engines do not need a fan whatsoever. Once you are moving above 10-15 mph, the frontal area pushing the wind does the cooling without a fan".

Interesting. Dad told me there was a Ford tractor that didn't use one. Not sure I'd want to try this on my Coupe. I have a 4 blade currently.
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Old 05-13-2010, 09:15 PM   #13
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Default Re: Two-Blade failures are caused by ......

When I bought my A back IN '62 it had the Ford four blade replacement fan on it. I am still using that fan today without problem.
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Old 05-14-2010, 02:47 AM   #14
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Mine broke while I was setting at a stop light. Man what a racket.
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Old 05-14-2010, 09:43 AM   #15
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Default Re: Two-Blade failures are caused by ......

Ford made a replacement fan that was also OEM on late '31's. It has the same airfoil shape, but the bends are different. They are easy to identify; the last three or four inches at the end of the blades are single thickness steel. Sorry I don't have a picture to post.
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Old 05-14-2010, 10:23 AM   #16
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Thanks for the link, it answered my question.

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Old 05-14-2010, 11:18 AM   #17
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Default Re: Two-Blade failures are caused by ......

Since it is a known fact that the fans will fail, I would rather look at how to fix the problem. There has been a repair designed by an engineer that has been published in, as I remember, the MAFCA publications. I personally have made this repair to a number of fans and run them without any fear of further cracking. At this point I have in excess of 70 thousand miles on my two cars with no further cracking.
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