Crankshaft Balancing

Few mechanics truly understand engine balance because most of the primary and secondary balance considerations for an engine and crankshaft are calculated by the engineers who design the engine, not the mechanics who work on them.

In some cases, however, a mechanic may be required to replace a crank shaft, piston, or other component that directly affects balance. So understanding crankshaft balancing and how it affects engine performance and mechanical longevity is important.

What Primary and Secondary Balances Are

The primary balance of an engine is the first harmonic of the crank's rotational frequency while secondary is the second harmonic.

Primary balance is achieved by compensating for the masses in the rotating system around the crankshaft. So on most combustion engines, this means balancing the pistons, connecting rods, and crankshaft spin to create not only perfect timing, but to allow the spin of the crank to be as friction-free as possible. The closer the crankshaft is to "floating" on its bearings, the better the primary balance.

Secondary balance is compensating for other variables that can affect the crankshaft's spin. These include things like the kinetic energy and non-sinusoidal motion of the pistons as well as the motion of connecting rods and balance shaft weights. Most engineers focus on the non-sinusoidal motion of the pistons - or the varying time changes that occur during the pistons movement up and down in the cylinder due to the lateral motion of the conrod. Perfecting the length of the conrods and the depth of the rod journals on the crankshaft adjusts the travel and timing of the piston within the cylinder.

So when a mechanic installs a new crankshaft, that crank needs to be balanced to match the cylinders and rods being used. Most of the time, using factory-approved replacements will accomplish this, but when all parts are being replaced, they will be required to be matched in order to achieve balance.

Machine shops and specialty mechanics may get into balancing by removing or adding material to affect the primary balance. In most cases, secondary balance is a matter left up to the matching of pistons and conrods. For the home DIY mechanic, this is not likely going to be in the repertoire and is best left to professionals.