Optimal Splined Shaft Material for Low Cycle Fatigue 3

[novateague]

We're looking to maximize the amount of torque we can apply to a short splined shaft (~10,000 reversing cycles). We have been using 4140 previously.

What would be the most important material property to achieve this? We are looking at Aermet 100, Ferrium C61, MP35N... Seems 300M is recommended for shafts frequently on here.
 
Taking some inspiration from this forum, we already changed the interface radius to an ellipse shape and increased the cross-sectional area to the maximum.

As we speak, we are machining some of these parts from Maraging C350 as a test. The parts will be heat treated for the higher end of fracture toughness and then shot peened. Hopefully we can utilize some of the insane yield strength before the lower ductility becomes an issue.

 

[mfgenggear]

Nova
It is a possibility, there is an issue with the design. And material.
How is mis alignment
How is run out
What shock loads
What type of fit is the spline
Was the part heat treated to a specification
Was there certifications on material and heat treat
Was the splines verified with go and no gages.
Was the shaft sized correctly to the applied torque. Maybe the diameter needs re evaluation.
Does it need a shear neck
What was the bending moment. Was acceptable
To von misses.

 

[mfgenggear]

Link to material data sheet,
Heat treat

https://www.ssa-corp.com/documents/Data Sheet Maraging.pdf

 

[novateague]

The existing design works at our current loads.
We are looking to drop-in a higher performing material so we can raise the input torque.

It is a "legacy" design that we can't modify any further.

Torque is applied gradually with minimal shock loading. Splines have not contributed to failure.

Maraging 350 has MASSIVE yield and UTS advantages over the existing 4140 material. It does have much lower fracture toughness which may be a limiting factor.

Would an alloy like Aermet 100 (or 310) give more usable strength due to it having higher fracture toughness than C350?

 

[Tmoose]

is the loading bi-directional ?

parts like some suspension torsion bars are shot peened while preloaded, so are not interchangeable side-to-side on the vehicle.

"Has to be micro shot peened (0.3 mm diameter shot) due to the tiny custom splines."
What Almen intensity is obtained with the micro shot?

"The failure mode is always ductile torsion failure right at the radius where the input to the shaft steps up to meet the splines."
From that description I think "real" shot-peening could be done with careful masking.

Some times creative boring of shafts can reduce stress concentration.

 

[mfgenggear]

Yes typical not to aggressive almen 4-8
More than that it will distort.

 

[mfgenggear]

Typically spline shafts are case harden for wear, contact stress, the case is equivalent to 60 HRc
9310 typically core is 33-43 HRc
Base on that what. Size diameter for the applied torque.

 

[Tmoose]

I think Almen shot peening intensity needs a letter prefix to Identify the test strip used. A, N, etc.
"A" is a pretty common one.

 

[Tmoose]

If the load is one directional, maybe a good properly calibrated yank would leave the part with nicely aligned residual compressive stress right where it is most needed.

As others said, using fancy materials to try to compensate for bad geometry is a tough road.

There are genuine examples of designs "strengthened" by selectively and thoughtfully removing material.

 

[novateague]

The load is fully reversing (bi-directional).

Those are some really interesting suggestions about pre-stressing the part or shot peening under load - never heard of that before. On FEA, the stress was reduced without the center bored out. Maybe a certain diameter will show an improvement.

Normal sized shot wouldn't reach the splines or the radius, it's very tight unfortunately. We have to work within the current constraints.

Compared to 4140, there should be some improvement just due to better material properties. How much difference is the question...

I've never worked with UHS Steels that have minimal ductility.

 

[EdStainless]

But you said that the failures weren't in the splines, so why peen there?
We had to run some shafting at higher loads than normal.
We picked a material and then heat treated with three different tempers (strengths).
And then we tested.
We settled on the medium strength as it had the best balance of strength and toughness.
But then we were having failures in the splines (1250HP through an 1.375" shaft).
The couplings were hell to make also.

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P.E. Metallurgy, consulting work welcomed

 

[3DDave]

I thought this was familiar. The quest for finding better than best solutions.

"3D Printed Piston Lower Mass = More Power?"

 

[novateague]

@EdStainless

What material did you end up using for those shafts if you don't mind me asking?
Was it the 4335 Vmod you mentioned?

May end up being that a lower strength but tougher alloy will perform better than the C350.
We'll have to test and find out.

The splines have been shot peened in the past without detriment. The parts are sprayed on a rotating fixture where the over spray hits the splines - one less step of masking too.

 

[EdStainless]

We ended up using an aerospace 4340mod grade that was double melted.
We didn't have any luck with C350.
It just didn't have enough toughness.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[novateague]

Finally got a chance to test the new C350 parts.

Gave us about 35% more torque capability in failure testing over 4140. Haven't done fatigue tests yet.

Would love to get a hold of some Aermet or Ferrium alloys.