therefore, greater ability to pass the drop
(2) The gas-operated firing mechanism (fig.
test.†. The disadvantage of aluminum
firing pins is that the tips deform when
enters through the port and pressure
they strike the primer. The majority of
rapidly begins to build up behind the firing
aluminum pins in present designs are
pin. When the pressure behind the pill is
used in conjunction with firing plugs,
sufficient to shear the firing pill shear pin,
where the large diameter of the firing pin
the pin shears and the firing pin is
end does not determine primer indent.
propelled toward the cartridge where it
strikes the primer. The firing pin velocity
(4) Firing pins are designed with large
is affected more by the force required to
diameters to increase the force acting
shear the shear pin than the maximum
against them. However, a means of
force (pressure) attained in the system,
assembling the firing pin in the device is
since the maximum force against the
necessary.
For example, 1/2-inch
firing pin usually is attained after the firing
diameter firing pins may have to be
pin has completed its travel. For this
assembled from the cartridge end since
reason, the selection of the shear pin
the gas entry port is sometimes as small
material and shear pin diameter are vital
as 5/16 inch.‡: The smaller firing pills may
to the design of gas operated firing
be inserted through the gas entry port.
mechanisms. Many propellant actuated
(5) Firing pins use O-rings to prevent the gas
devices use the same combination of
entering the device from passing the firing
firing pin and shear pin. It was, therefore,
pin. However, the O-ring must be so
considered advisable to include (in table
positioned that in the assembling the firing
pin in the firing pin housing or guide and
pins that are used widely. The lengths of
during operation, the O-ring does not pass
the firing pins are not common to all, but
the shear pin hole, or the O-ring may be
the length-to-diameter ratios listed in table
torn in assembling the device or in
functioning.
c. Mechanically Operated Firing Mechanisms.
Table XII. Firing Pins and Shear Pins Used in
(1) The firing mechanism must be designed
Propellant Actuated Devices
to deliver sufficient energy to the primer
to provide the high reliability of firing
Firing pin
Shear pint
Primer
necessary in propellant actuated devices.
diameter
to be
This energy must be delivered without
struck
exceeding the stipulated range of lanyard
(in.)
diameter
Units in which used
pull. Also, the length of lanyard travel
(in.)
must provide sufficient over-travel to
0.500 ................
0.046
72M
Thrusters, M1, M2, M5,
assure release of the firing pin and to
T16.
permit separation of the lanyard from the
0.500 ................
0.040
50M
Thrusters, T7, T8.
mechanism.
0.343 ................
0.040
M42
Initiators, M6, M10.
(2) The
mechanically
operated
tiring
0.343 ................
0.040
M29A1
Initiators, T26, T31.
mechanism (fig. 35) of a typical initiator
†Force required to shear copper pins: 0.046 dia = 48 4 lb,
operates in the following manner: The
0.040 dia = 41 5 lb.
firing pin is locked to the sear (pin) by
three steel balls. When the sear (pin) is
(3) The shear pins indicated in table XII are
pulled, a spring contained in the housing
made of electrical quality copper, while
is compressed and exerts a force on the
the firing pins are made of stainless steel,
firing pin. As the firing pin enters the
or alloy steel. Aluminum firing pins have
relieved section of the housing, the steel
been used in propellant actuated devices
balls move outward and allow the sear
because of their lighter weight and,
†Propellant actuated devices must be dropped 6 feet onto a concrete block, without creating sufficient shock to shear the
firing pin shear pin.
‡A small diameter ridge is provided between the gas entry port and the Bring pin to limit the entry of the hose fitting,
preventing its contact with the firing pin.
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