George Miller
Well-known member
I just replaced the one I got few years back. The new one is much better. The blades are in line, taper is good, the old one the taper was off with the blades not in line.
aluminum fan...Q
- Thread starter hardtimes
- Start date
I just replaced the one I got few years back. The new one is much better. The blades are in line, taper is good, the old one the taper was off with the blades not in line.
mass A man
Well-known member
It's worth mentioning again, make sure you use a steel washer under the castle nut when using aluminum fan. Eventually the steel nut will dig into the casting, creating a "looseness nuisance".
I’ve done some thinking on this Model “A” fan failure problem in the past. In my 40 years as a mechanical engineer with considerable experience in metal fatigue, vibration, stress analysis, and rotating machinery – here are some of my thoughts:
1. The failure mode is metal fatigue which means a very small crack occurs at a critical high stress region (near the hub) after many cycles of high and low stress reversals. Then, the crack starts growing (as stress cycles are added) and the crack grows. The crack keeps growing until it reaches a critical length at which time failure occurs. The failure would not necessarily occur at high RPM. It could occur at idle.
2. A single overload could greatly reduce the fatigue life. This overload could cause residual stress in the critical region, requiring a much lower number of stress reversals to initiate a crack. The overload could be caused by using the fan to rotate the engine such as when timing the ignition. I wouldn’t be surprised that nearly all original fans have been subjected to this atrocity – and I wouldn’t be surprised if all of us haven’t been guilty of it one time or another.
3. So, what could cause a high number of stress reversals in the critical area of the fan? It has been suggested that blade bending cyclic loads with respect to the vehicle fore and aft direction may be caused by air pressure pulsating loads as the blade passes over the generator, top radiator hose, and timing case (see “fan failure analysis and repair” by Russell Baetke, from “The Restorer” Vol 6). However, when the vehicle is going over 15 MPH the fan is no longer moving air and any pressure pulsations would be low. I thought that engine vibration could be causing a blade load in the fore and aft direction. There is a characteristic 2 x RPM vertical vibration common to all 4 cylinder in-line engines which I thought could be causing a high frequency rocking vibration of the engine about the rear mounts which could translate into fore and aft vibration. However, after going out and experimenting with my “A” a little – I don’t that that’s it. Blade unbalance could be the problem. The blades are hollow and rust eventually occurs on the inside. Rust particles are slung to the outer ends of the blades in an uneven manner causing unbalance. Also, the overload explained above could cause the blades to no longer be in the same plane, causing unbalance. So what’s left? The only thing I can think of (at this time) is the cyclic axial centrifugal loads on the blades due to 0 or low RPM to say 3000 RPM and back to 0 or low RPM the fan is subjected to over the years. Back when these cars were used nearly every day, and considering the fan is brought up to some intermediate or high RPM and back to low RPM when shifting from low to 2nd and then from 2nd to high – several thousand cycles could be accumulated in one year. This would still be a low number of cycles from a fatigue standpoint, but if the critical area sees high stress from high RPM centrifugal force, and considering the likely high overload (explained above) – I’m thinking his could be the main explanation for the failures.
4. The new cast aluminum fans: Forged aluminum fans would be much better from a fatigue life standpoint. Cast aluminum has a very low fatigue endurance limit. Of course, forged fans would be more expensive. Having said this, I run the cast aluminum fans on my “A’s” and would not consider running an original fan unless I “repaired” it as per the article referenced above. I recently purchased an original fan from eBay. It was a very nice looking original. The seller said he inspected it for cracks and found none. He wasn’t looking close enough! There was a crack in the characteristic location (5/8” from the center of the hub). I would keep a close eye out for fatigue cracks in the cast aluminum fans just as I would for the originals. As far as testing the cast aluminum fans – running them up to high RPM till they fail does not tell you anything about their fatigue life. They would need to be run from 0 to (say) 3300 RPM thousands of times to test them for the cyclic centrifugal forces.
Well, I hope you are not too “fatigued” from reading all this. I tried to keep it short (believe it or not). But, this is a complex and important subject.
1. The failure mode is metal fatigue which means a very small crack occurs at a critical high stress region (near the hub) after many cycles of high and low stress reversals. Then, the crack starts growing (as stress cycles are added) and the crack grows. The crack keeps growing until it reaches a critical length at which time failure occurs. The failure would not necessarily occur at high RPM. It could occur at idle.
2. A single overload could greatly reduce the fatigue life. This overload could cause residual stress in the critical region, requiring a much lower number of stress reversals to initiate a crack. The overload could be caused by using the fan to rotate the engine such as when timing the ignition. I wouldn’t be surprised that nearly all original fans have been subjected to this atrocity – and I wouldn’t be surprised if all of us haven’t been guilty of it one time or another.
