Figure 38-4. Fifth Wheel.
similar manner, except that it is controlled by the
operator from the truck tractor cab.
The height to which the landing gear lifts the front end of
the semitrailer above the ground can be varied by
38-4. Landing Gear. The landing gear (fig. 38-5)
adjustable clamps at the top of the two leg members.
is a retractable support under the front end of a
This adjustment is necessary if the semitrailer is to be
semitrailer to hold it up when it is uncoupled from the
used with truck tractors having different rear end heights.
truck tractor. The two leg members of the landing gear
usually are attached to the underside of the trailer
When semitrailers are coupled and uncoupled, it is
chassis by pivot pins.
These leg members are
important that the landing gear be coordinated with the
connected together by cross braces to resist any side
fifth-wheel lock. If the landing gear is elevated before
strains. Two small steel wheels, mounted on the end of
the fifth-wheel connection is locked fully, the front end of
these leg members, facilitate moving the semitrailer
the semitrailer will drop to the ground when the truck
about when it is uncoupled from the tractor. Bracing
tractor is driven away, with the possibility of damaging
members connect the lower end of the leg members to a
both the load and the semitrailer. Before the semitrailer
movable bracket containing a nut that rides on a
is uncoupled, the brake coupling should be discon-
wormshaft located lengthwise between the side
nected so that the semitrailer brakes are applied to
members of the trailer frame. Rotation of the wormshaft
prevent it from moving. Automatic controls sometimes
moves the bracket forward or backward (depending on
are used to coordinate the desired features. Auxiliary
the direction of rotation) and consequently lowers or
locking pins strike against the lower fifth-wheel plate on
raises the landing gear wheels. The landing gear (fig.
the truck tractor and lock the fifth-wheel connection,
38-5) is shown in its lowered position (semitrailer
preventing the trailer from being separated until the
uncoupled from truck tractor).
landing gear is lowered fully. These auxiliary locking pins
are connected by an actuating rod in such a way that
The wormshaft is supported by bearings in a housing
they cannot be raised to unlock the fifth-wheel
mounted on the underside of the semitrailer chassis. It
connection until the landing gear is lowered fully and the
is rotated by means of a set of bevel gears at the forward
semitrailer brakes are applied.
end of the housing.
These gears are turned by a hand-operated lever at the
end of a shaft that extends out to the side of the