Plastic Screw Design Equations and Calculator

Plastics Design Resources

Theoretical equations to calculate the strength of plastic screwed joints.

Plastic Screw Design Equations and Calculator

This calculator requires a Premium Membership

Preview Plastic Screw Design Calculator

Torque Applied to Screw Head

Torque Applied to Screw Head Formula

Torque in threads

Torque in threads Formula


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 formula:

Stresses in screw shaft formula


Area of screw and Area Moment of inertia formula

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:

Maximum torque on screw head