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Crankshaft balancing

What weight?

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Iain Downs09/12/2018 08:33:48
488 forum posts
377 photos

Hi, all.

I'm building a small steam engine (single cylinder, 2 inch bore) and am currently looking at how to balance the crankshaft. I should add it's to my own amateur design...

I'd worked out how to balance the crank proper, but then my informal thesis advisor, JasonB, told me that was insufficient. That there was a lot more to add imbalance to the system than just the crank.

Dang!

So how would I go about working out the right balance weights on the journals?

I'm using a professional design tool (OnShape albeit in an amateur way) and it may be that this has some tools to help - it can calculate centre of mass of moments of inertia for a part or assembly, but I'm not sure what I'd do with that.

Many thanks in advance..

Iain

Kettrinboy09/12/2018 10:22:09
75 forum posts
39 photos

Hi Iain

Looking at your flywheel on the other thread you could perhaps incorporate some weights in the flywheel as well as on the crank webs as you might not be able to put enough weight just on the crank itself , but some on the flywheel as well might help , heres a pic of a flywheeI I made , the weight is held by a single screw and can be modified in size , I think balancing although you can calculate it to an extent , in practice it is just trying different weights in different positions until you get it to an acceptable level.

regards GeoffBalanced flywheel

duncan webster09/12/2018 13:36:19
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2204 forum posts
32 photos

have a read at my recent article in ME, no doubt a moderator will soon come along and tell you the issue number. It tells you all you need to know. I've found 3D CAD very useful for working out the centre of gravity of odd shaped things, loco driving wheel in my case

Edited By duncan webster on 09/12/2018 13:36:32

Iain Downs09/12/2018 20:34:34
488 forum posts
377 photos

Would you believe that the last ME I actually have filed is that very issue? (Val 220 No 4586 11- 24 May 2018).

Which shows that I'm behind in my filing of course, but convenient.

I shall read and attempt to absorb .....

Iain

John Olsen09/12/2018 22:09:32
985 forum posts
86 photos
1 articles

You can't actually perfectly balance a single cylinder with balance weights, unless you add extra geared balance shafts. So what is done in practice is that you balance all the rotating mass, and a proportion of the reciprocating mass. What that proportion should be is determined by experience, and typically for single cylinder motorcycles would be about 60% So you weight the piston, gudgeon pin and the small end of the connecting rod, and also the crosshead and piston rod for a steam engine, and make up a mass that weighs say 60% of that. hang that off the crankpin and set up the balance weights to balance that.

Practically speaking, you are likely to find that it is difficult to get enough balance weight even to give a 60% balance factor on the reciprocating masses. In practice, it does not seem to be very important to balance small steam engines. Speeds are generally low, and vibration does not seem to be a big problem. I did make some attempt with my steam launch engine, but even with half circle balance weights on all the crank webs the reciprocating mass is mostly unbalanced. Even so, that engine will happily run at 600rpm while not bolted down, without trying to walk around.

Balance is much more serious on full size locos, but they are up against the same sort of problems. A two cylinder loco is really a pair of singles, since the two crank are at 90 degrees, and well out of line with each other, so they can't easily be used to balance each other. So it is reasonably easy to balance the rotating masses, but hard to balance the reciprocating ones.

What happens when you try to balance a reciprocating mass with a rotating balance weight is that you change some of the out of balance force to a plane at right angles to the cylinder axis.

John

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