3. So, what could cause a high number of stress reversals in the critical area of the fan? It has been suggested that blade bending cyclic loads with respect to the vehicle fore and aft direction may be caused by air pressure pulsating loads as the blade passes over the generator, top radiator hose, and timing case (see “fan failure analysis and repair” by Russell Baetke, from “The Restorer” Vol 6). However, when the vehicle is going over 15 MPH the fan is no longer moving air and any pressure pulsations would be low. I thought that engine vibration could be causing a blade load in the fore and aft direction. There is a characteristic 2 x RPM vertical vibration common to all 4 cylinder in-line engines which I thought could be causing a high frequency rocking vibration of the engine about the rear mounts which could translate into fore and aft vibration. However, after going out and experimenting with my “A” a little – I don’t that that’s it. Blade unbalance could be the problem. The blades are hollow and rust eventually occurs on the inside. Rust particles are slung to the outer ends of the blades in an uneven manner causing unbalance. Also, the overload explained above could cause the blades to no longer be in the same plane, causing unbalance. So what’s left? The only thing I can think of (at this time) is the cyclic axial centrifugal loads on the blades due to 0 or low RPM to say 3000 RPM and back to 0 or low RPM the fan is subjected to over the years. Back when these cars were used nearly every day, and considering the fan is brought up to some intermediate or high RPM and back to low RPM when shifting from low to 2nd and then from 2nd to high – several thousand cycles could be accumulated in one year. This would still be a low number of cycles from a fatigue standpoint, but if the critical area sees high stress from high RPM centrifugal force, and considering the likely high overload (explained above) – I’m thinking his could be the main explanation for the failures.
4. The new cast aluminum fans: Forged aluminum fans would be much better from a fatigue life standpoint. Cast aluminum has a very low fatigue endurance limit. Of course, forged fans would be more expensive. Having said this, I run the cast aluminum fans on my “A’s” and would not consider running an original fan unless I “repaired” it as per the article referenced above. I recently purchased an original fan from eBay. It was a very nice looking original. The seller said he inspected it for cracks and found none. He wasn’t looking close enough! There was a crack in the characteristic location (5/8” from the center of the hub). I would keep a close eye out for fatigue cracks in the cast aluminum fans just as I would for the originals. As far as testing the cast aluminum fans – running them up to high RPM till they fail does not tell you anything about their fatigue life. They would need to be run from 0 to (say) 3300 RPM thousands of times to test them for the cyclic centrifugal forces.
Well, I hope you are not too “fatigued” from reading all this. I tried to keep it short (believe it or not). But, this is a complex and important subject.
hardtimes
Well-known member
Bill,
Thanks ! Important, indeed! That's why my 'original' uncracked fan hangs on the wall.
I've got a four blade steel fan that seems to do very well after years of wear. But, I'm going to try a new aluminum, based on what you and others have reported.
Hmm, I do seem to remember tho that in a prior discussion of model a fans, that someone said that they know of an aluminum fan that broke apart. Probably back when they were made 'out of balance and not parallel', eh !
Thanks ! Important, indeed! That's why my 'original' uncracked fan hangs on the wall.
I've got a four blade steel fan that seems to do very well after years of wear. But, I'm going to try a new aluminum, based on what you and others have reported.
Hmm, I do seem to remember tho that in a prior discussion of model a fans, that someone said that they know of an aluminum fan that broke apart. Probably back when they were made 'out of balance and not parallel', eh !
Just a comment on the original 4 blade fans. I have had 3 that have had cracks and ready to come apart at any time. I will not use any original 2 blade or 4 blade for driving any of my A's. Use a repro aluminum or a plastic fan. I have sold several 2 and 4 blade fans at swap meets and have warned the buyer to bead blast it, paint and restore it and display it on your garage wall only. I'm sure some of you have read about Chuck Christianson's experience in the MAFCA mag. about losing a blade on one. He is the Tech Editor. Be careful.
Keith True
Well-known member
I have an aluminum fan that grenaded on me years ago.It had voids in the casting on one blade.I was doing around 60 MPH when it explodes.I had a horrible time trying to break the other blade off so I could keep going.There were no voids on that side.I've been using 4 blades for years now.If they are not rusty,or cracked they seem to work fine.They are just a steel fan,like a modern car.I see lots of fan failures working on stuff now.Fans are a big seller at the junkyards.My only personal experience with a 4 blade failure has been the rivets coming loose and shearing.I have always caught that,because I look at the fans on every A that comes in here,and I look at my own A's at least once a week,usually more often.When I check the oil,I look at the fan.If somebody wants to look at the engine,I look at the fan.I know the fans get checked more often than the oil does.
Paul in Colorado
Senior Member
When I got my 1930 coupe, it included a box of extra parts. In the box was a three-bladed fan, which originally had four blades. One blade had apparently broken off while driving, so they do break. I did not look at it closely enough to see if there was a pre-existing crack, as evidenced by corrosion in the break area.
