What should allowable shear stress be?

[ProcEng2]

I have seen 0.5Sy, 0.577Sy, and 0.4Sy used in various calculations for valve keys in shear.
What should be used as a shear allowable stress and why?

 

[CoryPad]

0.5 comes from the Tresca failure criterion.
0.577 comes from the von Mises failure criterion.

Both of these assume the part will begin yielding at the max stress. The von Mises criterion has been found to be a good model for polycrystalline metals undergoing multiaxial loading.

0.4 must be a conservative factor to prevent any yielding.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ProcEng2]

Thanks, Cory.

In our application the key is in pure shear.
How would multi axial loading apply in this condition?

 

[metengr]

ProcEng2;
CoryPad provided the necessary explanation for what you listed as values of shear stress used in design.

As far as which of the three shear stress criteria you should use, that is a completely different matter. Can you please provide more information related to your OP?

 

[ProcEng2]

Cory:
provided the necessary explanation for what you listed as values of shear stress used in design.

As far as which of the three shear stress criteria you should use, that is a completely different matter. Can you please provide more information related to your OP?

Reply:

Cory, our application is a motor operated valve key, which is between a splined adapter and the shaft. The key is subject to torsional loading from torque from the splined adapater, which is driven by the operator. The torque is transferred through the key to the shaft. The key is
the weak link in the valve. The stress in the key is
calculated from 2T/D x W X L where T is the torque, D is the shaft diameter, W is the key width (square key), and L is the key length. The stress is then set to the allowable stress to determine what the maximum torque can safely be placed on the key without yielding the key. If the key were too strong, then something else in the valve would break, not strong enough, then the valve would not operate possibly. Over time, as river water gets intot he valve bearings required torque increases, which is why we have to know what the key can take...

 

[CoryPad]

Multiaxial loading doesn't necessarily apply to your part, it just explains why there is a difference between Tresca and von Mises.

How accurate does the calculation need to be? Lot to lot variation may be huge. Can't you get shear strength data for your keys? Pay a test lab a small fee to provide the data if needed.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

Your application is for an actuator in a valve. I simply reiterate what others have stated that 0.5 should be guidance value in the absence of any test data.



" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke