Some interesting axle strength calculations

[acannell]

Some things to discuss...not sure how to interpret this actually.

Heres what I've learned.

Torsional yield strength of circular shafts is based on shear yield strength.

http://www.engineeringtoolbox.com/to...fts-d_947.html (equation 4)

Shear yield strength for all steels is about 0.6 of the yield tensile strength.

http://en.wikipedia.org/wiki/Shear_strength

If we choose 4340, there is an enormous variation in tensile strength depending on how its hardened. But lets say 180,000psi tensile yield strength.

https://books.google.com/books?id=S0...%20psi&f=false

So 0.6 of that means a shear yield strength of 108,000 psi

I measured my 90 Daytona's axle shafts and they appear to be 1.037" diameter.

So using the equation #4 linked above, we get a yield torque of 1972 ft/lbs. Thats the theoretical point a perfect shaft in perfect torsion will start to permanently deform, not break.

The breaking point would be use the ultimate shear strength, which is about 0.75 ultimate tensile strength. So if we say UTS is 230000psi (should be close), that gives an USS of 172500, and a breaking torque of 3150 ft/lbs.

In 1st gear an A568 has an overall ratio of about 11.5.

http://www.thedodgegarage.com/trans_guide.html

So if our theoretical launch is taking place with say at least 200ft lbs of torque at the flywheel, thats about 2300 ft lbs at the wheels. Split in two assuming equal traction. So 1150 ft/lbs per wheel.

If you crazy racer guys are somehow getting near full engine power at launch and its more like 400 ft lbs, thats more like 4600 ft lbs. Split in two around 2300 ft lbs per wheel.

No we throw in the reality of stress risers in our non-ideal splined axles/hub interfaces, and its starting to look like even very good 4340 axles + very high power launches are close enough to start breaking.

If we downgrade to 1040 axles, which I'm guessing the stock ones are made of, the breaking strength goes down to about 2250 ft lbs, or about 40% less than the 4340.

http://www.alloyusa.com/front-axle-shaft-kits.html

2250ft/lbs times 2, and then divided by 11.5 is 391 ft lbs...and that before stress risers. I think there are plenty of TD's putting out that much torque. And thats the breaking torque, not the yield torque.

Even if traction is definitely not 100%, during a controlled wheel-spin sub 2 second 60ft launch, I would imagine that the engine output is high enough to put out that much torque. Some 60ft to horsepower calculators could prove it as far as minimum required torque to launch so much weight 60 ft in so much time. Its got to be way up there for some of the nastier builds.

Has anyone figured out how to make the axles bigger? I'm guessing there is some kind of knuckle upgrade to something with a bigger hub, but what about the splined ends in the diff?

[ShelGame]

The shafts will always break at the root of the spline just before it goes into the CV joints center section. That's where the stress riser is from the spline, and from the joint with the CV. They should never break in the middle of the shaft.

Your analysis above all look correct to me.

It's one of the reasons many racing axle shafts are made with 300M. It's a modified 4340 with an UTS of 300kpsi.

[acannell]

The shafts will always break at the root of the spline just before it goes into the CV joints center section. That's where the stress riser is from the spline, and from the joint with the CV. They should never break in the middle of the shaft.

Your analysis above all look correct to me.

It's one of the reasons many racing axle shafts are made with 300M. It's a modified 4340 with an UTS of 300kpsi.

what surprises me is how close the ballpark breaking strengths are (not yield but actual ultimate breaking strength) and the torques our modified TD's might actually put out during a launch..

and thats before the stress risers!

and then throw in more torque going to one wheel...

and perhaps some sort of shock loading effects (not sure how to even ballpark quantify those)

do automatics multiply the torque at launch even more than the manuals?

I suppose all this makes some sense. 1.037" diameter is pretty small.

[ShelGame]

w
do automatics multiply the torque at launch even more than the manuals?

Yes, they do! The torque multiplication is proportional to the speed difference, minus the efficiency (just like a gear ratio). Theoretically, if you are brake torquing, the torque is infinite! In reality, it just dumps a ton of heat into the tranny fluid while brake torquing as the converter slips. But, the torque output can be 3-4 times the gear ratio in 1st gear as the car initially starts to move.

You also need to take into account what the tires can hold. If the tires will not hold the torque against the pavement, they will spin and the torque seen at the axle shaft will be much lower.

