Why did my axle break?

[jim galli]

In 54 years of using and abusing Model A's I've never experienced a broken axle. So I'm curious what the combined talent here can teach me. Did it break because the rear roller bearing is worn out and it flexed too much? How would someone measure that?

This is a car that gets used. Myself and 2 high school buddies did 1000 miles in this last October. All our gear plus junk we picked up along the way. Some dirt roads. Maybe 5% of the drive. Happily a photographer friend and I had just loaded our 8X10 camera outfits and were heading out for a shoot and the axle failed in the driveway. Pretty good luck, eh? We loaded everthing in the trunk of the '38 coupe and carried on unphased.

Yes, I did search first. Thank you ahead of time for your ideas.

 

[larrys40]

Jim,
I have had one break as well. I think metal fatigue is probably most likely. I was lucky they line failed in a parking lot albeit 500 miles from home. I changed it out the next morning .
Axle keyway and hub we’re fine . Just it’s time . It broke inside the housing and couldn’t be seen. Many times keyway stress from looseness can contribute near the key slot.

I’ve rebuilt hundreds of axles and che k them for stress cracks. You just don’t know.
Anyway, glad it didn’t happen on the road .
Replace it, check the other side.
All the best
Larry shepard

 

[Tom Endy]

What I have noticed is that a crack will occur at the inboard end of the locking key. I suspect it is due to the axle nut not being torqued down to 100 ft. lbs. This allows the key to rock fore and aft with acceleration and braking. The crack starts out ever so slight. For this reason I inspect the axle shaft key slots very carefully during a differential overhaul.

Eventually the crack will increase until it travels the full circumference of the shaft, and then separate.

Tom Endy

 

[M2M]

What I have noticed is that a crack will occur at the inboard end of the locking key. I suspect it is due to the axle nut not being torqued down to 100 ft. lbs. This allows the key to rock fore and aft with acceleration and braking. The crack starts out ever so slight. For this reason I inspect the axle shaft key slots very carefully during a differential overhaul.

Eventually the crack will increase until it travels the full circumference of the shaft, and then separate.

Tom Endy

Tom, how do you rate the new axles that are available? Are the axles sold by the various suppliers all from the same source? If no, which ones do you recommend?

 

[Tom Endy]

I would not buy one. The gear end is welded on and I have seen some that have come apart. I also had one in a job where the gear end was too large. Whoever put it together must have notice it was locked up. However Henry's 40 horses broke it loose and it wallowed out the inside of the carrier. I had to replace the axle and the carrier.

They are also too expensive at about $400. There are plenty of serviceable originals still around. Try Bert's in Denver.

Tom Endy

 

[M2M]

I would not buy one. The gear end is welded on and I have seen some that have come apart. I also had one in a job where the gear end was too large. Whoever put it together must have notice it was locked up. However Henry's 40 horses broke it loose and it wallowed out the inside of the carrier. I had to replace the axle and the carrier.

They are also too expensive at about $400. There are plenty of serviceable originals still around. Try Bert's in Denver.

Tom Endy

The gear is welded on??? I know that new axles for Model Ts have no gears so you need to remove the gears from old axles and fit them to the new ones (by pressing them on I think). Model A axles couldn't have been made the same way? New Model T axles are about $100 each.

 

[Purdy Swoft]

I've never had a model A axle break .

Here is my thought .

The outer seal that is just past the outer wheel bearing wears a pretty bad groove in the axle .

looking at the pics , it looks like your axle broke in that area

 

[Mike Peters]

The rear axle broke a year ago on our 28 Tudor. Same place as yours. I think the axle nut was tight, but it still broke. Just check your axle nut torque from time to time and hope for the best. Lots of good used ones around, just check carefully for cracks just behind the keyway.

 

[SteveB31]

Tom Endy, the welded on axles have not been made for aa couple years. The new axles made now are all one piece and made by the same company that currently makes axles for Ford, and in the USA. $495 each though.....

Steve @ Bert’s

 

[Synchro909]

Tom Endy, the welded on axles have not been made for aa couple years. The new axles made now are all one piece and made by the same company that currently makes axles for Ford, and in the USA. $495 each though.....

