At what hardness level? Anyway 440C is mainly used in the 55-58RC hardness level and at that level it is quite brittle. So, I would not use it where shear and tensile stresses involved. It is mainly used where compression state of load exists such as in ball bearings (the balls and the races) or where friction resistance under compression load is desired.
I shear load is involved use AISI 420 at 44-50 RC. It still have enough toughness and wear resistance.
This part is a shaft that is being driven by a gear box-servo motor combination, and it is made of SS 440C with between 52-54 Rc. We originally wanted to make this shaft from SS 440F but that was not available, so then we asked for SS 440C with 60 +/- 1 Rc, but the supplier gave us what I mentioned above, and we know this because we had the shaft analysed. I am currently trying to calculate whether the shaft can actually operate with the applied torque, and I have a value for shear stress, now I just need the value for yield shear strength of this material...thanks for your help...
Tom
The shear yield strength [τ] = 0.577 [·] [σ] (the tensile yield strength), assuming the material behaves according to the von Mises yield criterion, which is a good assumption.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The von Mises criterion is not a good assumption for Type 440C stainless steel at a hardness of 52-54 HRC. This alloy will fail by brittle fracture. Is this shaft going to be subject to dynamic loading? A calculation using only static loads will not capture this failure mode.
The shaft is loaded while turning. We have actually had on brittle fracture failure, and the machine while being tested this week appears to have failed again, however we will not know until the machine is taken apart this afternoon. We are changing shaft materials from SS 440 C to PH 17-4 (H900). I have done a static analysis and found that this new shaft will experience some permanent deformation, at least based on a shear yield strength of 120kpsi. I will try to redo my calculations considering dynamic loading. Thanks
Ok...I can admit when I am stumped...and I am stumped...I am not sure how to to this analysis considering dynamic loading. Right now, I am using the maximum torque from the gear box, and then using the torsion formulas from my old Solid Mechanics notes. So I have:
(Torque)/(Second moment of area) = (Shear Stress)/(radius)
I am solving for my shear stress, and comparing it to the shear yield strength of SS 440C.
Here is a quote from MIL-HDBK-5 concerning 17-4-PH.
"The impact strength of 17-4PH, especially large size bar in the H900 and H925 conditions, may be very
low at subzero temperatures; consequently, the use of 17-4PH for critical applications at low temperatures
should be avoided."
NIL-HDBK-5 doesn't give any guarantied shear values for 17-4PH.
I think you should go with Custom 455 H1000.
Can you give more info on the environment.
Is it possible that the fracture is actually a fatigue (dynamic) fracture?
To israelkk:
We had the shaft analyzed at a lab, and they said it was a ductile fracture due to overloading. My problem right now, it that our shaft is undergoing bending and twisting, and I am having a little trouble bring the two together to figure out to what torque I should limit this shaft to in operation.
Environment info:
between 5-10 degrees C
To make sure you are clear, the shaft we are using is made of 440C but we will be switching to 17-4 PH in the future...however I am hoping I can stop this from happening before the shafts get made...as I believe these shafts will not hold up...
Thanks,
Tom
I would exercise caution using 17/4 or 17/7 in any condition other than H1100 or above for 17/4 or condition TH1050 for 17/7. We have used both materials since they came on the scene. We had considerable more trouble with 17/7 than 17/4 for shafting. We had several failures on 17/7 shafts that cycled between 70°F and 500°F. If my memory is correct these were in Condition “C”. I don't know the current thinking on 17/7 shafting.
All the 17/4 we use is treated to at least to H1100 based on our original work coupled with ARMCO’s recommendations many years ago. Aside from the problems with 17/7 the only problems with 17/4 and 455 has been temper embrittlement after exposure to 600°F and occasional excursions to 900°F. This shouldn’t affect your application at your temperatures.
We actually used Aquamet 22 or Aquamet 17 for our RW pumps and CTW pumps in place of the original 416 SS with tremendous results. It comes TGP and straight in box ready to use.
We use a lot of 17/4 and 22/13/5 for boat shafting with not problems. I have a 3" x 11' Aquamet 17 (17/4 H1125) shaft on a boat that I have an interest in. Nearly all the local shipyards use Aquamet 17 on boats, even for Alaska. I seen some that had sheared in the coupling or prop but no brittle failures.
If you are seeing any plastic deformation (and if your lab says they see ductile overstress failure mode), then you can use the von Mises yield criterion to analyze your shaft. Your equation is proper for this analysis.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
You need to consider the maximun shear stress created by the combined bending and torsion loads. Your equation does not consider the bending loads. The correct equation is:
nominal max torsion stress = (16/(pi*d^3))*(M^2+T^2)^0.5
d = shaft dia
M = bending moment
T = torsion load
On top of this you must consider:
Stress raisers such as shoulder fillets & keyways
Surface finish factors
Size factors
Temperature factors
Reliability factors
Overload factors
120 ksi is an extremely high shear stress level for dynamic loading, for any alloy, especially if you have not yet considered the above list of factors.
A thorough treatment of the subject can be found in Shigley & Mischke's Standard Handbook of Machine Design, MCgraw-Hill.
