5.4.2.2 Maximum Allowable Deflection For Axial Lead Parts

When axial lead parts are vibrated in an axis perpendicular to their bodies
and the PWB, they tend to react in two major motions (bending and twisting). Bending motion occurs when the part
vibrates up and down along a perpendicular axis through the body of the part.
When the part is in bending it tends to fatigue at the lead bends and the lead
ends. Twisting motion occurs when the part attempts to spin about an imaginary
point located on the part body. This motion also causes lead fatigue. The
method used to analyze these parts calculates the maximum deflection allowed
by the twisting motion and the maximum deflection allowed by the bending
motion and uses the smaller of these two values to calculate the maximum
allowable PSD input level along with the expected board deflection for equation 5-3
in Section
5.4.2.4 . The smaller value will be the limiting factor in the motion of
the part. This method then, limits the stress to the highest level obtainable
without causing structural damage to the part leads. Equation 5-7 is the formula for the maximum
allowable deflection due to twisting motion and Equation 5-8 is the formula for the maximum
allowable deflection due to bending motion.