Sorry, ronmar I did indeed misquote you. You said generator weight, not "engine weight." My only excuse was I continue to think in terms of balancing an engine, and so read what you said inaccurately. Apologies.
Continuing, I don't believe that the mass and acceleration of the piston and rod alone account for all of the vertical forces involved, so I would very much have liked to see you calculate what they were, for accuracy sake. Saying they would have to be 50 lbs to cause the case to move vertically is not something I necessarily disagree with, but it isn't relevant to me since I see the the added weight's contribution as part of the picture, not all of it.
We started out with a statement that we had 8.8 pounds of weight opposing 1000 lbs. (1% you said) and by now have moved on to say it was 135 pounds of force opposing 750 lbs. A very different picture of the forces involved, and an important conceptual modification. But the 135 figure is also wrong and needs to be increased, I think you agree.
If you don't mind, in the interest of everyone's else's general understanding, I'll try a lazy method of calculating it, please object loudly and correct me if it doesn't seem accurate or fair:
You took 65% of the conrod mass as rotational and gave a figure for that as 13.64 pounds (I believe). I'd like to grab the remaining 35% if that's okay and call it reciprocal. That seems to be 7.34 pounds (unless I misunderstood your earlier statement.)
Now if the 8.8 pounds you calculated earlier yields 135 pounds of force and we add another 7.34 pounds to that weight, we might guess that the force increases proportionally. So that looks like another 113 pounds or so of force. Total force tending to lift is now 248 pounds. Not enough to do it yet on its own, of course, as you say, but very substantial.
However, what goes up must come down. We also have a force of 248 pounds force headed down every tenth of a second or so and through the crankcase trying to compress whatever is beneath the engine. If the engine is bolted solid to massive concrete, bedrock, etc. we naturally can't bounce.
If it's not, we may bounce more or less depending on what we are on and how attached. That is usually because of an equivalent form of spring compression beneath the engine, and it is caused by the engine appearing to weigh 1000 pounds one moment, and 500 pounds about a tenth of a second later to whatever it is on.
In a worst case resonant scenario, our spring will store 1000 pounds of energy, and release that against an apparent 500 pound weight every tenth of a second. In this situation, without damping, as in the case of the Tacoma Narrows Bridge which destroyed itself in a moderate wind, our engine may toss itself overboard.
That's unlikely to occur, unless someone intentionally tuned for resonance, or was incredibly unlucky in their choice of mounting. Luckily with damping, restraint, and/or a different spring resonance, we will see less vertical movement, but not necessarily no vertical movement, unless an engine is completely restrained from downward compression.
Restraining is not the same thing as balancing. You can definitely stop vertical and horizontal movement by preventing it, without balancing at all. Is that desirable? That's an open question. Noise and power pulsing transmitted through the ground is often reduced by a compressible mounting system.
I have not added in calculations for the effect I also asked about because of the offset center of mass of the genset .The reciprocating forces would then act on a fairly long moment arm offset to the side of the engine. But we've already got enough going without it to get things moving vertically. I think it isn't necessary to make the case further.
An engine can obviously move vertically, depending on many factors. The forces involved are not trivial -- an 8.8 pound piston and conrod can generate far more force than the numeric relationship of its weight to the overall weight of an engine.
My original question that started this was whether chalking on top of the flywheel would help in balancing an engine's reciprocating masses. I didn't understand why one would chalk in front of the engine. I do now understand that in some specific cases rotary balance is poor, (as in the case of a non-counterbalanced engine) and a particular engine is suffering from excessive fore and aft movement. If that engine is normally mounted to be restrained vertically, and ground pulsing is not a problem, then balancing fore and aft 100% for that case is fine.
My engine is well restrained fore and aft, and less restrained vertically. It has a buried railroad tie in graduated sand mount. The ties cannot move horizontally but are freer vertically. The genset frame is welded heavy channel.
I do have a somewhat objectionable amount of ground transmitted pulsing, as well as channel edge vertical flexing and I wondered if balancing could help reduce this. Balancing fore and aft didn't make much sense to me in this situation. Adding a rubber mat under the channel, as sometimes described to reduce noise might add to the vertical component, so I wondered if I could get better vertical balance with the chalk method on top of the flywheel. Hence the original question.
The proof of the pudding is in the eating. The real way to tell what is happening is to try it. At some point I'd like to loosen the mount and add rubber underneath, then chalk from above, to check periodic vertical movement, and if present, add weights to shift the balance percentage as described elsewhere. If not, report to ronmar, on my fool's errand.
As a side note, and reason why I haven't so far, one difficulty I have is that the ground actually moves vertically now in the vicinity of the engine, so a steady rest placed on the ground is going to be well, less than steady. I'll have to figure out a way to get a stable marker before I can get any results, one way or another..
