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Plastic Screw Design Equations and Calculator

Plastics Design Resources

Theoretical equations to calculate the strength of plastic screwed joints.

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Torque Applied to Screw Head

Torque in threads


F = axial force in screw [lbs, N]
R = radius of screw head contact surface [in, mm]
r = pitch radius of thread [in, mm]
f1 = friction between screw head and part
f2 = friction between threads
fa = friction derived
fb = friction derived
p = thread pitch [in, mm]
α = angle of thread in radial direction [degrees]
Mh = Torque applied to screw head [in-lbs, N-mm]
MT = Torque applied to screw thread [in-lbs, N-mm]

Stresses in screw shaft:


ro = outside radius of screw core [in, mm]
ri= inside radius of hollow screw; (solid: ri = 0) [in, mm]
σy = tensile yield strength [psi, N/mm2]
A = Area of Screw [in2, mm2]
Ip = Polar moment of Inertia [in4, mm4]

Maximum torque on screw head:

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