Glossary term
Runout
Geometric deviation of a rotating surface from ideal circular or axial alignment.
Definition
metricRunout is the measured deviation of a rotating surface or feature from ideal circular, coaxial, or axial alignment relative to a datum.
Runout combines geometric error, eccentricity, datum error, bearing motion, shaft bend, fixture error, and sometimes measurement setup effects. It is controlled in GD&T because rotating parts such as shafts, gears, pulleys, spindles, brake rotors, cutting tools, bearings, and encoder disks need predictable motion relative to their functional axis.
Runout is usually measured by rotating a part about a datum axis while an indicator, probe, or sensor records variation at a surface. The observed variation is often reported as total indicator reading (TIR), the difference between the maximum and minimum readings during one revolution.
Radial runout is measured perpendicular to the axis and affects round rotating surfaces such as shafts, pulleys, gears, and bearing seats. Axial runout is measured parallel to the axis and affects faces such as brake rotors, flanges, disks, and thrust surfaces. Total runout controls variation over an entire surface, while circular runout controls each circular element independently.
Engineering relevance
Runout can cause vibration, uneven wear, seal leakage, gear noise, bearing overload, tool chatter, poor surface finish, encoder error, and cyclic loading. In high-speed machinery, a small geometric error can become a significant dynamic problem because it creates periodic displacement, force, or unbalance at rotational frequency.
The value depends strongly on the datum and setup. A part can show different runout when held between centres, on a mandrel, in a chuck, or assembled into a bearing pair. Fixture error, dirt, burrs, thermal expansion, probe angle, and bearing clearance can all contaminate the measurement.
Common mistakes
A common mistake is treating runout, roundness, concentricity, eccentricity, and shaft bend as the same thing. Runout is a functional measurement relative to a datum during rotation; it may include several underlying geometric errors. Another mistake is specifying tight runout without controlling the datum feature, assembly interface, bearing condition, or measurement method. A strong runout review states datum, rotation method, probe location, radial or axial direction, TIR value, speed if relevant, fixturing, and acceptance criterion.