A “Toolbox” for Forensic Engineers

418 Forensic Materials Engineering: Case Studies
voltage, tripping the set of springs that are finely balanced in a metastable
state of mechanical equilibrium. This state is maintained by a friction force
between a bent arm and a bent leaf spring, so that any small movement of
another rigid arm held in a solenoid will trip the device and cause the bent
leaf spring to open the circuit.
13.4.4 Modeling the Technical Effect
It is helpful to redraw the action sequence shown in Figure 13.17 in a simplified
diagrammatic way, so that the technical effect is clear and transparent.
In addition, the balance of forces in the primed device should show exactly
how the device is held in a metastable equilibrium. This would be the starting
point, in any case, for quantitative analysis during the detailed design work
which should precede prototyping, followed by tooling up for production
models. The device is shown in schematic form in Figure 13.20, with the
balance of forces in the primed device shown in Figure 13.21. The lower
diagram identifies the principal forces acting against one another when the
contacts are touching and the external circuit is complete. The schematic
may then be compared with the situation as represented by the actual embodiment
presented in the TRC patent-in-suit (Figure 13.17). Because various
parts of the framework within which the device sits have been added, there
are several extra forces shown, which are primarily reaction forces at fixing
points. (Their addition, however, does not change the basic balance exhibited
by Figure 13.21.)
A more comprehensive diagram of the TRC patent is shown in
Figure 13.22. The solenoid, which holds an armature within it when current
is flowing and the voltage is normal, is linked mechanically to the two leaf
springs which lock together at a fulcrum. One of the leaf springs also makes
an electrical contact, which is broken when the armature is released, following
Figure 13.20 Schematic of TRC device primed and showing the coil spring
and two leaf springs that power the action (inverted).