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MIL-T-62255C(AT)
deformation of the sub-frame and the vehicle chassis frame rails. The vehicle chassis frame and
fifth wheel sub-frame assembly and its mounting shall be capable of accepting the loads imposed
during coupling, uncoupling, ascending and descending ramps up to 14 degrees from horizontal
and the effects of weight transfer up and down the specified grades and inclines and have
sufficient structural strength that when a 20 foot, 20 ton container is dropped at the front of the
40 foot semitrailer, 6 inches, that the truck tractor structural member will accept the moments
imposed. Calculations shall be furnished to the procuring activity.
3.5.2 Fifth wheel hydraulic system. The fifth wheel unit shall include a hydraulic system
and mechanism arranged to elevate the fifth wheel; shall incorporate a hydraulic or pneumatic
system to open and close the semi-automatic lock for SAE J700 kingpin; shall include a device
to prevent false locking in the lock; shall embrace remote control to permit all fifth wheel
operations, including operation of the power take-off from within the vehicle cab; and shall
include all components, such as power take-off, pump, hydraulic reservoir and suction and return
filters to remove contaminants (filters shall be accessible for service), and hose and fittings, to
comprise a complete installation. Means shall be provided to prevent payload descending in
event of hydraulic failure, and a manual means provided to lock and unlock the fifth wheel
assembly and to bleed down the elevated fifth wheel. When bleeding down the fifth wheel
hydraulic system, the hydraulic fluid shall return to the reservoir. The pneumatic system, if
furnished, shall be provided with an alcohol aspirator with an unbreakable transparent container
and capable of operation at minus 50F. Fluid shall not leak out the vent of the reservoir when
the fifth wheel is down and with payload. Instruction plate shall be provided.
3.5.3 Clearance. The clearance from the centerline of the kingpin to the pogo stick type hose
tender when furnished, with the fifth wheel at all elevated positions, shall be not less than
70 inches and not more than 98 inches. The landing wheel clearance from the center line of the
kingpin to a vertical plane on the outside edge of the rear tires, with the fifth wheel in the lowest
position, shall be not more than 57 inches, and the chassis frame rails shall be not more than
30 inches. Under no condition shall the kingpin be positioned to the rear of the centerline of the
axle. The unladen level height from the ground to the top of coupler plate of the installed fifth
wheel unit with the unit in its lowest position shall be 48 inches plus 1 inch minus 0 inch.
Actuation through the full lifting range shall not cause the center of the kingpin lock to shift
more than 10 inches, measured along the longitudinal centerline of the vehicle (see Table II).
33

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