MAST: Maximum allowable stem shear torque calculation.

[bao2ye]

Hi, Great valve engineers and experts,

I have a question and need your valuable suggestions.

I am using the basic equations to calculate the MAST of ball valve stem. However, the calculated results are lower than the test results.

The shaft drive end is 2 keyways design. I used the equation:

MAST = yield shear stress * (radius of shaft)^3/ B, where B is obtained from the Petersan's handbook.

Can you please tell me if my method is correct or not?

Thanks in advance

 

[Xun2006]

For maximum allowable shear torque calculation, I used maximum allowable shear stress, which equals shear factor(I pick 0.667, different factors from different strength criteria) times shear Limit(from ASTM standard). In reality, your material shall meet/exceeds the shear limit to qualify the ASTM material standard, and your material may fail exceed the certain strength criteria defines. That's what in my mind, someone may point out what I am wrong or add more comments.

Thanks

 

[bcd]

Hello. The method you are using will always calculate lower MAST valveus than testing for two main reasons. The formulas will identify the torque where the highest point of stress in the cross section reaches the yield limit for the material. 99+% of the rest of the cross section is still below the yield, so you can apply more torque before permanent deformation. Also, you need to go abovwe the yield in order to cause permanent damage. Somehwere near the ultimate strength.

My point is that it is very difficult to establish the MAST by calculation with any certainty of accuracy. You need to do testing and compare to calculated results to establish a correction factor for each cross section shape.

 

[sathish66633]

Hi boa2ye,
i ve done quite few no. of MAST calculations. as per my knowledge i ve been using the allowable stress @ the design temp.(which would be roughly 65% SMYS as per API std.)you can even take up the values from ASME BVP. Again, the C/S of the stem plays a major role (If it is not perfectly cylindrical as in special cases). as such constrains, it is advisable to convert it the equi. area rectangle and calculate the torsion constants.

 

[al1979]

Use formula in Roark's handbook with 2 key-ways. It will give more closer results.