axle shaft

[Pat]

I was wondering what should be the torque capacity of axle shaft. Last year we design them for 650 Nm (approx 475 ft/po) and broke both of them during summer testing. I think part of this is because of a bad design (center of shaft larger than spline major diameter) but I would like to have a starting point for a new design. We are using atv CV joint and have custom made shaft out of 4340 and the diameter was 0,795 in.

I remember seeing much smaller shaft in Detroit and the team said they had no problem with them. Also for those who have them custom made, do you have them heat treated? I have been told that they would bend if they where heat treated after being made and that it is very difficult to machine them after being treated above 35-40 RC.

Thanks
Patrick

[Pat]

I was wondering what should be the torque capacity of axle shaft. Last year we design them for 650 Nm (approx 475 ft/po) and broke both of them during summer testing. I think part of this is because of a bad design (center of shaft larger than spline major diameter) but I would like to have a starting point for a new design. We are using atv CV joint and have custom made shaft out of 4340 and the diameter was 0,795 in.

I remember seeing much smaller shaft in Detroit and the team said they had no problem with them. Also for those who have them custom made, do you have them heat treated? I have been told that they would bend if they where heat treated after being made and that it is very difficult to machine them after being treated above 35-40 RC.

Thanks
Patrick

[Chris Allbee]

Just as a bit of information, most commercially available axles i have looked at are heat treated to ~RC50. This amounts to about a 20% increase in strength and a decrease of about 2% in ductility.

In the past we have heat treated to about RC36-38 and have always had problems with deformation and (eventually) breaking. I'm going with a proper hollow axle this year, heat treated to RC50. If anyone has a good arguement as to why I should be slapped around for that decision, feel free to let me know!

[djbrodie]

You guys,

Can calculate this. Use shafts in torsional bending. You will find that a hollow shaft of the same weight will be nearly twice the strength of the solid shaft. It's all to do with the second moment of inertia.

Engine Team leader 03
Full Boar Racing
Swinburne

[Frank]

hang on,

are you talking front axle, like a dead axle

or the drive axle at the rear?

[Craig Taylor]

Patrick,

Yes, an axle shaft must be heat treated after machining. And, yes, they will bend during heat treat. A competent heat treater will both 1) hang the shaft vertically in the oven and quench, 2) be able to straighten the shaft to within .003 runout after the tempering draw.

The hardness after draw is critical to the life of the shaft. Too soft, (35-40) and the shaft will yield. This shows up as a spiral, or helix bend in the shaft prior to failure. Too hard, and the "toughness" goes away, and the shaft will fail in a flat plane mode. Other keys to shaft design: 1) the shaft basic diameter MUST be smaller than the spline, 2) if gun drilled the bore MUST be honed and polished to eliminate stress risers, 3) the smaller the diameter relative to the length, the harder the part can be (it then acts as a torsional spring, deflecting but not yielding)

The shafts that we produce for DSR and FSAE are made to the above specifications. To date none have been broken (Yes, I know, drivers are animals, and can break anything; therefore someone will eventually break one.) The reason that we went into production on a customizable "generic" range of shafts was to try to satisfy the many requests for custom shafts, which, with a six to eight week lead time and excessive costs became impractical. We are now using these same shafts on SCCA DSR cars with 1100CC motorcycle engines at 165 HP. Again, someone out there will break one, but it has not happened yet.....

Hope that this proves useful. If you have any questions, feel free to e-mail me at craig@taylor-race.com

Craig Taylor
Taylor Race Engineering

[Frank]

we've got a final drive ratio of about 20:1

so that's 1200Nm max

you say 650Nm..you didn't forget the internal 2:1 gear reduction between the crank and g'box?

we use GKN 21mm solid shafts (they are soft as.. like 35 Rc), crack test 'em regular now, no probs

i reckon you'd be quite safe wif 19mm OD @ 44 Rc

has anyone else found the GKN's soft?

has anyone used less than 19mm OD?

