spline shaft material 6

[azgearpro]

I have been working with a high performance company. They are having a problem with shaft failure. They have been using 300m you know the material that you cant hardly find for aerospace because the race industery is buying it all up. Well the shafts are still failing, and they came to me to find a stronger material, does anyone have any ideas?

 

[TVP]

Do you know what strength/hardness level has historically been used for the 300M part? There are higher performance steels like maraging steels (Vascomax C-250, C-300, C-350 from Allvac) or Aermet 100 (ultrahigh strength steel from Carpenter) or precipitation hardening grades of stainless steel like Custom 456 (Carpenter) that can achieve very high strength levels. Parts like this can see substantial improvements in performance without switching steel grades if the manufacturing processes are optimized. Items to address include:

1. Reduce surface roughness
2. Remove residual tensile stresses
3. Impart residual compressive stresses (shot peening or roller burnishing)
4. Reduce stress intensity (larger radii, better blending, etc.)

 

[metengr]

Is it really a problem with strength? You have not mentioned the cause of shaft failures? I would investigate this problem before deciding on selection of an alternative alloy.

 

[EdStainless]

TVP gave you a good list of alloys. One thing that all of them have in common is fairly low toughness. You really need to do a detailed failure analysis. I doubt that brute strength will solve the problem. I think that it is very likely a tolerance and manufacturing methods issue.

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[rayband]

You have received some excellent advice here, to which I would add, check the steel cleanliness. All pieces of VAR/VIR steel are not created equal. From experience I can tell you that occasionally inclusion segregations can be found despite the certifications. If you are a low volume occasional buyer, beware. You may be getting what can't be sold to others. Do some NDT like mag particle and also metallographic exam if you can as part of your failure analysis. Include AMS 2300 on you PO's if you don't already.

 

[TVP]

Ed,

I agree with everything except for the characterization of the mentioned alloys as having low toughness". A VIM/VAR 300M alloy will have a plane-strain fracture toughness of ~ 66-77 MPa m^0.5 when the tensile strength is ~ 1900 MPa. Similarly a 250 grade maraging steel will be ~ 70 MPa m^0.5, while Aermet 100 will be in excess of 100 MPam^0.5 at that tensile strength. My threshold for low toughness would probably be < 20 MPa m^0.5.

 

[EdStainless]

I should have used a few more words. What I was trying to express is that even though these are strong and tough materials, their toughness (and fatigue resistance) don't increase as fast as their strength does.
We built a number of test stands (using splined shafts) from Maraging 250 and 300. While they handled overload very well they were sensitive to impulse and fatigue.
This is why I feel that the real problem may not be the parts strength.

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Rust never sleeps
Neither should your protection

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[azgearpro]

The stress it is under can be impulsive. These trucks can put down 600+ hp and 1500+ foot lbs of torque at 1600 rpm. I have noticed companys like sunstrand nitride alot of there spline shafts, but I have delt with the down side of chipped nitrided parts. Also what about shot peening the weak point? I looked at Carpenters material site there are alot of good options for materials. How pricy is Aermet 100 (ultrahigh strength steel from Carpenter)? Thnks guys for all your help you all really know your stuff.

Aerospace Gear and Spline specialist

 

[Tmoose]

Got any pictures of the failed parts and surfaces?

 

[EdStainless]

AZ, you really need to find some shafts that have started cracking, but haven't toatlly failed. You need to pin down the mechanism and initiation locations.
A100 is probably a good option, but without addressing the root cause you could just keep breaking more expensive shafts.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metman]

On the toughness of maraging steel I am on the fence but in this instance lean toward EdStainless's take. We replaced a 4340 splined shaft (about 4" dia.) which had all of the issues proposed by TVP provided for plus various exotic heat treatments. The replacement I suggested was c-350 type because of the higher fatigue resistance. We finally discovered reverse fatigue loading with dynamic testing. The C-350 more than doubled the fatigue life which is about all I could hope for since now we knew it was a geometry problem. The point being that even though the maraging steel is more forgiving in fatigue, it still was failing eventually in brittle fashion because of the mechanism of failure. Geometry was inadequate.

 

[tripleZ]

I'm assuming expense is going to be a factor, but for a test comparison I'll throw out something like an Uddeholm Vanadis grade (it isn't cheap). I use the stuff (Vanadis 4 primarily) for tooling in a pressing operation in which I'm cyclically loading the face of my punches with about 55 tons per square inch of force. The stuff holds up extremely well to axial loads, unfortunately I don't have the ability to test torsional strength. Minus the expense, its excellent to machine and heat treats extremely well. It's got a toughness similar to an S-7, but the wear properties (at least in the 57-59 HRc range) are vastly superior. IMO, the Uddeholm sales staff is extremely easy to work with in regards to applications issues. Just my 2 cents.

 

[metman]

Depending on cost vs properties needs, you might consider H11 or H13 Tool Steel. If I remember correctly, they have some very good strength/toughness properties.

 

[azgearpro]

Thanks guys you really know your stuff, I found out through chemical analysis what material they were using that is still breaking. The analysis gave a material closes to the analysis and it was bs.835m30, I have never heard of it all I know is it is a British material. Does anyone know what American material it compares to?

Aerospace Gear and Spline specialist

 

[Heckler]

What was the failure mode?

http://aircraftmaterialsuk.com/data/alstst/s28.html

http://www.springfixlinkages.com/tech_alloy_steel.htm

Best Regards,

Heckler
Sr. Mechanical Engineer
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Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[Fabrico]

That's nice, you had no idea of what you were working with.

"Does anyone know what American material it compares to?"

You're the "Aerospace Gear and Spline Specialist working with a high performance company". . .

 

[azgearpro]

Thats right "gear and spline specialist" not material. I have never worked with British standard material, we specialise in material such as inco718, vasomex,4340,17-4,15-5,h-11,a-286,8620,9310,300m,titanium,and other american standard material. I see alot of 8620,4340,17-4 turbine shafts.

Aerospace Gear and Spline specialist

 

[metman]

Aha 9310! This might be a good choice. Lots of Nickel which helps greatly in fatigue strength. How do the properties compare with 300M?

 

[TVP]

835M30 according to BS 970-2 is not the same as SAE 9310-- it has twice the Ni and twice the C. It is a very high strength quench & temper grade, not a case hardening steel like 9310. As I mentioned in my first response, there are some even higher strength steels available, but investigating other design-related issues would probably yield better results.