Axle rotation direction

[Lastminuteracin]

I read in one of the "prepare to" books that axle shafts should always been run in the same orientation and never reversed. Since my Anson came in a box I have no idea as which was left or right. Do you think this will this be a problem?

[kea]

500 HP, maybe, otherwise, No.

[billtebbutt]

I read in one of the "prepare to" books that axle shafts should always been run in the same orientation and never reversed. Since my Anson came in a box I have no idea as which was left or right. Do you think this will this be a problem?

Chuck in a lathe to check for straightness, and off you go!
bt

[Rick Iverson]

500 HP, maybe, otherwise, No.

Chuck in a lathe to check for straightness, and off you go!
bt

Yep

[Lee Johnson]

I read in one of the "prepare to" books that axle shafts should always been run in the same orientation and never reversed. Since my Anson came in a box I have no idea as which was left or right. Do you think this will this be a problem?

The issue Carrol Smith was undoubtedly trying to address is the fairly well known (maybe not so well attended to!) phenomenon of fatigue crack (generation --> growth --> failure). This is a pretty complicated phenomenon, so Carroll was erring on the side of caution in his discussion.

Many steels have a stress level, Endurance Limit Stress (ESL), under which they will have essentially infinite cyclic life. The fatigue life for a part exposed to uni-directional stress cycles of (0 psi --> +X psi --> 0 psi, etc) will be longer than the life of the same part exposed to reverse direction stress cycling (0 psi --> + X psi --> 0 psi --> -X psi --> 0 psi, etc). Switching the rotational direction of an axle shaft will have this type of effect on the life of a stress cycled part.

The stresses in a drive axle for a Super Vee (200ish bhp) could be just as high as those in a 500+ bhp F5000 or CanAm car axle (that Smith might have been considering in his book). This would be possible because the Super Vee axle is likely to intentionally designed be a smaller diameter to save weight. So a similar precaution about avoiding reversing axle rotation direction could apply to the smaller car.

A good step to take while not knowing the the history of the car & its axle shafts would be to get the axles Magnaflux checked for cracks (removing any paint first for maximum sensitivity of the test). If crack free, then maintain uni-directional use in the future & likely have them re-checked somewhat frequently.

[Rick Iverson]

The issue Carrol Smith was undoubtedly trying to address is the fairly well known (maybe not so well attended to!) phenomenon of fatigue crack (generation --> growth --> failure). This is a pretty complicated phenomenon, so Carroll was erring on the side of caution in his discussion.

Many steels have a stress level, Endurance Limit Stress (ESL), under which they will have essentially infinite cyclic life. The fatigue life for a part exposed to uni-directional stress cycles of (0 psi --> +X psi --> 0 psi, etc) will be longer than the life of the same part exposed to reverse direction stress cycling (0 psi --> + X psi --> 0 psi --> -X psi --> 0 psi, etc). Switching the rotational direction of an axle shaft will have this type of effect on the life of a stress cycled part.

The stresses in a drive axle for a Super Vee (200ish bhp) could be just as high as those in a 500+ bhp F5000 or CanAm car axle (that Smith might have been considering in his book). This would be possible because the Super Vee axle is likely to intentionally designed be a smaller diameter to save weight. So a similar precaution about avoiding reversing axle rotation direction could apply to the smaller car.

A good step to take while not knowing the the history of the car & its axle shafts would be to get the axles Magnaflux checked for cracks (removing any paint first for maximum sensitivity of the test). If crack free, then maintain uni-directional use in the future & likely have them re-checked somewhat frequently.

Nicely done

[Lee Johnson]

Nicely done

The pedantic engineer in me comes out at times!

Lee

[Lotus7]

The issue Carrol Smith was undoubtedly trying to address is the fairly well known (maybe not so well attended to!) phenomenon of fatigue crack (generation --> growth --> failure). This is a pretty complicated phenomenon, so Carroll was erring on the side of caution in his discussion.

Many steels have a stress level, Endurance Limit Stress (ESL), under which they will have essentially infinite cyclic life. The fatigue life for a part exposed to uni-directional stress cycles of (0 psi --> +X psi --> 0 psi, etc) will be longer than the life of the same part exposed to reverse direction stress cycling (0 psi --> + X psi --> 0 psi --> -X psi --> 0 psi, etc). Switching the rotational direction of an axle shaft will have this type of effect on the life of a stress cycled part.

The stresses in a drive axle for a Super Vee (200ish bhp) could be just as high as those in a 500+ bhp F5000 or CanAm car axle (that Smith might have been considering in his book). This would be possible because the Super Vee axle is likely to intentionally designed be a smaller diameter to save weight. So a similar precaution about avoiding reversing axle rotation direction could apply to the smaller car.

A good step to take while not knowing the the history of the car & its axle shafts would be to get the axles Magnaflux checked for cracks (removing any paint first for maximum sensitivity of the test). If crack free, then maintain uni-directional use in the future & likely have them re-checked somewhat frequently.

Expanding on this quite correct dissertation above:
Even if a constant drive rotation is maintained, our axles actually experience minor load reversals every time we lift off the gas due to engine+driveline inertia. Very likely just a fraction of the drive torque, but a reversal nonetheless. (And even higher reversals for cars with inboard rear brakes).
I find it hard to imagine that the generally low power we put through these 1" diameter driveshafts comes anywhere near the infinite life point on the S-N curve. I suspect we're at a much higher risk of failures due to external stress risers from poorly machined spline runouts or nicks from rocks or errant tools.
I spend more time worrying about tripod orientation than I do about axle rotation.

[Lastminuteracin]

Tripod orientation? Tell me more please.

[Lotus7]

Tripod orientation? Tell me more please.

the relative angular orientation of the tripods at each end of the axle, (in line or not)

[Lee Johnson]

Expanding on this quite correct dissertation above:
Even if a constant drive rotation is maintained, our axles actually experience minor load reversals every time we lift off the gas due to engine+driveline inertia. Very likely just a fraction of the drive torque, but a reversal nonetheless. (And even higher reversals for cars with inboard rear brakes).
I find it hard to imagine that the generally low power we put through these 1" diameter driveshafts comes anywhere near the infinite life point on the S-N curve. I suspect we're at a much higher risk of failures due to external stress risers from poorly machined spline runouts or nicks from rocks or errant tools.
I spend more time worrying about tripod orientation than I do about axle rotation.

Ian --

You make some good points about load reversals: I know from (bitter) experience about inboard rear brake torque reversals ; I think the biggest, but still relatively small, load reversal from lifting off is from the mass of the car being decelerated (would be worst in 1st gear).

Would it work to address your comment about nicking of axles to put heat-shrink tubing over the axles before assembly?

Is it best practice with tripod joints to have them aligned @ each end (or to have them 60 degrees out of phase)?

All --

I failed to mention that axle shafts shown to be crack-free could benefit from shot-peening. It would have to be the real thing, not glass-bead blasting or the like.

[Larry H]

I would mag-check the threads on the stub axle very carefully.