I just taught balancing this week, so it may be a good time to share.
Primary balance (or imbalance!) is the rotational balance, such as with a rotating shaft or flywheel or rotor of some kind.
Static balance is in a single plane and what we correct when we balance our bicycle wheel:
Dynamic balance is when the imbalance weighting is offset so that each axle feels a different imbalance. It is possible to have something perfectly static balanced and yet have a dynamic imbalance as you spin it up.
This is why we put balance weights on the inside and the outside of automobile rims when we dynamically balance them.
Finally, the primary (rotational) imbalance force felt on the axle is the out-of-balance weight X distance from axle X the RPM squared.
So as the RPM goes up the imbalance force go up exponentially.
So a little out of weight on our rim is barely noticeable at 10mph, will be a huge pounding force at 50mph.
So statically balancing your flywheel would be a good thing, because it is narrow dynamic balance is not such a problem. What about the crankshaft? If you check, the crankshaft on any single cylinder engine is hugely out of balance. Why?
Secondary Balance, that's why.
Secondary Balance is any non-rotation balance in a machine, such as the reciprocating forces of the piston and connecting rod. The engine builders have used the crank imbalance to balance the piston/rod forces.
Bad news is that this relationship is very speed dependent. A single cylinder may be very well balanced at idle, or at speed but not at both unless they employ yet another balance mechanism. Some large single cylinder engines employ a balance shaft or reciprocating balance weight to counter the piston/rod forces.
Balance shafts:
Contra-rotating "syncro" balance weights:
Reciprocating balance weight(Briggs 12hp) :
Hope this helps!
-Gearhead437
