Need help analysing a failure 2

[MJMinear]

Hi, I'm new here. I just found this site and it looks like a great place to kick some ideas around.

I have a project I've been working on for some time now and I thought I had the solution worked out but ther was a failure recently and I'm having a hard time figuring out why. Hopefully, if I can figure out why it failed, I'll also be able to figure out how to modify it to prevent future failures.

Ok, heres the situation. I have a part that I designed for my race car. It's a wheel hub. The OE parts were not up to the task of road racing with big sticky tires. The part I designed is forged out of 4340 steel, machined, through hardened to ~RC52 and then the critical dimensions are finish ground. The part has the wheel flange and shaft forged from a single piece of material. By my calculations, the maximum stress this part will see is 190,000 psi. The stress peaks at the radius between the shaft and the wheel flange as you might expect. According to the material data sheet I reviewed at my fabricators shop, the material used at the heat treated condition is supposed to be 250,000 psi. The failure appears to be a sudden break. Although there is some discoloration and what appears to be bits of red rust and patches of black oxidation in the break. This might mean that the part cracked earlier and eventually completely failed. But the hammered "ring" around the perimeter of the break that I would expect to see if that were the case is not there.

Questions:

1. Any ideas why this part broke?

2. Would induction hardening instead of through hardening be tougher?

3. Is the material just not strong enough, either due to an inferior piece of 4340, or is 4340 the wrong material?

Thanks in advance for any insight you may be able to provide. I'm sure there is more info you might need to help, so feel free to ask for any info I left out.

 

[MikeHalloran]

Uh, 190ksi sounds like a pretty high design stress. Can you put a bigger radius there?

RC52 is harder than an axe or a chisel. You removed all the ductility with through hardening.

A few years ago, tiny wheels with large offsets became a fashion on automobiles. The fashion went away quickly because of wheel bearing life measured in days.
Are you running large wheel offsets with your wide sticky tires? That stresses hub flanges in addition to bearings.

A detail drawing of the hub, and a section through the hub/wheel/tire/suspension assembly, might help our actual experts figure out what's going on.


Mike Halloran
Pembroke Pines, FL, USA

 

[Ron]

Your design stress relative the ultimate strength of the material is high. You are at 75% of ultimate, so some yielding or minor cracking are possible with initial cyclic loading. Once a small crack occurs, you have too little safety margin to allow fatigue cycling, thus you get a quick failure.

Since your part is subject to repetitive loading, check the endurance limit of the steel and design for that.

 

[MJMinear]

Where might I find the endurance limit? Also, 250,000 psi is supposed to be the yeild strength, not ultimate. Although I don't know what the ultimate strength is, it should be higher than the yeild, right?

The radius at the base of the shaft is 0.250" on a 1.5" shaft.

Wheel offset and suspension geometry were all factored when calculating the stress. Stress was calculated at 1.6G. The most I've ever measured on my car was 1.45 G with a single spike to 1.5 G.

The thing that has me puzzled is that the part that I'm breaking does not fail in the stock part. In the stock part, the bearings fail. Additionally, my part is a larger diameter than stock.

 

[MikeHalloran]

Ah. We appear to be talking about a spindle, not a hub.

You probably can't afford to duplicate the tightly controlled induction hardening that OEMs use in that area.

A drawing would help to prevent further misunderstandings.



Mike Halloran
Pembroke Pines, FL, USA

 

[MJMinear]

Well, in this case they are the same thing. Yes a picture would help. Unfortunately, I don't have have any handy.

Picture this:

The flange and shaft look like a very short axle shaft from a solid rear axle. The bearing housing is bolted to the suspension upright.

What I did was machine out the stock housing to accept tapered roller bearings races(stock uses balls and has integral races in the housing and shaft).

In a typical spindle set up, the shaft is fixed and the housing rotates. In this case, the housing is fixed and the shaft rotates.

The pieces I'm having made cost me over $500 each now, would precise induction hardening increase the cost that much more? More importantly, if it would improve the durability, it might be worth it.

 

[MJMinear]

Here's a link to some photos of the part.

 

[MJMinear]

Here's a link to some photos of the part.

http://www.mjmracing.shutterfly.com/

 

[MikeHalloran]

Okay, I think it's better to call that part a stub axle.

The radius at the thrust face of the larger bearing appears to be not more than .05". Is that where the part broke?

How about some pictures of the broken part?



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi MJMinear

I found this site and reading your posts I would say that your 250,000psi is the UTS in which case your endurance limit according to this site is about 85000 psi ie 30% of UTS.

http://www.epi-eng.com/mechanical_engineering_basics/fatigue_in_metals.htm

desertfox

 

[MikeHalloran]

Wait, there's more.
The only .25" radius on the parts in the pictures is at the root of the wheel pilot.
A radius nearly as large as that _should_ be at the thrust face of the larger tapered roller bearing. ... but it's not there.

The obvious next question is whether your stress model even looks like the part.



Mike Halloran
Pembroke Pines, FL, USA

 

[MJMinear]

I think what you don't see and what I forgot to tell you is that the thrust surface for the outer bearing is a separate piece. The .25" radius is underneath it. The inside of that ring is machined to match the radius.

It is possible that the stress model is off, but I think it's pretty good.

I'll see if I can dig up a couple pics of the broken part and post them.

 

[MJMinear]

OK, here's a few photos of the broken part. They are big files so I hope this works.

 

[MJMinear]

Pic number 2

 

[MJMinear]

Pic 3

 

[MikeHalloran]

Okay, now draw a section through the assembly.
You need to include the bearing, the stub axle, and spacer.
The spacer, adjacent its internal diameter, has one radiused edge and one sharp edge.
The sharp edge of the spacer abuts a radiused edge inside the bearing.
The sharp edge of the spacer, where it contacts the shaft, constitutes, IMHO, a stress raiser.
Not by coincidence, that's where the shaft of the stub axle broke.



Mike Halloran
Pembroke Pines, FL, USA

 

[swall]

To me, it looks like you have a classic bending fatigue failure. I would take the piece to a metallurgical lab to verify this. The stresses on the part can be much higher than what the models show, due to residual stresses from manufacturing processes, such as grinding.

 

[desertfox]

Hi MJMinear

Having seen the values for the endurance limit for the steel you are using on the previous link I posted and combining it with this site which gives typical examples of different shaft failures I would agree with swall its a classic rotational bending fatigue failure.

http://www.asminternational.org/pdf/spotlights/jfap0502p011.pdf

see page 14 of the above link

desertfox

 

[MJMinear]

Mike,

The fracture is actually 100% on the raduis all the way around. Meaning it's completely under the spacer. No part of the fracture is at or outside the edge of the spacer.

Looking at the articles linked above, this appears to best fit the low stress riser, high bending stress example.

 

[tr1ntx]

You were right in your suspicion, that thing had cracked all around and the gray patch in the middle was all that was holding it when it let go. Classic rotational bending (cyclic) stress failure. I agree with Mike Halloran, the through hardening removed the ductility. Also, whoever said you don't have enough of a safety factor is right too.

Without spending any mental effort nor time on it, I will posit that this might be one of those cases where you're better off with un-heat-treated material. One time, the younger, fresher engineer who had preceded me at a job had made the tailshafts on some 78" dia screw classifiers out of 4340HT. The same thing happened, they broke. I figured out the issue and changed it, but they were no where near the 190ksi you're talking about, so that might not be applicable. Perhaps you will have to revisit the design geometry.