When a cable is found to be unserviceable and a
neck of a push-pull rod for inspection to ensure that the
spare cable is not available, an exact duplicate of the
stem has engaged a safe number of threads. The stem
damaged cable may be prepared. This will involve
must be visible through the hole. Push-pull rods are
cutting a length of cable to the proper length, attaching
generally made in short lengths to prevent bending
the necessary end fittings, and testing the assembly.
under compression loads and vibration.
To determine the proper length to which the new
Push-pull rod linkage must be inspected closely
cable will be cut, you should first determine the overall
for dents, cracks, and bent tubing. Damaged tubes may
length of the finished cable assembly. This may be
have to be replaced. End fittings are checked for
accomplished by measuring the old cable assembly or
damage, wear, and security of attachment. Worn or
by reading the measurements provided in the MIM for
loose fittings must be replaced.
the aircraft concerned.
When you are replacing a damaged push-pull tube,
REPLACEMENT.Replacing cables in the
the correct length of the new tube may be obtained by
loosening the check nut and turning the end fitting in or
aircraft, especially those routed through inaccessible
spaces, can be difficult. One method is to secure a
out, as necessary. When the push-pull tube has been
snaking line to the cable to be replaced, remove the
adjusted to its correct length, the check nut must be
pulleys from the brackets, and pull out the old cable
tightened against the shoulder of the end fitting.
while pulling the snaking line into the cable system run
Normally, only one end of a push-pull rod is adjustable.
at the same time. Attach the new cable assembly to the
The adjustable end has a hole (witness hole) drilled in
snaking line, and pull the snaking line out to pull the
the rod. The hole is located at the maximum distance
new assembly into place. Replace the pulleys and
the base of the end fitting is allowed to be extended. If
attach the new cable in the system.
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
Quick disconnects are used in cable systems that
tightly clamp the inner race of the bearing to the bell
may require frequent disconnecting. One type of quick
crank, idler arm, or other supporting structure. Nuts
disconnect is made with steel balls swaged to the ends
should be tightened to the torque values listed in the
of the cable, slipped into a slotted bar, and secured with
spring-loaded sleeves on each end of the bar. Figure
16-21 shows the procedures for disconnecting and
After installing a new push-pull rod in a flight
connecting this type of quick-disconnect fitting.
control system, the control surface must be checked for
correct travel. Procedures for accomplishing this are
RIGID CONTROL SYSTEMS
described later in this chapter. If the travel is incorrect,
the length of the push-pull rod must be readjusted.
Rigid control systems transfer useful movement
through a system of push-pull rods, bell cranks,
Bell Cranks and Walking Beams
walking beams, idler arms, and bungees. The simplest
rigid control system may consist of push-pull rods and
Bell cranks and walking beams are levers used in
bell cranks only.
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
Push-pull rods are rigid tubes equipped with eye
tube systems to decrease the length of the individual
fittings at each end or with a clevis fitting at one end
tubes, and thus add rigidity to the system.
and an eye fitting at the other. The eyes contain a
A bell crank has two arms that form an angle of less
pressed-in bearing. The rods are generally hollow and
than 180 degrees, with a pivot point where the two arms
neck down to a smaller diameter at each end where the
meet. The walking beam is a straight beam with a pivot
fittings are attached. One or both of the fittings are
point in the center. Bell cranks and walking beams are
screwed into the necked portion of the rod, and are held
mounted in the structure in much the same way as
in place by locknuts. When only one stem is adjustable,
pulley assemblies. Brackets or the structure itself may
the stem of the other eye fitting is riveted into the neck
be used as the point of attachment for the shaft or bolt
at its end of the rod. A hole is drilled into the threaded