OT-New John Deere tractor with broken axle - Metalurgy Question

[FredC]

A mechanic friend has been working on this new 60 hour tractor. The tractor had only pulled a shredder till the point where the rear axle broke. The tractor got stuck in wet sand and that is where the axle broke off with the wheel laying next to the tractor. The John Deere mechanics said the owner had tied something to the wheels and caused the axle to break. Owner says no. I looked at the tractor frame and fenders and saw no evidence that anything had wedged in there to break the axle.
My friend the mechanic asked me to look at the axle and I found that it was twisted, the metal grain looks like what I would expect. The splines and bearing races, and sealing surfaces are very hard and a new file does not cut them. They have a little bit finer grain pattern. Under the case the steel has a courser grain pattern and can be cut with a file. In between areas do not have the hard case, not sure how that was done, but it makes since to me that they would not need to be surface hard.
Three of the 4 photos I made are attached. Is there something obvious in the heat treatment that would indicate bad heat treat? My mechanic friend says that nothing you could do to a 65 horse - 4 wheel drive tractor should break an axle. I can see his point of view on this as the same tractor could have a 100 horse engine, so how can a 65 break it.
I see the dealer's point of view that the axle is definitely twisted near the splines and is heat treated as expected. I can bring the axle here again if another photo is needed. As far as I know only John Deere mechanics and no factory rep has seen this tractor.
Also the inboard tapered roller bearing is broken with split rollers and races broken, I kind of think that was done afterwards as the axle fell out. If the bearing had come unglued first I think it would have just spun on the axle and there are no marks indicating that

 

[howieranger]

The fracture looks similar to axles that failed on Porches that we serviced from accelerating over RR tracks. I wonder if the soft sand and one wheel grabbing caused the failure with too deep of hardening.

 

[FredC]

Last photo here. The axle is about 2 inches in diameter at the splines larger towards the wheel hub. The case looks to be about 1/16 deep. It looks deeper from the outside of the splines but I think that is to be expected.

 

[Hodge]

That axle should not have broken, it really should have been one of the toughest parts on the tractor. I have Deere's (two cyl) that have been put through all sorts of hell over many decades of use and abuse. I noticed that axle wasn't solid, good indicator of cutting corners, pinching pennies. I suspect it started with a stress riser, possibly splines, threads, or some other feature not visable.

 

[ewlsey]

It's a cheap ass design. The right way to make the axle is to neck the axle down between the flange and the splines. The end of the splines is a stress concentration. By necking it down, you avoid the stress riser. Of course, you have to increase some other dimension to get the same strength.

However, most companies don't bother designing them correctly, because it costs too much. You have to machine the necked portion, and most likely forge the whole rough shape instead of just using some bar stock.

I'm not sure why tractors can't figure out how to use full floating axles. Trucks have been doing it for decades.

 

[sable]

I am pretty sure that the hole down the centre should increase strength as long as it is truly central.

A neighbour had a similar break on an MF ,it didn,t go all at once and finally let go when he was tanking along at about 25 miles an hour with a loaded trailer ,tractor was written off.

I would think it would be a forging if not cut from bar ,rather than a casting.

 

[magneticanomaly]

The "steps" visible on the fracture surface are called "beach marks", and indicate a progressive or fatigue fracture. At some point, the remaining metal area was not sufficient to hold the load, and failed all at once. Grain size looks about right. The hardened zones are induction-hardened.

I suppose your question is aimed at determining whether JD owes you a new axle.

Fatigue fractures begin at some defect. If that defect was a non-metallic inclusion, scratch or gouge made at the factory, or a definite design defect, maybe they do.

If the point of initiation was a rust pit, or a chip or crack from some sudden accidental overload, probably they don't. Careful examination of the fracture surfaces under some magnification may tell. I think I can see where the point of initiation is, in the pic, but not clearly enough to guess what it is.

I think I see a spiral crack in the splines, a little distance from the fracture. That MIGHT indicate a history of periodic severe overloads, perhaps from a teenage boy trying to make the tractor jump when daddy not at home, perhaps from roll-starting at high speed in too low a gear, or even from foreign object getting jammed in gear mesh. One of the cracks grew, in fatigue, to final failure.

You could pay a consulting forensic metallurgist and a lawyer a few thousand bucks to take JD to court or just cough up a few hundred for a new axle.

If I were JD, and an intelligent layman sent me pics with a clear metallurgical defect at the origin of a fatigue fracture, I'd fix the tractor for free for the good PR.........My first name may b e John, but my last is not Deere.

JM2c

 

[dkmc]

I'm not sure why tractors can't figure out how to use full floating axles. Trucks have been doing it for decades.

