5.4.3 Maximum Allowable PSD Level For Tombstone Type Parts Vibrated in the Z Axis
5.4.3 MAXIMUM ALLOWABLE PSD LEVEL FOR TOMBSTONE TYPE PARTS VIBRATED IN THE
One part type that is critical in bending is a large tombstone type part.
In most cases, they are bent over to lower the center of gravity which reduces
some of the stresses associated with their use. This technique is beneficial
in some ways but the high center of gravity still tends to cause motion of
this part during Z axis vibration. The tombstone type part is bent in the
shape shown in Figure 5-4. The center of gravity of these parts is normally
high off the PWB and the motion is in a plane perpendicular to the body of the
part. If the part body is bonded to the PWB, it does not pose a vibration
Figure 5-4. Tombstone Type Parts
The part is not treated like other parts that have forces applied
perpendicular to the PWB, because the stresses are not related to PWB bending.
The first step in calculating the stresses on these parts is to determine the
motion that they will experience.
Due to its odd shape, calculations must determine the position of the part
in the X and Z axes during its oscillations. Those values will be designated
as XT and ZT, (See Equations 5-1l and 5-12
respectively). Because this is not a purely liner motion, a factor for
ZT2 must be calculated using Equation 5-13. The parameters for
these calculations are the weight of the part in pounds (W), the height from
the PWB to the lead bend in inches (l2) the length from the lead
bend to the part edge in inches (l1), the length from the edge of
the part to its center of gravity in inches (a), the moment of inertia (I) for
the part, and (d) the diameter of the lead.
The next step in this process is to calculate the deflection of the center
of gravity of the part due to a l G (static) load, as shown in Equation
5-14. This depicts the deflection of the part due to its own weight.
The natural frequency of the part can then be calculated using Equation
The moment in the lead wires produced by its body weight can now be
calculated using Equation 5-16. Using the moment, the stress on the
lead wires can be calculated using Equation 5-17.
Finding the magnification factor (Q) of the lead wires is the next step. Q
is dependent upon the damping ratio () and is
approximately 62.5 for part lead wires. If the damping ratio is known,
Equation 5-18 can be used to determine the actual magnification factor.
The G level on the part can now be calculated. Using Crandall’s equation
and wo equal to .04, Gout is calculated as shown by
The stress on the lead wire can now be calculated by Equation
With the stress level on the lead wires now determined, the maximum
allowable G level that this part can withstand (Gmax) can be
calculated using Equation 5-21. The endurance limit of the lead wire material
is provided in Appendix B.
Using the Gmax value just calculated, the maximum allowable PSD
input level can be calculated using Equation 5-22.