Reply To: Crankshaft balancing………..

[John OlsenParticipant @johnolsen79199]

Some of the above information is actually wrong. So far as I know, the best you can do with a parallel twin with cranks at 90 degrees and no balance shafts is balance each crank as if it was a single. This actually gives quite satisfactory results in practice. I have a Leak compound twin, 3 and 5 inch bores and three inch stroke. I have run it up to 600 rpm with no tendency for it to try to dance around. It was not bolted down. That has balance weights on each crank to fully balance the rotary masses and partially balance the reciprocating masses. If you fully balance the reciprocating masses you end up with an out of balance force in the horizontal plane of the same magnitude. This will not balance with the other cylinder since that is 90 degrees out of phase. Secondary forces are not worth worrying about unless you can get good primary balance, since the primary forces are much larger.

Even if you went to a 180 degree crank the two cylinders being on separate axes will give a reciprocating couple. To get rid of that you need the extra shaft. I think myself that it would be better to go to more cylinders if you really think you need good balance…but note that most steam engines in the smaller sizes have a single cylinder. Since they don't turn very fast and tend to weigh a bit, vibration is not a great problem. (extra mass does not reduce the forces, but it does tend to reduce their effect.)

A 90 degree V twin will of course give you perfect primary balance and self starting, which is one reason why this has been done for steam launch engines and of course the Heisler locomotive. Large triple expansion engines were often built with foru cylinders, eg two LP cylinders, since this arrangement permits good balance and self starting.

John