spaghetti (electrical insulation). The commonly used name for a type of insulating tubing slipped over the end of an electrical wire. The tubing is put on at the point the wire connects to a terminal, where it is soldered to a component, or where the wire is spliced. Modern spaghetti is often made of some form of polyvinyl chloride tubing, and it can be either transparent or color coded. The name spaghetti comes from the hollow tube of pasta the tubing resembles.
spalling. A type of damage in which chips are broken from the surface of a case-hardened material such as a bearing race. Spalling takes place when a bearing race is put under a load great enough to distort the softer inner part of the metal and cause the hard, brittle surface to crack. Once a crack forms in the surface, chips break out.
span loading (aerodynamic measurement). An aerodynamic measurement found by dividing the span of an airfoil by the total weight the airfoil supports.
spanner. The name generally used by the British for a wrench. In American usage, a spanner is a special type of wrench with a hook-shaped arm and a hole, pin, or projection in its hooked end. A spanner is used to turn ring-shaped nuts.
spanner nut. A special type of round nut with holes, notches, or projections around its periphery. Spanner nuts are turned with a hook spanner or pin spanner.
span of an airfoil. The length of an airfoil measured between its tips.
spar (airplane wing component). The main spanwise, load-carrying structural member in an airplane wing.
spark. An electrical discharge between two electrodes or points within a circuit having a difference of electrical potential. A spark normally has a very short duration and produces intense heat, light, and electromagnetic radiation that cover a wide band of frequencies. A spark is used inside the cylinder of an internal combustion engine to ignite the fuel-air mixture. A discharge of lightning is an extremely large spark.
spark coil (reciprocating engine ignition system component). A step-up transformer used to produce a high voltage. This high voltage causes a spark to jump across the electrodes of a spark plug inside the cylinder of a reciprocating engine. The primary winding of the spark coil consists of comparatively few turns of heavy copper wire, wound over a soft iron core. When primary current flows through this winding and through the breaker points, a magnetic field is formed that extends out beyond the secondary winding.