Run-out or runout is an inaccuracy of rotatingmechanical 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 . In the case of bearings, run-out will cause vibration of the machine and increased loads on the bearings. 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:
Radial run-out is caused by the tool or component being rotated off centre, i.e. the tool or component axis does not correspond with the main axis. Radial run-out will measure the same all along the main 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 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. Runout will be present in any rotating system and, depending on the system, the different forms may either combine increasing total runout, or cancelreducing 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.
Radial run-out
Radial run-out is the result of a rotating component running off centre, such as a ball bearing with an offset centre. This means that the rotating tool or shaft, instead of being centrally aligned, will rotate about a secondary axis. In general, cutting tools are more tolerant of radial run-out since the edges are parallel to the line of cutting tending to keep the tool tip aligned. However, a rotating shaft may be less tolerant of radial run-out since the centre of gravity is displaced by the amount of run-out.
Axial run-out
Axial run-out is the result of a rotating component not being parallel with the axis, such as a drill chuck not holding the drill exactly in line with the axis. In general, cutting tools are less tolerant of axial run-out since the tool tip tends to dig in and further increase run-out. However, a shaft may be more tolerant of axial run-out since the centre of gravity is displaced less.
Measurement
Typically run-out is measured using a dial indicator pressed against the rotating component while it is turned. Total indicated run-out is a technician's term for the measured run-out of any rotating system, including all forms of run-out, at the measured point.
