MIL-HDBK-1038
Infrared control units transmit pulsed, encoded, waves of light to the
crane. There are sensors, decoders, and interface modules on the crane to
participate with the motor control equipment. The infrared signal's wavelength,
between 780 and 105 nanometers, is impervious to radio frequency interference. One
of the benefits of using infrared transmitters is that frequency allocations and
assignments are not required.
Two infrared transmitters can operate side-by-side without cross-talk
interference or one transmitter can operate multiple cranes. The range of the
infrared transmitter is 100 to 200 feet, depending on the model and manufacturer.
An infrared transmitter cannot operate a crane when the transmitter is directed
away from the crane or is out-of-range.
4.5.26
Radio Frequency Links. Radio frequency (RF) insulating links are
installed between the crane lifting hook and the hoist block to protect personnel
in contact with the hook or the load under the hook from:
a)
Voltages induced in the crane structure or wire ropes from radio
frequency (RF) transmitting antennas.
b)
Internal crane electrical faults.
c)
Inadvertent contact between the crane structure or wire ropes and
external power sources.
The metal spoke end pieces consist of a series of spokes located radially
on the body of the unit. The metal spoke end pieces are fabricated from high
strength steel, conforming to ASTM-A148, Grade 80-50, and are cadmium plated.
High tensile strength glass filament impregnated with epoxy resin is wound between
the end pieces. A layer of this same material is circumferentially wound over the
longitudinal strands to induce equal tension in all strands. The metal spoke end
pieces are sufficiently separated to provide the RF attenuation and high-voltage
rating required. The link is encapsulated with a polyurethane elastomer and also
has a polyurethane jacket, tinted orange. Figure 23 shows a cross sectional view
of a typical RF link.
4.5.26.1 RF Link Gauges. Each link should have an accompanying gauge. The gauge
is used to verify that elongation of the glass fiber has not occurred. Prior to
any load ever having been applied to the link, the gauge was used to indent two
trammel points; these points are used with the gauge to verify that elongation has
not occurred.
4.5.27
Grounding and Bonding.
The National Electrical Code has requirements for
both grounding and bonding.
4.5.27.1 AC System Grounding. AC systems are required in NEC Section 250-5 to be
grounded where the maximum voltage to ground on the ungrounded conductors is 150
Volts or less. On cranes, this would involve certain control circuits operating
at less than 50 volts, control circuits operating at more than 50 volts which
include wiring external to their panel's enclosure, and 208Y/120 VAC utilization
systems. The grounded conductor of the system must be connected to the structure
between the transformer and the first system disconnecting means or overcurrent
device.
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