Drive Shaft Twisting

[tok-tokkie]

I am very surprised by that twist. Seeing it in pictures forces me to revise my opinion. Expected it to be less ductile & therefore brittle.

[F1 Boy]

Hi,

I worked for Xtrac for 7 year, I've never seen a drive shaft twist so much without failing. The splines normally twist which in turn initiates fatigue cracks in the spline roots.

Motorsport shafts are normally manufactured from S155 grade steel.

[riff_raff]

F1 Boy,

I would agree that most racing axels are made from something like S155/300M/4340M. These materials don't have an elongation rate that would result in that much twisting without fracture failure. The shaft in strad's picture was probably made from some low strength material. It also is poorly designed, since there is no waisting beyond the splined area, which produces stress concentrations at the splines.

By the way, what did you do at Xtrac?

riff_raff

[Caito]

F1 Boy,

I would agree that most racing axels are made from something like S155/300M/4340M. These materials don't have an elongation rate that would result in that much twisting without fracture failure. The shaft in strad's picture was probably made from some low strength material. It also is poorly designed, since there is no waisting beyond the splined area, which produces stress concentrations at the splines.

By the way, what did you do at Xtrac?

riff_raff

Riff raff, could you elaborate on the waisting. Is that a reduction on the diameter where the splines end? Wouldn't that create a weak point? Do you have a pic of a well designed shaft?


Thanks!

[F1 Boy]

Riff raff,

I was the Materials Engineer there.

[riff_raff]

Caito,

Yes, waisting is simply a gradual reduction of the shaft diameter inboard of the splines. The purpose of this is to equalize the stresses in the shaft by better distribution of the strains. Spline teeth have small radii at the tooth roots, which tends to create stress concentrations.

Besides waisting, careful attention also needs to be paid to spline engagement L/D ratio and thickness of the wall backing up the spline teeth. Once the L/D ratio of the engaged spline teeth exceeds 1.0, or there is insufficient structure supporting the spline teeth, the spline teeth will not load share across their faces. This causes stress concentrations at the tooth edges closest to the load. Well designed splines use tooth geometry modifications such as crowning or lead correction to compensate for this condition.

riff_raff

[ringo]

L/D is lenght of the spline along the length of the shaft to the root diameter?

[strad]

That's all fine, but there are times it twists for a much longer distance than just the splines. The twist can reach almost the length of the shaft.

[Caito]

Thanks for the answer riff.

[riff_raff]

L/D is length of the spline along the length of the shaft to the root diameter?

ringo,

The L/D ratio of concern is based on the engaged spline length, not the overall length of an individual male or female component. When a spline engaged length gets long, the teeth do not load share along their length. One end of the spline will usually be more highly loaded due to torsional wind-up. As I noted, a general rule of thumb is that engaged spline L/D ratio should be kept below 1.0.

If longer engaged lengths are unavoidable, then spline lead correction can be used. This involves manufacturing one component of the spline joint with a slight axial "twist" in the opposite direction of the load. If done correctly, the spline teeth will deflect and load share evenly across their face at the prescribed design point.

regards,
riff-raff

[PhillipM]

Well, I've been a long time reader but I had to sign up to reply to this one, I've built a few safari race cars, and I can tell you right now that not only do they twist a fair way (although not 3 full revolutions ), that yes, we do also want them to twist, in fact we usually make them out of tempered spring steel with oversized splines and waisted centre sections specifically so they are as small diameter as possible for the strength needed.
Because they're a spring they absorb the shock loadings in the drivetrain meaning the gearbox and clutch can be designed to cope with lower peak loadings, they also help to prevent overloading the tyres on the rough sections.

The calculations before as someone has said are wrong because of the multiplication ratio from the geartrain, should be around 13-14 times higher in first or so, but I would expect the highest shock loadings would not actually come from the start, but rather from riding rough curbs with the power on out of the slower corners as the tyre loads and unloads, the shock as the tyre briefly spins up and then grips (even with a lsd in the back), would probably be much harsher than the start is.

FWIW, the driveshafts we use generally have maximum twist angles in service of between 38-48*, depending upon application.

[Callum]

IT's great to hear from you, PhillipM thanks.

[xpensive]

Perhaps it's a good idea to point out the difference between elastic twist (like a torsion-spring) and plastic twist (permanent deformation)?

Depending on the relation between the material's yield stength and maximum elongation value, the amount of permanent twist will vary, while the elastic twist will always be a relation between modulus and yield strength.

Two outrageously simplified xamples;
- A shaft made of mild steel, low yield strength with high elongation, is likely to leave a lot of permanent twist if torque reach such levels, but very little in terms of elastic twist.

- A shaft made from Titanium or high-strength aluminium, low modulus with high yield strength, could probably twist quite a bit elastically, but will not leave much in terms of permanent twist.

[riff_raff]

xpensive,

A driveline component subject to fatigue would never be designed to operate close to its elastic limit, let alone beyond its elastic limit.

For example, an axle made from 300M alloy steel with an Ftu/Fty of 280/238ksi, a stress ratio = -1.0, and Kt = 1.0, would have an allowable fatigue limit of 130ksi at 10^5 load cycles (about 1 GP race distance). 130ksi is only about 46% of the material's yield strength.

riff_raff

[DaveKillens]

Many years ago I read a good book on the development of the Ford GT40 for LeMans. This is an interesting period in the technical evolution of racing because Ford really put a lot into data collection, which up to that time was a vague science.

Anyways, one thing that stuck to my mind was the mention that the engineers discovered that the rear axle twisted a substantial amount when under torque. To the best of my memory, it was 270 degrees. Of course that stuck to my mind, having a rear halfshaft twist 270 degrees is something that makes you sit up and take notice.