Interrupter Gear

An interrupter gear is a device used on military aircraft and warships in order to allow them to target opponents without damaging themselves.

The term covers two related technologies: the first, more accurately referred to as synchronization gear, or a gun synchronizer, is attached to the armament of a tractor-type craft so that it can fire through the arc of a spinning propeller without the bullets striking the blades. Introduced during the First World War, the gun synchronizer was a significant development in the history of aerial warfare and remained in operational use until the Korean War, after which the universal adoption of propeller-less jet aircraft rendered such gears unnecessary.

The other, true interrupter gear stops the firing of the machine gun when some part of the aircraft is in the way. For much of the early history of the fighter aircraft this was limited to the propeller. This would change with the introduction of gun turret mounts on bomber aircraft.

Though their effects were the same, there was a subtle difference between the concept of the interrupter and the synchronizer. A machine gun fitted with interrupter gear had the trigger normally enabled and the interrupter mechanism would disable the trigger when a propeller blade was in the way. A machine gun fitted with synchronization gear had the trigger normally disabled and the synchronizer mechanism would enable the trigger when the propeller was clear, essentially with the rotating parts of the engine (the crankshaft and other parts connected to it with an inline engine, the crankcase with a rotary engine) firing the gun. In reality, the technical difficulties associated with reliably halting, or co-ordinating, the firing of a Maxim-type machine gun meant that no working interrupter system was ever developed — all successful implementations used the concept of synchronization.