Some interesting axle strength calculations

[acannell]

Reading some articles on driveshafts and found that Quarter Master rates their Ultimate Torsion driveshaft at 4100lbft. I think it's safe to say that we aren't going to exceed this.

Their driveshaft? How does that help TM's?

[Shadow]

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?

Definitely Not dumb luck, as there are still several of us that have had the same success with the stock axles and have tried over the years to help others understand the importance of powertrain geometry and movement, launch technique ect ect.

As you would surely understand my friend, it was received with the old "I'm doing all of that Right and what you're feeding me is a bunch of BS"! lol

What can I say, you can lead a horse to water..............

[Reaper1]

It gives a reference point. We know that a max effort aftermarket driveshaft rating. Work it backwards and see if you can match it to an engine output. From what I was finding 4130 3.5x0.083 is only recommended after around 1300 engine horsepower. Anything lower than that and either aluminum or 1040 steel is used.

And remember...all of this still has to go through splines and usually a solid rod just like ours.

[acannell]

It gives a reference point. We know that a max effort aftermarket driveshaft rating. Work it backwards and see if you can match it to an engine output. From what I was finding 4130 3.5x0.083 is only recommended after around 1300 engine horsepower. Anything lower than that and either aluminum or 1040 steel is used.

And remember...all of this still has to go through splines and usually a solid rod just like ours.

Okay for 4130, w/ 3.5 OD and 3.334 ID, assuming an ultimate tensile strength of 220kpsi, and therefore an ultimate shear strength of 165kpsi, I come up with a breaking torque of 20447 ft/lbs.

The yield torque in that same case would be more like 14000 ft lbs.

????

I just compared my calculator to some random online college quizzes and it gets the same answers. So I think their driveshaft rating is very conservative and may include other considerations...

EDIT: probably how they are constructed...welds and/or risers that are not as strong in torsion as the base hollow tube

[acannell]

These guys probably have books full of data about what torques axles actually fail at..

http://www.markwilliams.com/axletech.aspx

[Warren Stramer]

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.

Interesting thread. While I've not run the numbers like Acannell, (thanks by the way) There are two circumstances when I have twisted off axles, either when I get into moderate to severe tire shake, or if one of the wheel tracks at the starting line has much more or less grip than the other; With a live front axle/ solid diff. the majority of the torque will load the axle on the side with the most bite and it will almost always twist off at the spline root in the diff.
On a good track with very good traction I will rarely break an axle. Also, I have brake stalled my converter to over 5000RPM many time no axle breakage from that, only the first two conditions, and only twice in 18 years have I broke the right (long) side.

PS. I would pay a lot of money for some truly unbreakable axles.

[acannell]

Interesting thread. While I've not run the numbers like Acannell, (thanks by the way) There are two circumstances when I have twisted off axles, either when I get into moderate to severe tire shake, or if one of the wheel tracks at the starting line has much more or less grip than the other; With a live front axle/ solid diff. the majority of the torque will load the axle on the side with the most bite and it will almost always twist off at the spline root in the diff.
On a good track with very good traction I will rarely break an axle. Also, I have brake stalled my converter to over 5000RPM many time no axle breakage from that, only the first two conditions, and only twice in 18 years have I broke the right (long) side.

PS. I would pay a lot of money for some truly unbreakable axles.

I wonder what it would take to have much bigger splined ends in the diff and the knuckle? I am not that familiar with either area. Is the possibility of custom diff gears and a custom wheel hub all it might take? All else being equal, a 1.5" OD will get you 3 times the torsional strength versus 1.037" OD, for instance. But I'm not sure what the splined cup end root diameters are to start with, all I could measure was the shaft between the joints. Maybe have the splines polished to reduce crack formation sites around the risers?

Whoa...best axle document of the day...actually so rich I am not sure I am ready to read it:

http://www.asminternational.org/docu...a4/HTP00906P15

Note that to interpret the "yield strength" one would have to learn about JEL (Johnson elastic limit), which I have not done yet. I'm seeing that concept mentioned in more than one professional axle white paper so I think its probably worth getting up to speed on. Appears to be a way of establishing "yield point" as a certain amount of deformation.

Includes this gem:





If I'm interpreting it correctly, looks like those 1.125 OD 1040 cold drawn axle shafts broke at 4000+ ft lbs! Yield was roughly half that. That would go a long way in explaining how our axles are surviving higher powered TD outputs at all. That equates to an ultimate shear strength of around 159ksi. But 1040 at 159ksi? That would put ultimate tensile strength around 212ksi...

