What grade of steel would be best for a transmission input shaft? 7

[LJDCRF]

Currently we are using a material that I am told is a modified 4340 called 300M. The shafts that fail show helical cracks down the length of the shaft. The failures have all shown up in trucks that are subjected to large amounts of towing. The currently the shafts are machined from 300M billet and are hardened. I do not know how hard they are making them.

Because of the configuration I want to look into forging this design. Machining from billet creates a large amount of wasted material. The billet must be 5.25” in diameter and 9.380” long. Approximately 90% of this material is wasted.

The shaft is 9.380" long. The first 7.5 inches of the shaft has an OD of .970 the remaining length is a splined drum that measures 5.25 OD.

Reading the machinery’s handbook I found 4150 as a suggested material for hard gears that require strength and toughness, but its tensile strength is not much higher than 4340.

If the design continues to be manufactured out of billet, what materials are being used currently in high torque, cyclic loading configurations? What would be the optimal hardness?

If a forged construction is feasible what material should be used? What treatment should be used to finish the part?

 

[desertfox]

Hi LRDCRF

My ten pennies worth if I may, firstly how many of these
shafts have failed and over what period of time?
The reason I ask is looking at the last photo in your post
I notice what are termed river lines spreading out from right to left these I believe are classic of fatigue
failures and I was taught in a failure of stressed materials
class
that if you follow these river lines back ie
from left to right in your case that they lead you to the start of the crack. Also the helical cracks are cracks growing on the principle planes due to principle tensile stresses and as fatigue cracks only grow under tensile stresses I thoughtthat might be significant.
Anyway following these river lines back they seem converge
to the right just above the I.D. of the bore where there
seems to be a tear drop shape.
It maybe worth while looking at your stress cycling in addition to some of the excellent advice above.

regards

desertfox

 

[Neubaten]

In addition to desertfox observation, I´ll add that the main crack grow direction seems to be inclined from the shaft axis, which leads me to think into a fatigue crack caused mainly by torsion.

 

[desertfox]

Hi Neubaten

I agree a torsional load causes tensile stresses on the principle planes and leads to the failure shown in the picture.

regards
desertfox

 

[Neubaten]

Check this link, it´s an article by J.O. Almen about torsional fatigue failures, a bit old, but interesting IMHO.

http://www.shotpeener.com/library/pdf/1952007.pdf

 

[desertfox]

Hi Neubaten

Good article thankyou.