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personnel must check constantly for failures and for
signs of approaching failure in aircraft structural
units. Stress may take the form of compression, torsion,
tension, bending, or shear or may be a combination
of two or more of these forces (Figure 1-4):
•
•
•
Compression is resistance to being pushed
together or crushed. Compression is
produced by two forces pushing toward each
other in the same straight line. The landing
struts of an aircraft are under compression
after landing.
Torsion is resistance to twisting. A torsional
force is produced when an engine turns a
crankshaft. Torque is the force that produces
torsion.
Tension is resistance to being pulled apart or
stretched. Tension is produced by two forces
pulling in opposite directions along the same
straight line. Pilots put the cables of a control
system under tension when they operate the
controls.
Bending is a combination of tension and compression.
The inside curve of the bend is
under compression, and the outside curve is
under tension. Helicopter main rotor blades
are subjected to bending.
Shear is the stress exerted when two pieces of
•
•
metal fastened together are separated by sliding
one over the other in opposite directions.
When force is applied to two pieces of metal
fastened together by rivets or bolts, sliding
them across each other, the rivets or bolts are
subjected to shear. Stress will cut off the bolt
or rivet like a pair of shears. Generally, rivets
are subjected to shear only, but bolts may be
stressed by shear and tension. There is internal
shear in all parts being bent such as the
skin of sheet metal structures.
LEVERS AND MOMENT OF FORCE
A lever is a useful device found in tools such as jacks,
shears, wrenches, and pliers. To use tools and
balancing procedures correctly, the repairer needs to
understand moment of force (amount of leverage).
1-3
FM 1-514
Levers
Levers are classified as three types according to the
position of the applied force (effort), the resistance,
and the fulcrum (the pivot point) (Figure 1-5). In
Type 1 the fulcrum is located between the applied
effort and the resistance. In Type 2 the resistance is
located between the fulcrum and the applied effort.
In Type 3 the applied effort is located between the
R
MA =
E
–
MA = mechanical advantage
R = resisting force (weight moved)
E = effort (applied force)
resistance and the fulcrum.
R 4
MA = – = – = 4
E 1
1-4
Mechanical advantage is the ratio between the resistance
and the effort applied to a lever. This is expressed
in the following formula:
Proper use of mechanical advantage enables a relatively
small force to overcome a larger resisting force
by applying the effort through a longer distance than
the resistance is moved. For example, to lift a 4-pound
weight (R) which is 2 inches from the fulcrum of a Type
1 lever requires 1 pound of effort (E) applied 8 inches
from the fulcrum. The mechanical advantage of this
lever would be as follows:
Thus, the applied effort in the example would move
through a distance that is four times greater than the
distance the resistance would move.
Moment of Force
A moment of force is the product of a force or weight
and a distance. To find a lever’s moment of force,
multiply the applied effort by the distance between
the point of effort application and the pivot point
FM 1-514
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•
(fulcrum). If the moment of force of the applied
effort equals the moment of force of the resistance,
the lever will balance. If an object to be balanced on
a Type 1 lever weighs 4 pounds and is located 2 inches
from the fulcrum, it could be balanced by a 2-pound
effort applied 4 inches from the fulcrum on the opposite
side or by a l-pound effort applied 8 inches
from the fulcrum.
VIBRATION
Any type of machine vibrates. However, greater than
normal vibration usually means that there is a malfunction.
Malfunctions can be caused by worn bearings,
out-of-balance conditions, or loose hardware.
If allowed to continue unchecked, vibrations can
cause material failure or machine destruction.
Aircraft – particularly helicopters – have a high
vibration level due to their many moving parts.
Designers have been forced to use many different
dampening and counteracting methods to keep
vibrations at acceptable levels. Some examples are —
Driving secondary parts at different speeds to
　
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