Run-out

Run-out or runout is an inaccuracy of rotating mechanical systems, specifically that the tool or shaft does not rotate exactly in line with the main axis. For example; when drilling, run-out will result in a larger hole than the drill's nominal diameter due to the drill being rotated eccentrically (off axis instead of in line). In the case of bearings, run-out will cause vibration of the machine and increased loads on the bearings.^[1]

Run-out is dynamic and cannot be compensated. If a rotating component, such as a drill chuck, does not hold the drill centrally, then as it rotates the rotating drill will turn about a secondary axis.

Run-out has two main forms:^[2]

• Radial run-out is caused by the tool being translated off the machine axis, still parallel. Radial run-out will measure the same all along the machine axis.
• Axial run-out is caused by the tool or component being at an angle to the axis. Axial run-out causes the tip of the tool or shaft to rotate off-centre relative to the base. Axial run-out will vary according to how far from the base it is measured.

In addition, irregular run-out is the result of worn or rough bearings which can manifest itself as either axial or radial run-out.

Run-out will be present in any rotating system and, depending on the system, the different forms may either combine increasing total runout, or cancel reducing total runout. At any point along a tool or shaft, it is not possible to determine whether runout is axial or radial; only by measuring along the axis can they be differentiated.

Absolute alignment is impossible; a degree of error will always be present.