Snap-Fit Joints for Plastics - A Design Guide - MIT

Torsion Snap Joints C
Deflection force
A force P is required to deflect the lever arm
by the amount y (1,2). The deflection force can
act at points 1 or 2. For example see Fig. 21.
In this case,
P 1l 1 =P 21 2 = �GI p (x2)*
r
where
G = shearing modulus of elasticity
� = shear strain
IP = polar moment of inertia
= � r4
; for a solid circular cross section
2
*Note: The factor 2 only applies where there
are two torsion bars, as in Fig. 21.
The shear modulus G can be determined
fairly accurately from the secant modulus
as follows:
G=
where
20
Es 2(1+� )
ES = secant modulus
� = Poisson's ratio
Example of snap-fitting rocker arm (flexure and torsion about the Y axis)
Page 19 of 26 Snap-Fit Joints for Plastics - A Design Guide