[acannell]

Yes, they do! The torque multiplication is proportional to the speed difference, minus the efficiency (just like a gear ratio). Theoretically, if you are brake torquing, the torque is infinite! In reality, it just dumps a ton of heat into the tranny fluid while brake torquing as the converter slips. But, the torque output can be 3-4 times the gear ratio in 1st gear as the car initially starts to move.

Wow that puts things in definite breaking territory then even with 4340 shafts!

You also need to take into account what the tires can hold. If the tires will not hold the torque against the pavement, they will spin and the torque seen at the axle shaft will be much lower.

What do you think engine output is during a full slicks burnout? I would think a high power, sub 2 second launch would be at least as much power as that plus quite a bit more. In other words, even a zero traction situation can present a high load to the engine and therefore transmit torque through the axles. Or are burnouts done at much lower power levels?

So do automatics break axles more than manuals?

[mopar-tech]

So do automatics break axles more than manuals?

FYI- The Reliant comes out of the hole at 27 pounds of boost, 1.6 short times and is running the same 87 Shelby Z Daytona axles, never broke one.

Dumb luck perhaps?

[ShelGame]

Wow that puts things in definite breaking territory then even with 4340 shafts!



What do you think engine output is during a full slicks burnout? I would think a high power, sub 2 second launch would be at least as much power as that plus quite a bit more. In other words, even a zero traction situation can present a high load to the engine and therefore transmit torque through the axles. Or are burnouts done at much lower power levels?

So do automatics break axles more than manuals?

Technically, zero traction (literally) would mean there was NO load on the axle. The axle won't see any torque if there's nothing holding it on the other end (IE, the tire).

Personally I get 1.8x 60' times with a 'low' power non-IC 2.5. I run 24.5" slicks, and I almost always get a little slip out of them. They never hold completely.

[Warren Stramer]

I dug out three more used and abused axles that where not yet broken, but show the initial signs of spline twist. The failure mode is the same on all of them. Cracks starting where the spline valley transitions to the machined relief.

I also magnified three unmolested axles and they are fine, no cracks. So, I will grind and polish a relief back to where the splines engage the diff side gear splines and eliminate all those knarly stress risers. I will also make some small narrow paint lines to monitor twist.

Also going to try these axle bearing/seal supports, unless someone here has tried them and had problems with them.(If so, please speak up) I think anything to stabilize the axle stub should be beneficial.

[acannell]

I dug out three more used and abused axles that where not yet broken, but show the initial signs of spline twist. The failure mode is the same on all of them. Cracks starting where the spline valley transitions to the machined relief.

I also magnified three unmolested axles and they are fine, no cracks. So, I will grind and polish a relief back to where the splines engage the diff side gear splines and eliminate all those knarly stress risers. I will also make some small narrow paint lines to monitor twist.

Also going to try these axle bearing/seal supports, unless someone here has tried them and had problems with them.(If so, please speak up) I think anything to stabilize the axle stub should be beneficial.

I think I get it..they replace the seals but also act as needle bearings on the stub shaft? That seems like a pretty good idea. I wonder if any OEMS do that?

[135sohc]

I have never personally tried them but from the allpar days those seal/stabilizer things do actually work. They are really made as a bandaid fix for the OEM open differentials where the side gears are shimmed in the carrier but either from loose factory setup or high mileage/one tire fires the side gears are a sloppy fit and that allows the axle to whip around and wear the seal out.

[Ondonti]

Also going to try these axle bearing/seal supports, unless someone here has tried them and had problems with them.(If so, please speak up) I think anything to stabilize the axle stub should be beneficial.

Yeah ummm

The million times I have bugged people to use them, I got grief about them gawling up the axles blah blah blah they fail/seize blah blah blah too expensive blah blah you are an idiot for wasting money.

Not sure I respect those opinions. I certainly don't care if my broken axle has some gawling. I also don't care if it costs 30-50 bucks to protect an axle.

[Reaper1]

I dug out three more used and abused axles that where not yet broken, but show the initial signs of spline twist. The failure mode is the same on all of them. Cracks starting where the spline valley transitions to the machined relief.

I also magnified three unmolested axles and they are fine, no cracks. So, I will grind and polish a relief back to where the splines engage the diff side gear splines and eliminate all those knarly stress risers. I will also make some small narrow paint lines to monitor twist.

Also going to try these axle bearing/seal supports, unless someone here has tried them and had problems with them.(If so, please speak up) I think anything to stabilize the axle stub should be beneficial.

