The eyes contain a pressed-in bearing. The rods are
generally hollow and neck down to a smaller diameter
at each end where the fittings are attached. One or
both of the fittings are screwed into the necked
portion of the rod, and are held in place by locknuts.
When only one stem is adjustable, the stem of the
other eye fitting is riveted into the neck at its end of
the rod. A hole is drilled into the threaded neck of a
push-pull rod for inspection to ensure that the stem
has engaged a safe number of threads. The stem must
be visible through the hole. Push-pull rods are
generally made in short lengths to prevent bending
under compression loads and vibration.
Push-pull rod linkage must be inspected closely
for dents, cracks, and bent tubing. Damaged tubes
may have to be replaced. End fittings are checked for
damage, wear, and security of attachment. Worn or
loose fittings must be replaced.
When you are replacing a damaged push-pull
tube, the correct length of the new tube may be
obtained by loosening the check nut and turning the
end fitting in or out, as necessary. When the push-pull
tube has been adjusted to its correct length, the check
nut must be tightened against the shoulder of the end
fitting. Normally, only one end of a push-pull rod is
adjustable. The adjustable end has a hole (witness
hole) drilled in the rod. The hole is located at the
maximum distance the base of the end fitting is
allowed to be extended. If the threads of the end
fitting can be seen through this hole, the end fitting is
within safe limits.
When you are attaching push-pull rods with ball
bearing end fittings, the attaching bolt and nut must
tightly clamp the inner race of the bearing to the bell
crank, idler arm, or other supporting structure. Nuts
should be tightened to the torque values listed in the
aircraft MIM.
After installing a new push-pull rod in a flight
control system, the control surface must be checked
for correct travel. Procedures for accomplishing this
are described later in this chapter. If the travel is
incorrect, the length of the push-pull fod must be
readjusted.
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.
Figure 9-22.—Bell crank and walking beam.
9-28
