Static balance: the balancing of the rotating masses so that the resultant of all centrifugal forces on the crankshaft, when the shaft is not rotating, is zero, i.e. the shaft has no heavy point. Static imbalance is detected when the shaft, supported on knife edges or rollers, always rolls until the heavy side is at the bottom. It is corrected by adding or removing mass on the appropriate crank webs (counterweights) or by drilling/grinding material from the heavy side so the centre of mass lies on the axis of rotation.

Dynamic balance: even a statically balanced shaft can have a couple acting because the unbalanced masses lie in different planes along the shaft, so that when rotating a rocking couple is set up. Dynamic balancing is performed on a balancing machine where the shaft is rotated and the vibrations at the two ends are measured; correction masses are added or removed at calculated positions (usually on the crank webs) in the two end planes so that both the resultant force and the resultant couple are zero. In practice counterweights are attached to, or cast integrally with, the crank webs, and for large shafts the balance is checked during manufacture and correction machining is done on the webs.

The primary reciprocating force arises from the acceleration of the reciprocating masses (piston, rings, small-end part of connecting rod) and varies in magnitude once every revolution (at engine speed, first order). For a multi-cylinder engine, by arranging the firing order and crank positions, the primary forces of different cylinders can be made to cancel to a large extent. The methods used are:

1. Balance (counterweight) on the crank web approximately equal to the fictionally rotating part of the reciprocating mass (a rotating balance weight of about half the reciprocating mass placed on the opposite side of the crank throws) which balances part of the primary force.
2. Arrangement of crank throws so that the out-of-balance primary forces act at different phases and cancel each other in symmetrical multi-cylinder engines (e.g. six-cylinder inline engines), including using an even number of cylinders spaced evenly.
3. The use of balance shafts (counter-rotating shaft pairs) running at engine speed, carrying balance weights, which generate a downward force to cancel the primary force.

They are not completely successful because: (1) the primary force is a force (not a pure couple) that cannot be wholly eliminated for an inline engine unless balance shafts are fitted, and these themselves add weight and complexity; (2) the balance weights cancel only the rotating part of the primary force; the reciprocating part acts along the cylinder axis, so the horizontal component passes through the crank and the vertical component cannot be cancelled by webs alone; (3) remaining combined forces leave a residual imbalance which, though small, is not zero; (4) the counterweight approach only reduces the force, and the exact phase relationship varies with speed.

Secondary imbalance: the secondary reciprocating force varies at twice engine speed (second order) and arises from the finite length of the connecting rod causing the piston acceleration to have a second harmonic. Additions to control implantance are:

1. Counter-rotating balance shafts (Lanchester or reciprocating balance shafts) running at engine speed for primary and at twice engine speed for secondary, with weights arranged to generate cancelling forces. These shafts are gear- or chain-driven from the crankshaft.
2. Addition of counterweights of increased size to the crank webs.
3. For secondary forces, two shafts rotating in opposite senses at twice crankshaft speed with parallel axes are used so their horizontal components cancel and vertical components add to balance the secondary force.
4. Use of a flywheel of correct mass moment of inertia and a torsional damper/detuner to damp out the torque variations and torsional vibrations that such vibration produces.

These additions reduce frame vibration, main-bearing loading, and forces transmitted to the ship's structure, preventing excessive vibration and noise.