MIL-HDBK-1038
The pendent pushbutton station is suspended by a stainless steel wire
rope. This eliminates any carrying of the weight of the station by the electrical
cable used.
A festooned system, similar in design to a bridge-to-trolley
electrification system but running on a separate track, is frequently used with a
pendent pushbutton station. In this application a specially designed lead trolley
is free to be pulled by the crane operator or it may be motor driven if the track
is more than 40 feet above the floor. Connections between the festooned and
pendent conductors are made in a junction box on the lead trolley. If the pendent
pushbutton station is not suspended from a festooned system, it would usually be
suspended from the crane trolley. The disadvantage of this is that it limits
operator mobility.
4.5.25.2 Master Switches. Master switches are used in operator's cabs on bridge,
portal, floating, and container cranes to control the speed and direction of the
crane drives. While having the master switches spring returned to OFF position is
preferable, quite often they are not, even if they are infinitely variable in
movement.
The "dead-man" feature is incorporated into master switches by having a
mechanism which must be continually operated. This mechanism may be a pushbutton
on the top of the handle or a grip added to the side of the handle. Future
designs may incorporate a passive conductive or capacitive circuit in lieu of a
switch mechanism. To reduce operator fatigue, only one device has to be operated
at a time to enable all master switches to be functional. The operator
attentiveness monitoring "dead-man" circuitry also incorporates safeguards to
ensure that all master switches are in the OFF position before it can be
energized.
4.5.25.3 Radio Controls. In addition to master switches in cabs and pendent
pushbutton stations, portable transmitter units are used to control cranes. These
control units transmit radio signals to the crane; there is receiving equipment on
the crane and additional equipment to interface with the motor control equipment.
Most manufacturers digitally pulse encode the radio signal. This makes
the system less susceptible to interference due to other equipment. However, if
interference does occur, the crane will be shut down and remain out of operation,
from the radio control equipment, as long as the interference occurs. Functional
decoding equipment is installed on the crane.
The range of the portable transmitter is less than 650 feet. A unique
"address" signal is also transmitted to ensure that only one transmitter can
control a given crane. This permits several cranes in one building to be radio
controlled simultaneously.
In industrial applications, the radio signal is transmitted on a
frequency between 72 and 76 megahertz. However, this frequency band is reserved
for industrial use and is not available for Government use. Navy activities can
obtain dedicated frequencies between 30 and 50 megahertz. Most manufacturers can
provide transmitting equipment operating in the Navy's frequency band.
4.5.25.4 Infrared Controls.
Another type of portable transmitter control unit
emits infrared light.
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