A cannon bearing or cannon box bearing is an arrangement of bearings on a shaft, usually an axle, where two bearings are mounted in an enclosed tube.
The function of the cannon box is to preserve the alignment of the two bearings, even if the overall tube is allowed to move. The two bearings will retain their same relative position. The bearing tube can be attached to the vehicle frame through either a pivot or springs.
The name 'cannon box' derives from the appearance of the hollow tube. Also from the boring machines used to machine the accurately aligned bearing seats, the same machines developed for the boring of cannon and also used for machining the cylinders of steam engines.
Cannon box bearings are still found today, although much of the need for them was removed by the development of self-aligning ball bearings. These allow the inner race of a bearing to move independently of its outer, so that each bearing can align to both shaft and housing simultaneously, even if the housing moves around.