Engine Balance - Static & Secondary

Now when the Piston/rods have been balanced and assembled to the crank, then that whole assembly need to be balanced both statically and dynamically. And again when the engine is built and attached to a prop. The rotating mass is the total weight of the rods, pistons, rings, and complete flywheel/prop/hub assembly. A rotating assembly that is in perfect balance rotates concentrically around the center line of the axis of rotation.

A rotating assembly that is not in perfect balance tends to rotate in an elliptical (egg shaped) manner, around the center line of the axis axis rotation, with the elongated portion of the ellipse at the point of unbalance.The more unbalanced, the more exaggerated the ellipse.

Of course, the elliptical movement caused by the unbalance is restrained by the bearings. It follows that the greater the elliptical motion of the flywheels, the greater the wear on the bearings.

For example, as the speed of the engine doubles, the amount of force, per revolution, created by unbalance tends to increase by an approximate factor of four. Unbalance is typically expressed in ounce inches (oz. in.). This means 1 ounce (28.35 grams) of unbalance at 1-inch radius.

Example:

One ounce inch of unbalance at 2,000 rpm creates approximately 7 pounds of force per rpm.

One ounce inch of unbalance at 4,000 rpm creates approximately 23 pounds of force per rpm.

One ounce inch of unbalance at 8,000 rpm creates approximately 120 pounds of force per rpm.