Wednesday, January 05, 2011

Engine Balance and the Arms Race

Interesting question about the engine balance "arms race" I've shortened and edited it a little:

So here's my question.  Is there any computation or formula that you know of to convert the CH IPS velocity units into the moment units of fixed balancing machines, something that factors in the approximate weight of the whole engine and/or prop? 
Here's what prompts the question.  Some engine shops now balance engine crankshaft assemblies, and it's sometimes sort of an advertising arms race to split hairs more finely by advertising or claiming the lowest unbalance limit (expressed in moments).  Sooner or later, some owner will ask, "OK, your crank balance limits are in different units from the units in the prop balancing book I read.  How do your levels of engine balancing precision compare to what is acceptable or recommended for the prop?"


This is not in my area and I don't know a thing about standard practices in the balancing industry, but I can't resist: Reducing the problem to its most basic level:

What Roger calls Arm is the eccentricity of the mass or the distance between the center of gravity and the center of rotation. So in terms of unbalance it is simply the amount of mass eccentricity times the radius (mR). 

How do we detect this? We can spin the object and measure the vibration force because when mR >0 the centrifugal forces are greater than zero. We can measure ips, acceleration, g., or pilot comfort or whatever vibration measurement we wish to take. BUT these are all reactions to mR being greater than zero. We have measured the effects of imbalance and not the amount of imbalance itself.

Now Rogers question is astute. He wants to do just the opposite: Having measured the vibration in ips, g's, or human comfort level he is asking for a formula to convert any of these back to mR. I could be wrong but I don't think this is possible without knowing the mass. For example, If you tell me you are experiencing a 1 g force I cannot compute your mass (weight in this case). If Roger knows the mass of the crankshaft (and possibly the rpm, then I believe he could). He could empirically by making changes to the mass.

So if we talk about the amount of imbalance or mass eccentricity and not the vibration caused by the mass eccentricity we can express this in terms of eccentric mass and radius. So if Roger tells his customers that he balances crankshaft's down to "twenty milligrams per millimeter" (1 grain of rice 1 mm from the center of rotation) he will completely confuse the customer and possibly bullshit his way to leader of the arms race. The IPS guy has no idea what the amount of imbalance is - he just knows the amount of reaction there is to the imbalance. 

To add further confusion, everyone is assuming that the crankshaft is perfectly rigid which it is not;  that is why longer crankshafts have "counterweights" better described as "tuned pendulum absorbers". The crankshaft locally is not balanced as the cheeks are not opposite so we get local reactions to that imbalance. A 4 cylinder Lycoming or Continental engine has unbalanced reciprocating forces that are greater than any rotating imbalance. So ultimately it's a marketing question involving human nature and gullibility and thus so should the answer be framed.

1 gram is approximately the weight of 1 drop of oil so as the crankshaft rotates it is covered in oil so at the gram or sub-gram level the eccentric mass is always changing and thus crankshaft "balance" under operating conditions is dynamic and no amount of fixed mass will compensate. But for marketing reasons we could assume that some oil pools in recesses or is always present in oil galleys. Why not spin balance the crankshaft with the oil galleys filled with oil or the crankshaft wet to determine its mass eccentricity in real-life conditions and balance accordingly. You could advertise a 'wet balanced" crankshaft.! 

Any counter-weighted crankshaft will have eccentric mass at the pins (bifilar mounted counterweights) as different diameter pins are used to tune the counterweight. Possibly one could compensate for this and call it a "bifilar tuned wet balance". Wait there's more we can do: any crankshaft collects a patina, sludge, and  carbon deposits that certainly weigh in excess of our grain of rice. Why not advertise a "carbon compensated bifilar tuned wet balance."

Enough of this nonsense - 

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