Shaft torsion at cryogenic temperatures of -196C 3

[Burdy]

I cannot find any recommendations for materials being used as shafts in cryogenic conditions. Shear modulus / modulus of rigidity always appear to missing from the majority of materials I do search on.
Can anyone recommend any material with a shear modulus greater than 87 GPa (17-4PH)?
There appears to be a lot of information about cryogenic conditioning but very little data on the properties of material at those temperatures!
Hope someone can help.
Br
Bernard H

 

[CoryPad]

Shear modulus is related to Young's modulus (also known as elastic modulus):

G = E / [2 (1 + [ignore]&nu[/ignore]]

where

G = shear modulus
E = elastic modulus
[ignore]&nu[/ignore]; = Poisson's ratio (if unknown, assume 0.3)

Thus, look for materials with high E, which means they also have high G.

For a stiff shaft, other metals to investigate include alloys of Be, Co, Mo, and W. However, your shaft will also need strength and fracture toughness, which may be missing from these materials. Good luck.

 

[Burdy]

Thanks, for that.
I believe youngs modulus would also change at cryogenic temperatures and the data for these figures is limited.
Bernard

 

[CoryPad]

You are correct that elastic modulus is temperature dependent, but below 300 kelvins, the change is small. Figure 1.2d in Deformation and Fracture Mechanics of Engineering Materials by Hertzberg shows the elastic modulus change to be [ignore]&le[ignore]; 10%, thus you should be able to perform materials selection based upon standard modulus values, then perform testing to verify prototype/final designs.

 

[prex]

I don't understand why you need a high shear modulus: steels have all almost the same value, and at cryogenic temperatures there is not much change. Hence your parameter is fixed and you can't change it.
If on the contrary it is a matter of resistance and not of stiffness, then I suppose that the commonest choice of a stainless steel is not good (though I would give it a try): then you should turn into the field of high Nickel alloys (Hastelloy and the like). prex

http://www.xcalcs.com

Online tools for structural design

 

[CoryPad]

prex,

A cylinder in torsion will have minimum mass per unit deflection when constructed from a material with the maximum quantity G^0.5/[ignore]&rho[/ignore];, where G is shear modulus and rho is mass density. Therefore, one must find materials with high G. Your statement "Hence your parameter is fixed and you can't change it." is false.

 

[prex]

No, it isn't false, if you limit your choice to steels, as I specified: all steels have practically the same mass densities and elastic moduli.
If you want to extend your search to other metals, then Titanium may be a choice for you: both the elastic modulus and the density are roughly half as those of steel, so your ratio improves. On the contrary Copper is not good, as the density is higher than that of steel, but the elastic modulus is halved, and even worse it goes with aluminum (low density but much lower modulus). Zirconium alloys are not very different from Titanium, just a little worse from your point of view. High Nickel alloys, that I already mentioned, are not very dissimilar from common steels, just a little worse from your perspective (similar modulus, slightly higher density).
However there is a simpler way to obtain a high torsional stiffness with minimum mass: simply use a tube instead of a solid bar! prex

http://www.xcalcs.com

Online tools for structural design

 

[bcd]

Hi B,

I believe you have received an answer to your question, but not a solution to the problem. Previous posts are correct in stating that Young's modulus does not change significantly for materials that are suitable for use at -196 C. Materials that retain sufficient ductility at cryogenic temperature so they are mechanically useful are 300 series stainless, Monel, Bronze, Titanium, Aluminum, Hastelloys, Nickel, and 17-4PH cond. H1150M or 15-5PH.

My guess is that you may have a shaft wind-up problem. Excessive torsional wind-up will not be resolved with a simple material change. You will need to increase the shaft diameter in a non critical area to increase its stiffness.

Let me know if my guess is incorrect and you need more information.

DBay

 

[Burdy]

Dbay, you are correct in sumising that torsional wind up is my problem. the current use of 17-4 PH H1150M has a value of G as 87Mn/m. as does most of the 17-4PH family although the other material in use for warm applications - H1150+H1150 has a far greater yield strength.
I have to use a solid shaft because of the danger of cavities at these temperatures (we would only have to have a pin hole in a seal weld to cause untold damage), I must mention we are talking about equipment for pressure retention which could be as great as 255 Barg.
The wind up is still excessive despite the use of larger diameters in a compound shaft.
I have fixed input and output diameters, so was looking for a more rigid steel, 10% would help but there appears to be nothing greater than 17-4PH unless I go to VERY expensive materials which have even less cryogenic data than the steels!! 17-4PH is >£240 a kilo as it is.

Bernard

 

[Burdy]

Corypad,your explanation of how to get to the modulus of rigidity doesn't work out! the calculation will however give you youngs modulus of elasticity for TORSION.
If you work any material backwards I think you'll find this to be true, if not then could you pls go further?
B

 

[CoryPad]

Burdy,

I don't understand what you mean by "youngs modulus of elasticity for TORSION". I used accepted materials properties, names, and symbols to describe elastic constants. You can see a more detailed description at:

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/elastic_constants_G_K.cfm

I think you have a full understanding of your problem. If you need higher elastic/shear modulus, you will need to use a more expensive material which has less published data than your current material. I recommend investigating beryllium alloys - try Brush Wellman at:

http://www.brushwellman.com

 

[Burdy]

Thanks for the web address.
this is the same site but gives all equations for calculating E, G, k or v -

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/calc_elastic_constants.cfm#Table

Bernard