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attached to the component to be moved by the control rod.
The control rod end has a socket into which the sphere fits, and the sphere rotates on a film of
grease. The rod end can be out of alignment by a few degrees without affecting the operation of the bearing.
Self-aligning bearings are used in control rods installed in helicopter rotor systems.
self-centering chuck (machine tool component). A type of chuck used on a drill motor or lathe that has
all its jaws connected in such a way that they move together. When a drill or piece of work is placed in the
chuck and the jaws tightened, all of the jaws move inward together and hold the drill or the work centered
in the chuck.
self-demagnetization. The process in which the magnetic fields surrounding a piece of magnetized
material try to demagnetize it. If a permanent magnet sits for a long period of time, it will partially
self-demagnetize. But if a keeper, a piece of soft iron, is placed across its poles, the lines of magnetic flux
will not spread out from the poles, and the magnet will self-demagnetize far less than if no keeper were
used.
self-excited generator. An electrical generator whose field coils are supplied with current produced in the
armature. The voltage regulator for a self-excited generator is a variable resistor in the field circuit between
the armature and the field coils or between the field coils and ground. When the resistance in the field
circuit decreases, the field current increases, and the generator output voltage increases. Most aircraft
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 511
generators are self-excited, but DC alternators are not.
self-extinguishing. A combustible material is said to be self-extinguishing if it burns when it is exposed to
a flame or to a high temperature, but the burning process automatically stops as soon as the flame or source
of high temperature is removed.
self-healing capacitor. An electrolytic capacitor with the ability to automatically restore the oxide film
dielectric to the plate if the film is ever punctured by too high a voltage.
self-induction. The characteristic of an electrical conductor that causes a counter electromotive force
(CEMF, or back voltage) to be built up in it by alternating current flowing through it. The expanding and
collapsing magnetic field caused by the alternating current cuts across the conductor and induces the CEMF
in it. The polarity of the self-induced voltage is always opposite that of the voltage in the original circuit.
self-locking nut. A type of nut with a built-in locking device that makes the nut tightly grip the threads of
the bolt. This locking device may be a fiber collar, with a hole slightly smaller than the major diameter of
the bolt threads, or it may be a portion of the threads in the nut that is slightly out of phase with its main
threads.
A self-locking nut must be turned onto a bolt with a wrench and grip the bolt threads tightly
enough that vibration will not cause it to loosen or back off.
self-tapping screw. A type of screw with sharp threads that cut their own matching threads when screwed
into soft metal, wood, or plastic. Self-tapping screws are especially suited for holding together sheets of
thin metal, and for this reason, they are often called sheet-metal screws, or PK screws, after Parker-Kalon,
one of their major manufacturers.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 512
Selsyn system. The registered trade name for a DC synchro system. Mechanical movement of the
transmitter shaft moves a two-arm wiper across a tapped, circular, variable resistor. Direct current enters
the transmitter through the arms of the wiper and flows through the resistor out to the indicator. Here, the
current flows into three sections of a coil wound around a ring-shaped core. A small permanent magnet on
the pointer shaft is free to rotate inside the indicator coil.
As the wipers move across the transmitter resistor, the current in the three sections of the indicator
coil change. The changing current changes the magnetic field produced by the coil so the magnet to which
the pointer is attached moves in exactly the same way as the shaft in the transmitter.
selvage edge (fabric). The woven edge of fabric used to prevent the material unraveling during normal
handling. The selvage edge, which runs the length of the fabric parallel to the warp threads, is normally
removed from materials before they are used in composite construction.
semi. A prefix meaning partially or part (usually half) of something. A semicircle is half of a circle, and a
semiannual event is something that takes place every half year (twice in a year).
semiautomatic operation. An operation that is partially automatic. Some manual input is needed to start
　
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