Stronger steel than 4340...... 20

[Tagger]

I have a Ø6" shaft that necks down to Ø4.70" which is splined. The shaft is subjected to torsion and bending.The splined shaft goes through a cyclical bending as it rotates, and this eventually lead to the shaft breaking after a little over two years of operation. The shaft broke right at the point where the usable spline length ended. Basically the Major Ø of the spline is the Ø of the necked down portion of the shaft (Ø4.70). Torsional-wise the shaft can easily handle the torque, it is just over time the bending fatigued the shaft. Without getting into detailed analysis of the application, what class of steel would better handle the bending (give longer life). Assuming the operating parameters are the same as the previous shaft, (I can't guarantee the customer the shaft will last longer), what would be a better material? The current shaft is 4340 Q&T (don't know the details of the Q&T) to 269-321 BHN. The length of the spline was nitride case hardened to a depth of 0.015-0.020" and Rockwell 15N hardness 90 minimum. Thank you for your support.

 

[priyasachin]

Priyasachin
Dear Friend,

While there are many suggestions, I have one too.

I have seen such failure on turbo shaft of diesel locomotive. One possibility is that this type of failure could be controlled by using hollow shaft rather than solid shaft. The failure is due to tortiona;l vibration than anything else. The material 4340 in Q&T condition is sufficiently good material.

Regards,
MRCN

 

[diamondjim]

I would suggest that you turn the spline
od and put a large fillet before hardening
the shaft. Allow some extra stock for
final machining. This might provide better
transition in the hardening pattern as well.
I assume you also used a stress relief after
hardening. I assume your specs call out for
a clean steel and vacuum degassed material
etc. How long is the shaft?

 

[metman]

Tagger,
Sorry to be such a late-comer but am compelled to reply, hoping not too late.

You said on Jan 8 "I agree...the other alternative is the increase the diameter of the spline to closely match the diameter of the shaft (Ø6&quot. This would involve changing another component of the design to match the increase spline diameter."

If you can do this, the gain in torsional resistance, fatigue resistance, reduction of spline root stress, etc will be so dramatic that it pales all the other suggested fixes by a HUGE factor. Compare the Polar section modulus of 4.7 vs 6. It is a cubic function and therefore you DOUBLE the torsional strength. You said the 4.7 will handle the torsional stress but you need to increase the endurance limit.

If this is still an alternative, DO IT because this will save you oooooodles of headaches trying all these different suggestions. Not that there is anything wrong with the suggestons if you have the luxury of "trying them".

We had a similar "design" problem (torsion and bending with splines of 4340) and tried every single suggestion posed here-plus some-except for the astroloy version because instead we finally used maraging steel which doubled the fatigue life. This bought us some time, several years in fact (even though maraging shafts were still fatigueing in as short as 4-6 months in sever applications) to allow us to slowly introduce a new design of a complex gear case including a planetary that had to grow in order to grow the diameter of THE splined shaft that passes thru the sun gear. Our original design did not allow for an earlier unknown reverse torsional load on repeated start-ups. In other words it was a geometry problem such as yours.

For example: Let's say you neck down the shaft at the end of the splines. This could possibly give you a high enough stess at the necked down region to still give you a marginal endurance limit at that section.

And yes you can nitride maraging steel but why use exotic materials as TVP states when more practical solutions are available. A maraging shaft that size would probably cost upwards of $1,500 just for material assuming it is only a few feet long.


Jesus is THE life,
Leonard

 

[EdDanzer]

Although we never tried a maraging steel for our torsion bar application because of competitive cost constraints, E4340 if properly processed and designed should work well. The one factor not covered in this is the length. If you could post the length it may be help full in further investigation of the problem. It may be that the 4.7 diameter should be reduced if possible.
One reason for the induction comment is it works similar to drilling the ID but you do not loose moment of inertia. Polishing the diameter and radius before HT is required.

After some searching I have found the Induction information, it is from the March, 1985 Metal Progress magazine. It has a graph that show degrees of twist, torsional yield, and ultimate torsional strength, another graph has fatigue test data. Although this information was test data for SAE 1040 shafts, we used it very successfully on 4340.
The best draw temper for overall axle applications is about 350F. We specified 350° to 375° F so the heat treat shop could actually hit a target. This temper should allow over maximum twist in relation to length and exceed 1,500,000 cycles if not over stressed.
The case has a hardness of approximately 55 RC, and should be about 15% of the diameter in depth if possible and dropping to 30 RC at the core.
We found this to more than double the life of any carbon steel material that was quenched and tempered in a torsion bar subjected to bi directional twisting.