From what I remember reading about it, it’s a cam on the prop shaft. When the lobe on the cam comes to the top, it pushes a rod up that disengages the firing mechanism/trigger. I know I probably don’t have it all, or have it completely right, but it is basically that simple.
It is confusing because there were two methods employed but both became known as “interrupter gear”.
What you describe O.P. (correctly I might add) is technically an interrupter, it mechanically prevented the gun from firing when the prop was in a position to be struck.
Due to “mechanical lag” of all the associated linkage and the general combined vibrations and girations of airframe, engine and gun the poor pilot could operate the trigger for several seconds and still only fire a few rounds off.
He had a bead on the ememy, gritted his teeth, pulled the trigger and that classic WW1 pop pop pop ……….pop pop pop pop pop …….pop pop result was all he achieved.
The slow rate of fire was not the pilot being conservative with his ammunition, it was as expended as fast at the interrupter gear ‘prevented’ his gun from firing.
If everything worked and he didn’t hit his own propellor (hence the deflectors Loose Head) he was jolly lucky if he managed to strike an opponent at all.
A synchroniser worked slightly differently from the point of view that most of these systems ran via a pulse pump from a cam lobe on the rear of the engine and were hydraulically operated, including the gun operation.
Rather than ‘preventing’ the guns from firing mechanically when the trigger was pressed, the equipment actually fired the gun when the prop was out of the way.
Hydraulic fluid was compressed by a pump in pulses off the engine camshaft, via tubing this ran to valve gear at the base of the joystick where the pressure was absorbed by a system of relief valves and chambers.
Operation of the gun button closed these valves and opened others to allow the pressure pulse to flow up to the gun and operate the trigger to fire it.
There was a bit more to it than that and there were loads of teething troubles to start with but that’s roughly how it worked.
‘Knowing’ the gun and ‘tuning’ the rate of fire correctly in relation to the revolutions of the propellor was the black art of the armourer.
Most WW1 Squadrons operated a system of ground crew support for aircraft in groups of four. A fitter/rigger for each airframe plus an amourer for the guns and a mechanic for the engines with a Sgt well versed in all disciplines in overall charge of the four aircraft.
If my Grandfather was still alive I am sure he would be delighted to tell us how ‘it’ was done, he was the ‘Sgt in charge’ for Capt Albert Ball VC’s SE5a.
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