I may not have been critical enough of the cast aluminum fans.
The reason I'm using them is because I don't know of anything better right now that has the authentic look of the original.
As I mentioned, cast aluminum has very poor fatigue properties. This is mainly because the casting can contain small voids. If the voids occur in a critical stress area - it's bad news. The main advantage of the cast aluminum fans as I see it is that they are new. They haven't seen any fatigue cycles yet. I left the blade to hub region on mine unpainted so I can check regularly for cracks. Cracks will not necessarily occur in the same place as they did on the original fans, but if cracks occur they will still probably be in the blade to hub area somewhere. Bottom line - I think these cast aluminum fans are risky, just not as risky as the original ones with who knows how many thousands of fatigue cycles.
Bill
Keith True
Well-known member
What I would like to find are some of the old aftermarket 3 blade fans.I have seen a few,I just think they are neat.I like oddball things,like the Bool or Fool hubcaps,enclosed water pumps,(one long bearing,no packing nut)clear dist.caps,galvanized splash shields,Fulton upstarts,etc.I see people have done in-depth studies on how much air a 2 blade fan moves versus a 4.It's kind of meaningless to me,when the aluminum fan let go on my pickup I trimmed it up and ran it a couple more summers before I got up the ambition to pull the radiator and change it.I did watch my driving habits though,I never got into much real traffic,and when I did it just got a little warm.No parades either.
hardtimes
Well-known member
Although I am aware of the 'rivet' type four blade fan on a model a, I have personally not used one. The four blade fans that I'm using and familiar with are....TWO BLADES that are bolted together onto the fan hub with FOUR screws/lock washers/nuts. I'm now wondering whether any of this type four blade have ever failed.....anyone
Paul in Colorado
Senior Member
This was the type which I described in my post aboveAlthough I am aware of the 'rivet' type four blade fan on a model a, I have personally not used one. The four blade fans that I'm using and familiar with are....TWO BLADES that are bolted together onto the fan hub with FOUR screws/lock washers/nuts. I'm now wondering whether any of this type four blade have ever failed.....anyone
hardtimes
Well-known member
Paul,
Thanks for that clarification!
Hmm, what would it take to make a 'forged' aluminum fan...as opposed to the currently available 'cast' one. Can anyone explain if forged can be done?
jr-41ford
Senior Member
After reading this thread during the weekend, I ordered a new 2-blade aluminum fan. I will not be starting the '28 Roadster up until the new fan is installed.
Over the last few months, my fan belt has been making noises and I would spray it with belt dressing. I figure one or both pulleys are worn, since the belt is new.
When I change it out this weekend, will check the crank pulley too. If it needs to be replaced, it will be done.
How durable are the pulleys, upper and lower?
Over the last few months, my fan belt has been making noises and I would spray it with belt dressing. I figure one or both pulleys are worn, since the belt is new.
When I change it out this weekend, will check the crank pulley too. If it needs to be replaced, it will be done.
How durable are the pulleys, upper and lower?
hardtimes
Well-known member
If you check any of the vendor catalogues, you will find that the 'plastic' fan is the most expensive..by far. And , they express that this fan is most efficient at cooling.
I ran one on my '29 sport coupe touring car and never was concerned of losing a blade. However, I do believe that a while back, that I read where one guy commented that he had a plastic one come undone.
Tom Wesenberg
Well-known member
I never see a problem with the original pulley wearing out, but the crank pulley is very vulnerable to breakage just because it is thin cast and in a very vulnerable spot during engine R&R and during storage.
Are you using an alternator? The small pulley can be out of alignment and cause problems. The alternator belt also needs to be tighter than the generator belt due to the smaller surface area of the small diameter pully.
Plastic fans look the most out of place, cost the most, and use the most horsepower.
Last edited:
jr-41ford
Senior Member
Tom, I do run an alternator, will look at the belt alignment closer. Real easy fix to align it properly. Will also check the crank pulley as I said early.
I had an aluminum pulley on the alternator of my '41Ford flattie. It squealed, slipped, etc, drove me up a wall. Checked alignment, changed belts, nothing worked.
Compared it with a stock pulley and the aluminum pulley had worn about 3/8" out of the groove. Ordered a new polished steel pulley from Speedway, haven't had a problem since.
Since the new aluminum fan is with a pulley, will make sure my alignments are true.
Thank you Tom.....
I had an aluminum pulley on the alternator of my '41Ford flattie. It squealed, slipped, etc, drove me up a wall. Checked alignment, changed belts, nothing worked.
Compared it with a stock pulley and the aluminum pulley had worn about 3/8" out of the groove. Ordered a new polished steel pulley from Speedway, haven't had a problem since.
Since the new aluminum fan is with a pulley, will make sure my alignments are true.
Thank you Tom.....