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Current induced into the shading coils by the AC in the exciter coil demagnetizes the part of the
pole shoe around which it is wound. The AC alternately magnetizes and demagnetizes the corners of the
pole shoes, producing a rotating magnetic field which the squirrel-cage rotor follows. Shaded-pole motors
have a relatively constant speed, but their torque is quite low.
shaft horsepower. The horsepower actually available at a rotating shaft.
shaft runout (engine overhaul measurement). The amount the end of the crankshaft of an aircraft
reciprocating engine is bent. Shaft runout is measured by mounting the shaft in a set of V-blocks and
holding the arm of a dial indicator against the edge of the shaft at its end. When the shaft is rotated, the
pointer of the dial indicator shows the amount the shaft is bent or out of round.
shake (wood defect). Longitudinal cracks in a piece of wood, usually between two annual rings.
shank of a drill. The smooth part of the body of a twist drill that fits into the drill chuck. The three basic
parts of a twist drill are the shank, the body (the flutes), and the point.
shaving (metal-cutting operation). An operation in which thin layers of metal are removed from the outer
surface of the device being shaved.
shear. To cut by causing the parts to slide over each other.
shear failure (structural failure). The failure of a riveted or bolted joint in which the fastener, rather than
the material being joined, fails. The fasteners (rivets or bolts) shear off, rather than the material tearing at
the fastener hole. A joint should be designed so it will fail in shear rather than in bearing. When a joint fails
in shear, only the fasteners must be replaced; but if it fails in bearing, the torn material must be repaired.
shear nut. A thin nut used on a clevis bolt to hold the bolt in place. Shear nuts are used when the fastener
on which they are screwed is loaded in shear only. They do not have enough threads to allow their use for a
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 519
tensile load.
shear pin. A pin used to protect a piece of equipment by intentionally shearing if the equipment should
seize. A shear pin is used in the drive shaft of some engine-driven pumps. If the pump should seize, the
shear pin will break and allow the shaft to turn, but the pump is protected from further damage.
shears (cutting tools). A type of cutting tool used to cut sheet metal, paper, or cloth. Shears are similar to
scissors, except that they are usually larger and more powerful. Power shears, used to cut thick sheets of
metal, are driven by an electric motor.
shear section. A necked-down section of an engine-driven pump shaft designed to shear if the pump
should seize. When the shear section breaks, the drive can continue to turn without causing further damage
to the pump or the engine.
shear strength (sheet metal construction). The strength of a riveted joint in a sheet metal structure in
which the rivets shear before the metal tears at the rivet holes. A properly designed riveted joint will fail in
shear. The rivets will shear before the metal tears at the rivet holes.
shear stress. A type of stress that tries to slide an object apart.
sheave. A grooved wheel used as a pulley. A sheave can be used to change the direction a cable must be
pulled to move a weight and it can also be used to increase the force applied to a weight.
If a sheave is attached to a weight, the amount of force needed to lift the weight is only one half
the weight. But the cable will have to be pulled twice the distance the weight is lifted.
sheet metal. Metal that has been rolled into sheets whose thickness is less than 0.25 inch. Metal whose
thickness is greater than 0.25 inch is called metal plate.
sheet metal layout. The drawing or pattern used to cut a piece of sheet metal. The layout includes marks to
locate all of the bend tangent lines and the center marks for all of the holes to be drilled.
shelf life. The normal length of time an object or material can be expected to keep its usable characteristics
if it is stored and not used. Many products, such as batteries, rubber seals, shock cord, and certain types of
finishing materials, have a date stamped on them. This date relates to the shelf life. If these products are not
used by the date stamped on them, there is a possibility that they will not function as they should.
shell (atomic structure). The outer structure of an atom formed by electrons having the same energy level
as they spin around the nucleus. The K shell is nearest the nucleus. The L shell is next and, depending on
the number of electrons, an atom can also have M, N, O, P, and Q shells.
shell-type transformer. A type of electrical transformer encased in a steel shell. The shell completely
　
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