14 inch propeller shaft, 12 mm diameter

[chrome]

Mori,

It sounds like you have classic stress corrosion cracking problem of your alloy steel parts. It really starts with a pit (small hole with a sharp tip), where cyclic stressed causes the crack to grow by means of an accelerated corrosion process due to the extremely high acidity at the bottom of the pit. If you look at the fracture surface closely, you will see a fan shape fracture starting from a point, that's your pit that started it.

Instead of upgrading steel, why not try by stopping pit initiation in the first place? Sand blast a new OEM shaft, coat it with a high quality primer, then a high quality powder coating, assuming the OEM shaft itself comes uncoated from factory.

 

[moribundman]

Thats the prop-shaft that broke last year. I have now the same OEM shaft installed, but I don't trust it 100%. I want to be prepared for the day it breaks.

The OEM shaft is coated or painted (very thinly) black. As you can see, the shaft sheared off right at its thinnest perimeter, which is where the keyed area ends and where the collar of the bevel gear also terminates. You can also see where corrosion crept in.

When I buy a new prop-shaft, I do not know how old it is, or if there is corrosion present. I lack the equipment to test for microscopic cracks and existing damage. The prop-shaft and the bevel gears are fairly cheaply made (in Taiwan). That's why I was eying an upgrade versus improving upon the existing part. I am however open to suggestions. The prop-shaft isn't expensive. The one shown was replaced at no charge under warranty.

 

[chrome]

From the photos, it looks like you have circumferential fatigue cracking. If you see the rusty looking external ring extending to about halfway into the radius, that's the fatigue crack. Once the crack reaches a critical point, the remainder of the shaft sheared through plastic overload, ending in the cone & dimple you see.

The crack appeared to have started from the machining point at the base of the splines. This is where a sharp cut is made on a lathe that puts the bevel on the inner end of the splines.

To circumvent the propagation of the cracking, I suggest "blunting" the sharp cut taken at the inner end of the splines. Put it in a lathe, and use a rounded tool end (~1mm radius) and take a small cut to make a semi-circular groove all the way around. This will remove the stress riser caused by the sharp cut.

 

[moribundman]

Sounds like a good suggestion, thanks. I'll give this a go.


Quote:
the shaft sheared through plastic overload


It was quite noticeable. I was accelerating while going uphill, standing on the pedals, pushing hard. The shaft went all soft and failed relatively gently. I knew that same moment that I had sheared the shaft off.

I am not aware of other people with shaft drive bikes having prop-shafts failing. However, I do use my bike on some really steep hills and also off-road -- for which it really wasn't designed. I also wear out the gears two to three times faster than could be expected. Component life is estimated to be at least 6,000 miles, which seems overly optimistic to me.

 

[IndyIan]

Is the bike frame flexing enough to mess up the gear clearances causing them to wear faster? That could be bending the shaft as rotates as well? Maybe the whole bike is not up to the task for off roading?
You could have a look at a motorcycle wrecker and see if a shaft drive from there might be made to fit. It should handle the torque I'd imagine.
Ian

 

[mechtech2]

Like Chrome stated, smooth out the area at the spline base by any number of common sense means.
Kinda like the M1 Garand fix [by introducing a radius to stop breakage of the actuating arm that would always crack at the stress point].
This is the real problem, not the material.
And spray Rustoleum is a great coating afterwards for the main part of the shaft. Cheap and tough.

 

[XS650]

That's a spline, not a key. A very different coupling and a good one if done right, unlike a key which bites at best.

Chrome gave you good advice. Getting onesy twosy shafts made would be big $$ because of the splines

 

[moribundman]

Originally Posted By: IndyIan
Is the bike frame flexing enough to mess up the gear clearances causing them to wear faster? That could be bending the shaft as rotates as well?


The shaft is not exposed to much frame flex. The clearances within the meshing bevel gears are very large, and I doubt that there is enough flex in the setup to cause a problem. The one of each meshing bevel gears has also play inbuilt.

Quote:
Maybe the whole bike is not up to the task for off roading?

A I already implied that when I said this bike was not designed for off-roading.


Quote:
You could have a look at a motorcycle wrecker and see if a shaft drive from there might be made to fit. It should handle the torque I'd imagine.
Ian


Too far, too much time required, probably way too large, not worth custom fitting.

 

[moribundman]

Originally Posted By: XS650
That's a spline, not a key. A very different coupling and a good one if done right, unlike a key which bites at best.


A spline is not keyed? I'd say every splined connection is keyed, but not every keyed connection is splined. I could be wrong.

 

[XS650]

A spline is not normally referred to as a key. A keyed joint is something else all together. "Key" usually implies a separate piece that rides in grooves in the mating parts. Usually one key per joint.

 

[moribundman]

Well, I didn't call it a key (unless I'm overlooking it). I said a keyed bevel gear was attached to the prop-shaft. I find the term "key spline" in technical writings.

 

[punisher]

I was under the impression this was some evil 100+hp shaft drive superbike to be twisting a shaft like that. I don't see how there is any way that shaft twisted unless there was some manufacturing error, or it is made from some incredibly bad material.

When I saw "I was accelerating while going uphill, standing on the pedals, pushing hard. The shaft went all soft and failed relatively gently." I scratched my head a bit.

 

[chrome]

The shaft was sized with ample safety margins, but due to the fatigue crack propagating inwards into the material, it essentially decreased the shaft dimension, until the shaft became too weak to transmit the torque that mori was putting as he powered up the hill. Instead of just snapping (sudden fracture), the shaft essentially deformed plastically (twisted), then broke, leaving the tell-tale cone and dimple. When it deformed plastically, that was when mori felt it went "soft".

 

[moribundman]

Originally Posted By: chrome
when mori felt it went "soft".


No comment!

 

[Oldmoparguy1]

On the off chance that you will actually build a new shaft, you might look at using 300 series stainless steel. Extremely tough and doesn't require heat treatment.

Otherwise 4140 is hard to beat.

 

[tom slick]

Do you have access to machines for spline making?

 

[moribundman]

I don't have access to that. I'll actually have to find a machine shop that will put a radius on the spline base. Minimum charge for any service like that is $100 locally (I don't know any machinists who'd do me a favor). A prop-shaft costs only about $15. I have two spares.

 

[Kestas]

That fracture is not stress corrosion cracking (SCC). SCC is a unique fracture mechanism. This is simple fatigue with multiple origins around the circumference. Corrosion plays a very minor role in this fracture.

I agree that the stress riser should be dealt with. There should be a better way to blend the surfaces where fracture originated. High carbon steel is an excellent choice for shaft material. Improving the alloy choice is a secondary concern in this project.

What is the hardness of the shaft? You should aim for 32 to 35 HRC.

If you really want to go all out metallurgically, you could use an 8620 steel, carburize, heat treat, then shot peen the surface with glass beads to provide beneficial compressive residual stresses on the part surface.

 

[moribundman]

I have no idea how hard the prop-shaft is. I only know it's made from carbon steel. The distributor will not reveal any further specifics.

 

[tom slick]

I was hoping you were going to set up your proxxon with a indexing table.

Look up a driveline shop in the area. they should be able to supply the material and do the work.