enginehistory.org

[gryan]

I am inerested in the double V-12 engines produced by manufacturers such as Daimler Benz and Allison. These units were made up of a pair of V-12 engines mounted together on a common case with two crankshafts geared together to drive a single output. Sometimes they have been referred to as W-24s.

My interest is in how the crankshafts were geared together to a single output shaft. Does anyone have an illustration of the arrangement of the gear train in the Allison or Daimler Benz?

How did the gear train survive the alternate firing impulses delivered from each crankshaft? Surely that would lead to all sorts of torsional vibration issues. The gear train would first be subjected to one set of loads as the first crank (piston on power stroke) drove the entire system including the second crank. Then there would be significant load reversal across the gear train as the second crank provided the drive (one of its pistons firing and on power stroke). The first crank would now be driven by the system (as its pistons would all be on compression, induction or exhaust strokes). Some of the gears would alternate between driving and being driven; rapidly oscillating back and forward. This is exactly a condition to destroy a geartrain.

How did the manufacturers protect their engines and gears from this destructive vibration?

[rinkol]

I have a recollection of reading that the DB double engines used rubber to provide some flexibility in the coupling of the crankshafts to the gear train. I would also expect that the crankshafts would not be geared directly to each other, but would each drive a common propeller reduction gear. This would provide some isolation between the crankshafts. According to White's book on Allied Piston Engines, this was the arrangement on the A series Allison V-3420 engines. The B series engines had counter-rotating crankshafts and must have had a more complex drive train arrangement.

Other engines would have had to address similar issues - H type engines such as the Napier Sabre come to mind.

Robert

[gryan]

Thanks Robert

I have a drawing of how the Napier Sabre engine crankshafts were geared together and had their drive joined. It is a very clever system that always made certain the teeth would not suffer "chatter" (destructive instantaneous load reversals). The Napier system had balance beams which relied on the end thrust of helical gears to keep forces in balance across the gear train (avoided the instant load reversal every rev problem).

When BRM built a H-16, for Formula One car racing, they suffered trouble with destructive chattering in the gear-train. They managed to get around this by some design alterations (firing cylinders one at a time) and by "lifing" certain key parts of the engine.

I am keen to find out how the DB engine and others solved the problem.

Gerald