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

Where:

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
f_a = friction derived
f_b = friction derived
p = thread pitch [in, mm]
α = angle of thread in radial direction [degrees]
M_h = Torque applied to screw head [in-lbs, N-mm]
M_T = Torque applied to screw thread [in-lbs, N-mm]

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Stresses in screw shaft formula:

Where:

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

Maximum torque on screw head: