Crankshaft balancing………..

[Rik ShawParticipant @rikshaw]

……..how do you do?

[Rik ShawParticipant @rikshaw]

I am building a Bogstandards inspired twin cylinder steam engine that is 50% bigger than he designed it. I intend modifying the design of the crankshaft so that the fabrication will be a press fit construction with some split bearings.

The only thing I have a problem with getting my head round is the balancing of the finished crankshaft. It seems to me that with the two journals at 90 degrees to each other balancing would be impossible. On a smaller engine I suppose you might get away with the vibration but as the model increases in size I suspect the problem needs addressing.

A flywheel with more mass would help maybe but are there any simple solutions?

Rik

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Rik ,

An engine with two cranks at right angles can have almost perfect primary balance – much better than a single crank engine .

If you want to hear the theory then contact me by all means but I think a trial and error approach would work as well as calculation in this case .

Just start by realising that the forces causing imbalance have two components per con rod – one on axis and one at right angles . So for your engine two out of balance conrod assemblies have four components conveniently at 90 degrees to each other . They are not the same size but oposite pairs tend to cancel rather than reinforce and balance weights only have to deal with the differences .

So first approximation make balance weights as scaled up from original design but expect to be able to reduce them in size after some trial running .

Adding heavier flywheel does not improve balance at all but for other reasons will often promote smoother running .

Regards ,

MikeW

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Purely out of interest complex balancing problems can be represented on vector diagrams with the components of imbalance transformed from rotating and translational quantities into pseudo static ones with a fixed phase relationship – just the same as Andrew described for electrical systems .

Variations of same method work for numerous types of rotating system .

Back to tapping drill sizes ???

[Rik ShawParticipant @rikshaw]

Thanks Michael – I'll try it with a file then

Rik

[Russell EberhardtParticipant @russelleberhardt48058]

Of course, if you want perfect balance you could look at the inventions of Dr. Frederick Lanchester; harmonic balancers, counter rotating dual crankshafts, torsional dampers. If you can get hold of a copy of the book "Lanchester Motorcars a History" it makes fascinating reading.

Russell.

[Michael GilliganParticipant @michaelgilligan61133]

Rik,

This paper by Tony Foale is useful, and very practical

See especially p14, re: Ducati

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Another belated response.

Two cranks at 90 degrees, will, if put into static balance, should each be in Primary balance.

The fact that the cranks are at 90 degrees, will mean that they are also in Secondary balance, although, because the cranks are not in the same longitudinal plane there will be some couple acting on the engine. This will give it a tendency to squirm around a vertical axis.

What will determine how smooth, (in terms of vibration) the engine is when running is the proportion of Reciprocating mass that is balanced as compared to Rotating mass. You can spend a long time experimenting to optimise that!

The mass of the flywheel will have no effect on vibration (unless the flywheel is out of balance!). What it will improve with increasing mass is the smoothness of angular rotation speed. Whilst a heavy flywheel will minimise the speed variation at the flywheel end, it is likely to increase the speed fluctuation at the free end (Torsional vibration) This is because the heavy flywheel tends to act as fixed point about which the flywheel can vibrate in torsion.

In a two cylinder engine, unless the scantlings of the shaft are such that the pins and journals are thin and long, it is not likely to be a problem.

If you were making a high power eight cylinder engine, with nine main bearings, it might be another story!

Lanchester Harmonic Balancers are most often applied to four cylinder engines, although the trend towards three cylinder car engines makes their use more likely. The balance shafts, with their balance weights have to be timed to the crankshaft and each other, and geared to run at twice engine speed. If the balance shafts are not accurately timed to the crankshaft, the engine could turn out to be coarser than without them!

The function is that the secondary out of balance forces from the balancer shafts oppose the secondary forces of the reciprocating and rotating masses and so, to an extent, cancel them out.

On a twin cylinder steam engine, I have never heard of their use (but there may be examples), and are probably not needed.

[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

[Rik ShawParticipant @rikshaw]

Some VERY helpful advice in the replies here to my initial posting and thank you all.

John Bogs — I agree with your remarks about excessive speed, I always reckon that model steam engines look most impressive when run as slowly as possible.

Rik

[Ian S CParticipant @iansc]

One thing with the larger size, the relative weight of the moving parts should be less in comparison to the smaller prototype.

An engine that uses a system for dampening harmonic vibration , is the Continental O- 470 aero engine, and its derivatives.

Ian S C

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