Propeller balancing is the practice of taking vibration out of the propeller unit. Placed on a pivot axle, two-blade propeller balancing is accomplished by removing material from the heaviest blade to make both blades of the propeller weigh the same. Often, with multi-blade propellers, such as three-, four- and five-blade propellers, a propeller balancing level is used to accurately balance the propeller. Looking much like a bubble balancer used on automotive tires and wheels, the propeller is balanced by removing material from the heaviest blades until the level is zeroed.
Left unbalanced, a propeller could literally vibrate and shake a vessel apart. Bearing life is dramatically increased by propeller balancing as well. In an aircraft application, this is extremely important due to the propeller being attached to the engine's crankshaft. If propeller balancing is not accomplished, the engine could suffer crankshaft bearing failure, resulting in catastrophic engine failure. In a nautical application, propeller vibration could potentially sink the vessel if water seals are damaged.
Wooden aircraft propeller balancing is accomplished by sanding the heavier side of the propeller until it balances when placed on an axle between two points. With the propeller mounted on an axle, the axle is placed between two stands and allowed to rest. The propeller blades are then placed horizontally and released. If the blade begins to move, the propeller blade that begins to dip is the heaviest, so it is sanded. This operation is repeated until the propeller remains completely motionless when left to rest horizontally. When this occurs, propeller balancing has been accomplished.
With metal propeller blades, material is removed from the heavy blade by drilling into the propeller hub at the base of the propeller blade. As the drill bit makes its way into the hub, material is removed in the form of small curls of metal. In some instances, weight is added to the lighter side and then affixed to the propeller by welding, brazing or soldering. Often, the amount of weight by which the propeller is out of balance dictates the manner in which the unit is balanced.
In boat propeller balancing, the blades are often filed until the correct amount of weight has been removed to ensure the propeller is balanced. On very large ship propellers, balancing is often accomplished by adding slugs of Mallory metal to the lightest blades. Mallory metal is a very dense, and therefore very heavy, metal often used in balancing automobile crankshafts. When using this type of material to balance a propeller, the propeller is drilled, and the slug of Mallory metal is inserted and welded in place.