impulse coupling

impulse starter

impulse starterclick for a larger image
Initially the body of the starter is rotated while the rotating magnet remains stationary. This results in winding of the spring in the coupling. At the appropriate time the fly weights are released and the spring unwinds. The rapid movement of the magnet results in a better spark. As soon as the engine starts running, the flyweights are held in release position by centrifugal force and the magnet fires in the normal advance position. However, during engine start up the retard spark is produced when the magneto rotation is held back by the impulse starter of inertia coupling.
A spring-loaded clutch device between the engine drive and the magneto spindle that helps increase momentarily the rate of rotation of the magnet, which increases the voltage produced. An impulse starter is required while starting the engine. Also known as impulse coupling.
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During a 500-hour magneto inspection, the impulse coupling was discovered to have cracks in its flyweight base plate.
Impulse coupling found cracked at advance stop notches of base plate.
A vibrator switch triggers a rapid firing sequence that causes there to be nine to 12 sparks from the impulse coupling rather than one--creating a longer spark sequence that helps ease hot (and cold) starting.
Magneto impulse couplings disengage at an RPM as low as 180 and Sky-Tec's starters will turn up at about 120-140 RPM--so there's not a lot of room for improvement.
I had the idea it was about a 6 hp with an external magneto and an impulse coupling and weighed about 150 pounds.
Eventually, impulse coupling made things a lot easier.
Consider that hand propping an engine could only be possible with impulse coupling.
The stiff/frozen bearing was causing the impulse coupling to be ineffective.
The starter, or in the case of an airplane with no electrical system, the human arm, is required to swing the propeller and crankshaft, engaging an impulse coupling typically built into the left magneto.
While doing so will prevent the impulse coupling from winding up and snapping, Hughes cautioned doing so could damage vanes in the engine-driven vacuum pump, especially if it's a dry pump.