Craig, do you know what the hardness limit is for gun drilling? 35Rc? 38Rc? i heard the tooling is very expensive and people dont like doing it to 4340 once heat treated. Is the "honing" a usual process in the gun drilling?, or do you have to specify that as well?, and finally, can they grip a shaft that already has splines cut? (spline diameter larger than shaft as usual)

another question Craig, what OD have you specified for FSAE cars in the past, or was that a decision for the students?

thx

Frank

[This message was edited by Frank on November 05, 2003 at 06:29 AM.]

[Pat]

Frank, if I remember correctly, I didn't take the engine torque into account when I design the shaft. We have around 50Nm and a ratio of 20 =1000N*m at the wheel but the friction coefficient of the tire is around 1,5 with a normal force of 1600N/wheel in acceleration. F=uN ---- 1,5*1600 = 2400 * 0.254 (radius of tyre) So the max torque that can be transmitted by the tire to the ground is around 610 Nm.
Did you take your cv joint from a car?
I had a good look a Craig's product and they really good and light but changing the entire cv joint would require new hubs, new uprights and major modifications to the differential and it is a bit late for that now. We still got over 100 hours from the last shaft without heat treatment.

Craig, when you say 2) be able to straighten the shaft to within .003 runout after the tempering draw., do you mean straightening it on a press because our shaft manufacturers says they do that on various car drive shaft and that they could do it on our shaft. I was worried because of the plastic deformation that will result.

Thanks
Patrick

[Frank]

we had a strain gauge on the shafts last year, it uses a transponder

we think the shafts "see" something more like the "output" + 20% than the "frictional calculation"

there also seemed to be a very short (5msec) "spike" under braking.. we assume it's from windup

we'll run the strain gauge again this year with our new datalogger and I'll post the results

regards

Frank

[Craig Taylor]

Patrick: You understand correctly. Shafting is normally straightened after heat treat and draw (temper). Yes it does require plastic defomation to accomplish. Not a problem.

Frank: Would you be willing to share your strain gage data? We would really like to see it.

Thanks,

Craig

[Courtney Waters]

Frank,

Regarding Craig's (Taylor Race Engineering's) shafts:

If you use their shafts that take tripods on both ends, they have a .750" OD and .400" ID. They have detailed drawings on their web site.

Courtney Waters
UC Davis Formula SAE Alumni

[-]

For axle heat treatments there are some higher grade alloy steels that do not need straightening after heat treatment. The straightening is usually needed due to the creep of the materials under heat. Some of these higher grade alloys steels can be heat treated with out fixturing and come out straight as when the went in. This quality also works well with cutting splines while the material is in the annealed state, and even after heat treatment they fit perfectly into whatever they orginally were made to fit.

Also these steels have tremendous fracture toughness when properly heat treated, which is also a sought after quality in axle shafts.

Do your homework and you can find out what materials I am talking about.

[Frank]

Craig and all,

yes we would be most willing to do so.

I will post it when we do it again

we have a new datalogger this year, that is far more reliable and efficient.

we just finnished perfecting the "ART" of achieving reasonable shock histograms

tyre pyrometers the next few days

hopefully within a week we will have strain gauge data again

regards

Frank

[Craig Taylor]

To "New Member"

You are correct that some shafting steels do not warp during heat treat. They are the stainless-based or martempering type steels. They typically do not have the same properties as do the 4340 or4340-300M steels, which always need straightening after draw. Most competition shafts are made of 4340, with 4340-300M reserved for really extreme applications. 300M requires different fabrication, heat treatment amd finishing, and is priced accordingly. For SAE cars you do not need to go there.

Craig

[Pat]

Concerning heat treatment, I was speaking with a teacher this week and he didn't know much about axle but for this type of application, he was thinking that since it is a solid shaft, we might want to keep a lot of ductility at the center of the shaft. The main goal of the treatment is to prevent cracks from spreading. He was recommending an electrical induction treatment. It would also eliminate part of the warping problem. This treatment affect only the first 2-3mm on the surface. I'm also pretty sure that our shafts have been treated this way before on the 2001 car and we have not had problems with them but nothing was documented at that time.
Anyone used this treatment before?

Thanks
Patrick