Maximum allowable stem torque calculation
Maximum allowable stem torque calculation
(OP)
Hello,
New here, but long time lurker. I am trying to figure out how to calculate the MAST on a Double D ball valve stem.
Thank you,
BertremValve
New here, but long time lurker. I am trying to figure out how to calculate the MAST on a Double D ball valve stem.
Thank you,
BertremValve





RE: Maximum allowable stem torque calculation
RE: Maximum allowable stem torque calculation
Formula as follows:
Table 20, Figure 9, page 350 of 6th Ed.
Θ = angle of twist
T = twisting moment (input torque)
L = length of the bar (length of the double-D)
K = non-circular polar moment of inertia
G = modulus of rigidity of the material
r = radius of the bar (the curved part of the double-D)
w = centerline of bar to edge of the flat
h = r - w
Θ = TL/KG where:
K = 2Cr4, where C varies with h/r as follows:
For two flat sides where 0≤(h/r)≤0.8:
C = 0.7854 - 0.4053(h/r) - 3.5810(h/r)2 + 5.2708(h/r)3 - 2.0772(h/r)4
Maximum stress, τ, at the surface is:
τ = (TB)/r3, where B varies with (h/r) as follows:
0≤(h/r)≤0.6:
B = 0.6366 + 2.5303(h/r) - 11.157(h/r)2 + 49.568(h/r)3 - 85.886(h/r)4 + 69.849(h/r)5
If you want to be sure your stem doesn't twist past an allowable point (pulling a disc off of the seat on a butterfly valve for example), you want to run the first calculation. If you're just concerned about the stress, run the second calculation. After you have the result of the second calculation, compare the result to the maximum allowable stress of your design code.
To calculate your MAST...lower your input torque, T in τ = (TB)/r3 until you have an acceptable stress.