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BELL CRANKS AND WALKING BEAMS.
Bell cranks and walking beams are
levers used in rigid control
systems to gain mechanical advantage. They
are also used to change the direction of motion in
the system when parts of the airframe structure do not
permit a straight run. They are often used in push-pull
tube systems to decrease the length of the individual
tubes, and thus add rigidity to the system. A
bell crank has two arms that form an angle of less
than 180 degrees, with a pivot point where the two
arms meet. The walking beam is a straight beam with
a pivot point in the center. Bell cranks and walking
beams are mounted in the structure in much the
same way as pulley assemblies. Brackets or the structure
itself may be used as the point of attachment for
the shaft or bolt on which the unit is mounted. Examples
of a bell crank and a walking beam are shown
in figure 9-22. The two are similar in con-struction and
use. The names bell crank and walking beam
are often used interchangeably. IDLER ARMS. Idler arms are levers with one end attached to the aircraft structure so it will pivot and the other end attached to push-pull tubes. Idler arms are used to support push-pull tubes and guide them through holes in structural members. BUNGEE. Bungees are tension devices used in some rigid systems that are subject to a degree of shock or overloading. They are similar to push-pull rods, and perform essentially the same function except that one of the fittings is spring-loaded in one or both directions. That is, a load may press so hard (compression) against the fittings that the bungee spring will yield and take up the load. This protects the rest of the rigid system against damage. The internal spring may also be mounted to resist tension rather than compression. An internal double-spring arrangement will result in a bungee that protects against both overtension and overcompression.
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