[acannell]

LOL what about getting rid of the splines and instead, shrink-fitting the axles into the diff gears and hub? Heat the gears and hubs up to 250F with hot oil pumped through the tranny. Cool the axle shafts in dry ice. Then assemble it all quickly. Once the temperature stabilize the clearance goes away and you have a stress riser free connection. Very detailed info on how much torque the connection can handle should be available from machining land since that method is used for tool holders. Im not sure if its in the ballpark as far as torque.

http://www.amesweb.info/Interference...erenceFit.aspx

That would get you some interesting numbers as far as actually doing it...

[Shadow]

PS. I would pay a lot of money for some truly unbreakable axles.

They would have to come with a unconditional lifetime warranty/ guarantee for me to even consider it.

[Reaper1]

What if instead of focusing on making the splines stronger, larger, harder...whatever...feeding it little plue pills LOL

What if we focused more instead on making the axle tube itself TOUGHER. Not stronger...tougher. Meaning, it will deflect to a larger angle and not pass the plastic deformation stage. Basically, make it a shock absorber.

[acannell]

What if instead of focusing on making the splines stronger, larger, harder...whatever...feeding it little plue pills LOL

What if we focused more instead on making the axle tube itself TOUGHER. Not stronger...tougher. Meaning, it will deflect to a larger angle and not pass the plastic deformation stage. Basically, make it a shock absorber.

Along with the blue pills will it be getting side by side bathtubs at sunset??

How much it deflects is a property of the Youngs modulus, which for all steels is the same and isnt affected by hardening, composition, etc... So the only way to change how much it deflects is by changing diameter or length.

But its actually not deflection that breaks the axle anyhoo. Tensile strength determines shear strength and shear strength determines when the shaft goes plastic or breaks under torsion, and neither determine deflection. For instance, a 300M axle and a 1040 axle of the same diameter will deflect the same amount under the same torsion, but the 1040 will go plastic/break at a lower torsion.

Im thinking if we really want invincible then going up in OD big time would be the way. But since nobodys biting on that I guess its not realistic because of all the drivetrain changes it would need i.e. custom diff gears and a custom hub, as well as two custom CV joint cups, unless there are already some that could be adapted. Not to mention changes in seal diameters and the housing extension.

BTW that Coromill 172 cutter holder is a cool $3200 just for a 4" holder, not including inserts. So perhaps a plain jane involute cutter on an arbor ala horizontal mill would be a more affordable alternative for prototyping. The right cutter plus a cheap rotary table plus a mill should be able to pull it off?

[Vigo]

200ft lbs at least right?

More like 450.


This is interesting reading and it's helpful to understand certain things, but for me the main thing that would be helpful to be understood is how the people who are going quick on stock axles year after year are getting away with it. I think I already have a pretty good idea, but my point is that learning a whole bunch about imaginary parts builds without learning how to replicate the record of success with stock parts that already exist, is not going to help many (if anyone) in this community.

[acannell]

More like 450.


This is interesting reading and it's helpful to understand certain things, but for me the main thing that would be helpful to be understood is how the people who are going quick on stock axles year after year are getting away with it. I think I already have a pretty good idea, but my point is that learning a whole bunch about imaginary parts builds without learning how to replicate the record of success with stock parts that already exist, is not going to help many (if anyone) in this community.

I think those two things are linked. Find some truth in the imaginary calculations and it will lead to the answer regarding stock part longevity. Keep pulling on threads until some sort of pattern pops out.

If someone has some 60 foot times with accurate mph at 60 feet, we could back calculate and determine the minimum torque the axle saw with a high degree of certainty. That might be interesting. Even if its for a non-TM.

The logic being this:

Known vehicle weight at known mph = known potential energy.

Known time (60 foot time) to that energy = known average power required to add that energy in that amount of time.

Known wheel diameter/circumference = known average torque required to maintain average power.

Heres the key: since we obviously don't know the exact torque curve during the crazy 60 foot launch, and its likely got peaks and valleys and is not a constant torque, we will only be able to calculate the equivalent constant torque that would have the same 60 foot time/mph.

But that tells us the MINIMUM torque that the axle had to have seen. Because any other 60 foot torque curve, i.e., a non-constant value one (not a flat line), would necessarily have peaks, and therefore valleys. And peaks would mean higher than the constant torque value to get to to the same end-energy. So a constant-torque approximation tells us the minimum stress the axle saw. If its very high, thats interesting. If its very low, that is not interesting and not conclusive.

[Ondonti]

What if instead of focusing on making the splines stronger, larger, harder...whatever...feeding it little plue pills LOL

What if we focused more instead on making the axle tube itself TOUGHER. Not stronger...tougher. Meaning, it will deflect to a larger angle and not pass the plastic deformation stage. Basically, make it a shock absorber.