Steve @ Bert’s

Heck, that would be near enough $1,000 by the time it got here. I only want an axle, not the whole darned fatory!

 

[M2M]

Tom Endy, the welded on axles have not been made for aa couple years. The new axles made now are all one piece and made by the same company that currently makes axles for Ford, and in the USA. $495 each though.....
Steve @ Bert’s

Good news; but a pity they don't sell them without the gears (like the Model T ones). Without the gears they would be a lot cheaper.

 

[Bob Bidonde]

Axle failures like yours are not rare occurrences. This type of failure is the result of fatigue and a stress riser. The photo here is a textbook classic of how a fatigue crack initiates at a stress riser and grows to become a static overload.

The stress riser is at the end of the axle's key slot where the taper begins. Note how the crack grew outward across the section of the axle.

 

[M2M]

Axle failures like yours are not rare occurrences. This type of failure is the result of fatigue and a stress riser. The photo here is a textbook classic of how a fatigue crack initiates at a stress riser and grows to become a static overload.

The stress riser is at the end of the axle's key slot where the taper begins. Note how the crack grew outward across the section of the axle.

Would not having rear shocks on a car mean more fatigue and a stress on the axles?

 

[rotorwrench]

Ford had a lot of time to experiment with different steels to come up with a relatively ductile axle that could take thousands of stop and go cycles. These shafts do twist a bit during every cycle so eventually fatigue will get all of them. The design was such that it had a built in place for failure with the key way being cut in there like that. If the key way had radii cut in, it would have last longer but likely not a great amount of cycles longer. It was a "designed in" weak point. I don't think Ford would have ever thought that so many of these cars would still be on the road 90-years later.

 

[jim galli]

Thanks so much for all the answers. I'm lucky. It happened at home in the driveway, and I have a parts rear end in the back yard that still has it's drums on. When I get it all dis-assembled I'll do some good pics of the break along with whatever other evidence I can see then.

Yes, it's remarkable that we're running around in these almost 100 years later with so few problems. A testament to good engineering and materials.

 

[Y-Blockhead]

What I have noticed is that a crack will occur at the inboard end of the locking key. I suspect it is due to the axle nut not being torqued down to 100 ft. lbs. This allows the key to rock fore and aft with acceleration and braking. The crack starts out ever so slight. For this reason I inspect the axle shaft key slots very carefully during a differential overhaul.

Eventually the crack will increase until it travels the full circumference of the shaft, and then separate.

Tom Endy

I wonder how many folks are out checking the torque on their axles this morning...

I actually did check mine ~2 weeks ago while doing my "bolt check" routine.

 

[frank55a]

I had the right axle break on our 29 Tudor coming back from the 86 MARC/MAFCA meet in Wisconsin. We had driven about 3500 miles on that trip and were about 5 miles from home when I went around a sharp corner onto a bridge. It had rained a short time before and the road was dry except for a small puddle about 4 feet in diameter. I was only doing about 10MPH. When the rear right wheel hit the wet spot it spun up and then when it wheel hit the dry pavement it was like trying to bring the wheel to an abrupt stop while the axle was still spinning. It twisted the axle and broke it. Needless to say over the years I have collected a few spare axles just incase. Anyone need an axle contact me.

 

[ursus]

Axle failures like yours are not rare occurrences. This type of failure is the result of fatigue and a stress riser. The photo here is a textbook classic of how a fatigue crack initiates at a stress riser and grows to become a static overload.

The stress riser is at the end of the axle's key slot where the taper begins. Note how the crack grew outward across the section of the axle.

I have some axle shafts that are in nice shape except for an annular groove worn into the shaft where it rides in the seal inside the housing. Is this a potential site for breakage or not a concern (aside from oil leagake)?

 

[ronn]

maybe the HS buddies were doing hole shots, while you were out picking up lunch...........?

 

[Joe K]

Would not having rear shocks on a car mean more fatigue and a stress on the axles?

Hard to say.

Use of shocks may actually INCREASE cyclic loading - best shock restraint is called "critical damping" - a skosh more damping (technical term) results in almost impact, while a skosh less damping can result in wheel hop.

Um - extending this thought, the best (least force on the axle) damping condition MAY be where wheel hop is imminent - and impact is minimized and extended.

Joe K