Circular runout is a GD&T control that limits how much each cross-section of a surface moves an indicator while the part makes one full turn on a datum axis. It is checked one section at a time and catches both off-centre wobble and out-of-round shape, which is why it is the everyday control for shafts and anything that spins.
Put the part in V-blocks or between centres on its datum diameter, touch an indicator to the controlled diameter and turn the part once. The full indicator movement (FIM, also called TIR) must not exceed the tolerance. Then move to the next section and repeat; each section is judged on its own.
That single reading includes off-centre error (the section centre not on the datum axis) and form error (the section not being round). It does not include taper or bend between sections, because the indicator is reset at each section. Total runout adds those.
Circular runout can also be applied to a face perpendicular to the axis, where it controls wobble of each circle on the face, like a brake disc.
For a diameter, at each cross-section the zone is two concentric circles, the tolerance apart radially, centred on the datum axis. For a face, it is two circles at the same radius on the face, separated axially by the tolerance.
Because the zone is centred on the datum axis, it cannot float like circularity.
Runout always needs a datum axis, from one diameter, two diameters used together (A-B), or a face plus a diameter.
| Modifier | With circular runout |
|---|---|
| MMC or LMC | Not allowed. Runout is always applied regardless of feature size, and its datum references are at RMB. |
| Diameter symbol | Not used: the value is a full indicator movement, not a zone diameter. |
Example: a Ø35 mm seal diameter, circular runout 0.03 mm to common datum axis A-B (the two bearing journals)
Result: 0.021 ≤ 0.03, so the diameter passes circular runout.
Every GD&T symbol page: GD&T symbols chart
Circular runout resets the indicator at each section and controls each one separately. Total runout moves the indicator along the whole surface during rotation and controls everything at once, including taper and bend.
TIR (total indicator reading) is the older shop name for full indicator movement, the reading runout is judged by. The GD&T symbol is circular runout.
Yes. It always references a datum axis, the axis the part spins on in service.
Yes. On a face perpendicular to the axis, circular runout controls the wobble of each circle on the face.
No. Runout tolerances always apply regardless of feature size.
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