Hence the longer axle breaking less.
The full dreamer mode on this seems pointless because I don't think anyone here sees a future in upgrading every single part of a transmission that we can't even buy synchros for. This just leads to "throw it all away and go RWD something" in the end. Break a few shafts because you are still shockloading the drivetrain and you will learn to hate those axles with zero transmissions available in the junkyard. With automatics, yes, there are already full on custom internal transmissions in a few people's hands so that would be a realistic market. Are they breaking axles too? Hmm. Ask boots what they are going through before wasting time!

[mopar-tech]

This is interesting reading and it's helpful to understand certain things, but for me the main thing that would be helpful to be understood is how the people who are going quick on stock axles year after year are getting away with it.

Proper driveline angles, avoiding rapid loading/unloading (wheel hop) and restricting drivetrain movement.

[turboshad]

Proper driveline angles, avoiding rapid loading/unloading (wheel hop) and restricting drivetrain movement.

And some launch technique. I broke one axle and decided to change how I launch to reduce the shock. Now I bring my leg straight up pulling my knee to my chest. My leg can only move so fast and it controls the speed of the clutch engagement. No broken axles after I switched.

As others have said and hinted to the stock axles are not breaking from pure torque, it's either shock loading or fatigue.

[acannell]

And some launch technique. I broke one axle and decided to change how I launch to reduce the shock. Now I bring my leg straight up pulling my knee to my chest. My leg can only move so fast and it controls the speed of the clutch engagement. No broken axles after I switched.

As others have said and hinted to the stock axles are not breaking from pure torque, it's either shock loading or fatigue.

Yes it would seem fatigue may be the key here.

For those with broken axles, take a look at the fracture surface, and you may be able to tell if long term fatigue played a major role, or if it was more of a one-time overload, or a mix:

http://www.maintenancetechnology.com...achine-shafts/

The condition or roughness of the fracture surface is one of the most important points to look at in analyzing a failure because of the difference between overload failures and fatigue failures. With overload failures—because the crack travels at a constant rate—the surface is uniformly rough. Fatigue-induced cracks, however, travel across the fracture face at ever-increasing speeds. As a result, the typical fatigue fracture face is relatively smooth near the origin(s) and ends in a comparatively rough final fracture.

That link has more pictures for this purpose as well.

[Reaper1]

Another way we could tell what kind of torque the axles are seeing is using a torque gauge. I think we could pull this off fairly cheaply. You need 2 tone rings (think ABS ring) and 2 sensors (again, think ABS or CPS, etc.). Both of the rings are installed on the same shaft and indexed to each other. Data log the sensor output and compare the timing of the signal differential between the 2 tone rings. You can extrapolate an twist angle from this, and hence a torque. I know for fact that P3 Orion's use this sort of system in their turbo-prop gearboxes because the engine can generate way more torque than the box can actually handle, so it's part of the flight engineer's job to monitor this and adjust the power as needed.

If you knew enough about your set-up and got the electronics down, it might even be possible to create an algorithm that would torque limit the engine to keep your axles alive and it could even act as a traction control. Many many possibilities.

[acannell]

Another way we could tell what kind of torque the axles are seeing is using a torque gauge. I think we could pull this off fairly cheaply. You need 2 tone rings (think ABS ring) and 2 sensors (again, think ABS or CPS, etc.). Both of the rings are installed on the same shaft and indexed to each other. Data log the sensor output and compare the timing of the signal differential between the 2 tone rings. You can extrapolate an twist angle from this, and hence a torque. I know for fact that P3 Orion's use this sort of system in their turbo-prop gearboxes because the engine can generate way more torque than the box can actually handle, so it's part of the flight engineer's job to monitor this and adjust the power as needed.

If you knew enough about your set-up and got the electronics down, it might even be possible to create an algorithm that would torque limit the engine to keep your axles alive and it could even act as a traction control. Many many possibilities.

This is reasonable but would require some custom electronics as well as some careful work to mount the tone rings and the sensors in the right spot.

But I think there would be a measurable signal. If the engine is outputting 200ft lbs, and in 1st with an A568, thats about 2200 ft lbs out. Assuming our literal axle shaft is 12" and 1.037" OD, we should see a deflection end to end of around 15 degrees. It would require calibration of the axles to a few known torques. Since the wheel rpm is so low in 1st gear, 15 degrees is zillion clock ticks with even a slow microcontroller so it should be very easy to detect.

[Warren Stramer]

Out of curiosity I dug up some of my axles I had run in the past and removed for one reason or another and inspected the spline roots under magnification, This is what I found.............If you can enlarge the photos you will see a crack starting near the spline valley on nearly every spline. (I don't know how to make these photos larger here) Right click on photo and "open link in new window"?? maybe?
As an experiment I'm going to take an axle that has not yet been subject to high torque loads and machine an undercut right at the spline end to the valley root depth, polish and shot peen that area, and see if it helps at all, as that is where my left side axles fail.