Circular Runout — Runout Tolerance

Limits composite variation (roundness + concentricity) in one circular cross-section.

Definition

Circular runout limits how much a surface may vary from its nominal position relative to a datum axis, as the part is rotated about that axis — one cross-section at a time. It is a composite control: a single dial-indicator reading over one revolution bundles together the section's roundness error and its coaxiality to the datum, without separating them, and that bundling is exactly why the control is so practical. Every prior symbol in this library is verified by static measurement; runout's defining characteristic is that the inspection itself involves physically spinning the part. The datum reference is mandatory — rotation about the datum axis is the whole method.

What one FIM reading combines

The full indicator movement (FIM) over one revolution at a fixed section is a single number that includes everything that moved the stylus: out-of-roundness of that section, any offset of the section's center from the datum axis, and any wobble of the surface. It does not attempt to decompose them — a section that is perfectly round but mounted 0.03 off-center and a section that is centered but 0.03 lobed read identically, and both fail a 0.05 callout only if their combined excursion exceeds it. That compositeness is a feature for rotating parts, because what the function experiences — a bearing journal under a load, a seal seat under a lip — is precisely the combined effect. It also makes runout the fastest honest check in the whole GD&T toolbox: one indicator, one revolution, one number.

Reading a feature control frame

↗ 0.05 A–B reads as: a circular runout tolerance of 0.05 FIM, evaluated as the part rotates about the common datum axis established by datums A and B. Runout never carries a material-condition modifier — it controls a surface, not a feature of size — and the arrow symbol takes no diameter symbol: the zone is a radial separation between concentric circles in one section, not a cylinder. Note the common datum A–B: runout is usually checked between two journal datums so the part spins on its functional axis rather than on one end.

Measurement

Bench center + dial indicator — mount the part on centers or in a V-block that reproduces the datum axis, zero the indicator on the controlled surface, rotate the part one full revolution, and read the total indicator movement; it must not exceed t. Move the indicator to the next section and repeat. This is the classic, fastest, cheapest geometric inspection there is.

CMM rotation analysis — probe each section and evaluate the radial variation in the rotated coordinate system. Used when the part cannot be spun (large or asymmetric geometry) or when the reading must be decomposed into roundness and offset components for process diagnosis.

When to use it

• Bearing journals and seal seats, where the combined round-and-concentric excursion is what the mating part actually feels
• Rotating surfaces with a functional surface relationship to a datum axis — pulleys, gear blanks, rotor laminations
• Any drawing where concentricity was about to be called out — runout is the surface-based, gaugeable control that does the intended job
• First-article and incoming inspection, where a one-revolution dial check gives a pass/fail answer in seconds