Awesome! I was thinking of paint to use as a twist indicator.

As for those seal/bearings. I also would like to see somebody try them, however when they were brought up a while back the response was exactly like this:

Yeah ummm

The million times I have bugged people to use them, I got grief about them gawling up the axles blah blah blah they fail/seize blah blah blah too expensive blah blah you are an idiot for wasting money.

Not sure I respect those opinions. I certainly don't care if my broken axle has some gawling. I also don't care if it costs 30-50 bucks to protect an axle.

However, I can't see how they would do that damage. There is splash lube out there already and it can't be worse than the bushing that's out there now that has slop in it. The only thing I can think of is if the bearing or the shaft has a harness issue or some other flaw where a roller locks up and starts messing things up. Also, RWD applications have been using a similar design for...oh...probably as long as the automobile has been around. I would love to give it a shot.

[lengel]

I dug out three more used and abused axles that where not yet broken, but show the initial signs of spline twist. The failure mode is the same on all of them. Cracks starting where the spline valley transitions to the machined relief.

I also magnified three unmolested axles and they are fine, no cracks. So, I will grind and polish a relief back to where the splines engage the diff side gear splines and eliminate all those knarly stress risers. I will also make some small narrow paint lines to monitor twist.

Also going to try these axle bearing/seal supports, unless someone here has tried them and had problems with them.(If so, please speak up) I think anything to stabilize the axle stub should be beneficial.

Wow! Where did you get those from? I was looking at this same issue a few nights ago and decided I'm going make a new diff bearing retainer plate that would use a radial ball bearing. Any way I think it would only help things.

[krut]

Wow! Where did you get those from? I was looking at this same issue a few nights ago and decided I'm going make a new diff bearing retainer plate that would use a radial ball bearing. Any way I think it would only help things.

32125LRB is part number. I just ordered my set from parts authority. They ran about $36 each.

[acannell]

heres a science graph, as you can see, yield strength vs. shaft OD is extremely non-linear. An increase of OD by 33% increases yield strength by 133%.

[acannell]

Re making axles...this article from fourwheeler (not exactly the pinnacle of truth..some of their equations are flat wrong) shows an axle shaft being machined on what appears to be a typical CNC mill with a 4th axis and an involute cutter.

http://www.fourwheeler.com/how-to/tr...nder-pressure/



Those sorts of cutters are described in detail here:

http://www.mmsonline.com/articles/speeding-up-splines



That would lower the axle prototype cost quite a bit...looks like the star of the show is the Coromill 172 insert holder.

http://www.sandvik.coromant.com/en-u...s/default.aspx

I cant seem to find online pricing, which is unusual for machine shop stuff..and not a good sign. But if its like other insert holders, I would expect it to set one back several hundred dollars for the holder and a set of inserts.

You could actually do the 4th axis with an indexer or manual table, since it doesnt rotate during the cut.

[bgbmxer]

I see more people break axles from wheel hop then those who are running slicks without wheel hop involved

[black86glhs]

I see more people break axles from wheel hop then those who are running slicks without wheel hop involved

What he said. Wheel hop kills axle shafts.

[Reaper1]

Remember that while the wheels are spinning the torque seen by the axles is actually low as there is less resistance to the tires spinning once they are spinning (dynamic friction).

The Reliant was VERY light. This reduces the resistance against the movement of the vehicle at launch and reduces the instantaneous torque load. That's what is going to break your axle. That's why shock loads such as wheel hop and dumping clutches breaks axles and auto's typically don't...they apply the torque in a softer manner.

Think about this...if you have a big 1/2" drive impact and you adapt it down to a 1/4" drive extension and you want to break a bolt loose...if the bolt actually breaks loose, then you probably won't break the 1/4" extension, however, if the bolt stays put...I hope you have some PPE on because that extension is going to shatter like glass!

Now...freewheel the impact and have the other side of the extension attached to a torque absorber (some sort of breaking device). Apply a load slowly and you will probably twist the extension like a candy cane before it breaks (depending on the toughness of the metal). Lock it up and again you will shatter it.

Eliminate shock loading and a good majority of the breaking issues will probably go away.

[mopar-tech]

The Reliant was VERY light.

Oh? 2350 isn't that light, l-body territory.

Getting it below 2000 lbs was the goal, not sure if its possible though.