If tractors get to the point where tractors figure out anything for themselves, the use of computers in farm equipment will have gone too far.
Trucks, better keep an eye on them too...
Next thing, they'll be wanting to vote.

 

[Ringleboy26]

That failure looks correct for a overload. All the axles I have broken in my dumptrucks look like that. The 45 degree angle tells you that it sheared from being over torqued.

If he was in sand he prolly let the tractor hop a few times which leads to busted axle shafts. The axles in my macks are porpoisly designed to fail before it damages the rearend. A 300 dollars axle is easier to stomach than a 2800 dollar rearend.

 

[The Extractor]

Looks just like the shaft failures Ive had on my wheelin rigs when I beat the snot out of them.

 

[David J.]

Other possibility is he engaged the diff. lock with one wheel already spinning.
Depending on the tractor model??, this is not that hard to do.
David

 

[Joe Rogers]

I could see a chance that the spinning wheel got some serious speed and then finding some traction with a grab to 0 rpm. The torque of the drive trains rpm change could snap an axle just like a "neutral drop" used to on the street cars.
Joe

 

[Jim Williams]

Looks like the typical spiral fracture demonstrated in some early mechanical engineering courses. The instructor took a stick of chalk and twisted it until it broke. The next week I was rebuilding an outboard motor foot that had been run on the ramp under power. The broken shaft demonstrated the same spiral fracture. The instructor was delighted when I brought it to class to show. Looks like an overload of some sort to me.

Jim

 

[kenh]

Yup, diff lock engagement could cause the breakage.

 

[Garwood]

If I were John deere I would pay for the parts and labor and ask him to try to break it again.

 

[FredC]

By the way the tapped hole in the spline side is about 1/2 13, I did not check it though.
Differential locked with the other wheel spinning, that is an interesting thought.
I remember reading about some drag racer who would paint a stripe down new axles. When the stripe made a full revolution he would throw them away. Tractor axles are to short and stout for that.
Again this is a new machine with 60 hours and essentially no real work done till this stuck in the sand failure. I had thought about the possibility of a shipping accident before the owner took possession. I will ask if it has a user engaged diff lock.

 

[HuFlungDung]

I've seen quite a few drivelines off of semi tractors where the spline is destroyed in a similar fashion. They say that it occurs from deploying the clutch when the engine is really lugging down in a high torque situation. Best prevention is to throttle down and let the engine quit, and it will absorb the shock that will otherwise occur when the driveline untwists.
In a farm tractor, the front wheels may begin to lift off the ground if the tire is not rotating, and naturally, you'd release the clutch in a panic so it stops rearing up And that is when the pent up torque releases and unwinds the axle.

Those low gears can produce insane torque multiplication, imagine what you can do with a few thousand pounds on a torque wrench 10 or 15 feet long.

 

[ewlsey]

Hu is right. I've always heard the absolute worst thing you can do for your driveline is let the wheels "hop" when they don't have traction. It's a constant loading and immediate unloading of the drive line. Tractors don't do it so bad since they have deep groove tires and really low gear ratios. But semis can and will.

If the rear axles start to hop, you need a tow.

 

[sticks]

This ^ and this ^^ since I type so slow. But here goes ...

I've twisted off more rock shafts (NH) than axles, but by hitting something solid while in motion. More often than not there was an existing crack, and it was just finished off by the latest impact. (Same multi-event fracture shown on a JD 2wd spindle once.)

Diff lock pedal was/is on the left side of the op platform of my 1520 and 5210 JDs. Best to be out of gear or clutch-in when activated. Depends a little on how far each wheel was 'buried' however deep once engaged, too. Consider the gear ratio (engine to wheels) and torque multiplication therein. 65hp means a lot of torque applied to an axle turning 28" or larger rims by planetary reduction and in a lower gear. ("something's gotta give" ..)

Getting stuck can have one mashing pedals in frustration. Hard to guess what'd be rough enough if you/we weren't there when it happened. 6k+ lb tractor won't just climb out of a hole when axle deep. Still surprising if MFWD was engaged, but was it at the time?

 

[abarnsley]

Time to tell guy he just can't run a tractor..

He broke it... fess up.

Running in sand, sucks power through a normal drivetrain.

Add wet sand, TALL deep lug tires, and extreme gear reduction... A hop or 2 SNAP..

Learning experience..

If he worked for me.. 1 axle would be a lesson.. Second axle... hit the road jack.

Axles as stated above, are supposed to fail before rest of drivetrain.. Cheapish part.

Strictly operator error, trying to blame a mfg for his mistake/carelessness.

Full floating axle on a tractor??? Why?? To add more parts to fail/service??? No flange to seal, fewer bolts.

Big truck axles (full floating) snap the same way, when abused the same way...