Maximum Screw Torque

Equation for screw survival

Engineering Assumptions:

For screw root diameters assuming a typical 1 inch screw with 3:1 ACR where the feed channel depth is 0.180 for 8 L/D’s, straight compression section 8 L/D’s long (where the next to last flight will then measure 0.091 channel depth, and the meter 8 L/D’s long at 0.060 inches then,

as given by Machinery’s Handbook, page 263,

T = Ss x Zp

Where, T is the torsional or twisting moment in inch-pounds.
Ss is the allowable shearing stress in pounds per square inch.
Zp is the polar section modulus.

Zp for a circular cross-section, page 266, is given as: 0.196 D3.

There is little published data on the torsional yield stress. The industry rule of thumb is that the yield stress is about two thirds the tensile yield stress. Spirex has published their use of 60% rule.

Our standard screws are made from 135M from Crucible Materials Corporation with a published tensile strength at 145,000 psi and yield strength at 125,000 psi for a hardness of 28-32 HRC.

So, using the conservative 0.60 figure,

0.60 x 125,000 x 0.196 D3 = T

For the following screw root diameters, T =

0.640 = 3,851 inch-pounds
0.810 = 7,805 inch-pounds
0.880 = 9,996 inch-pounds