13.1 Circular and Total Runout
Key Takeaways
- Runout is a composite surface control: every runout tolerance must reference at least one datum axis (the axis of rotation) and applies only to surfaces of revolution
- Circular runout controls each cross-section independently with a 2D circular tolerance zone; it bundles circularity, coaxiality, and axis-offset errors at that one section but says nothing along the length
- Total runout controls the entire surface simultaneously with a 3D zone (two coaxial cylinders, or two coaxial cones for a tapered surface); it adds cylindricity and taper control along the full length
- Circular runout is measured one plane at a time; total runout requires the indicator to sweep the full surface while the part rotates, so total runout is always at least as restrictive as circular runout at the same value
- Runout never controls a flat surface's orientation to a datum and never applies without a datum axis - a flat face perpendicular to the datum axis needs profile or orientation, not runout
13.1 Circular and Total Runout
Quick Answer: Runout is a composite surface control that always references a datum axis (the axis of rotation). Circular runout inspects each cross-section independently against a 2D circular zone and bundles circularity + coaxiality + axis-offset at that section. Total runout inspects the entire surface at once against a 3D zone (two coaxial cylinders) and additionally controls taper and cylindricity along the full length. Runout is roughly 5% of the Y14.5-2009 Senior exam, but the questions are application-level: they test whether you know what each control does and does NOT catch.
The Common Foundation: a Datum Axis Is Mandatory
Both runout controls in ASME Y14.5-2009 are defined relative to a datum axis - the axis about which the part is rotated during inspection. The feature control frame always carries at least one datum reference, and that reference must establish an axis. A runout callout with no datum reference, or with only a planar datum, is invalid by definition. This is the first thing to check when a Senior question shows a runout frame: if the datum cannot produce an axis, the callout is wrong.
The tolerance zone for circular runout is a 2D zone: at any single cross-section perpendicular to the datum axis, the surface must lie within two concentric circles separated by the tolerance value t. For total runout the zone is 3D: the surface must lie everywhere between two coaxial cylinders (or two coaxial cones for a tapered surface) separated by t along the full length of the feature.
Circular Runout: Section-by-Section
Circular runout is applied to a surface of revolution and inspected one cross-section at a time. The inspector rotates the part about the datum axis and reads an indicator at one plane; then repositions the indicator to the next plane and repeats. At each plane, the full indicator movement (FIM, formerly TIR) must not exceed t.
Because each section is judged on its own, circular runout controls the combined effect of three error sources at that section:
- Circularity (roundness) of the surface at that section
- Coaxiality of the feature's axis with the datum axis
- Axis offset - a shift of the section's center from the datum axis
It does NOT control anything along the length of the feature: a shaft could be a perfect cylinder at every section yet be tapered, barrel-shaped, or bowed along its length, and circular runout at every section would still pass. That longitudinal error is the domain of total runout.
Worked Example: Stepped Shaft
Consider a shaft with two journals (A and B) that establish the datum axis, and a mid-shaft diameter controlled by a 0.05 circular runout to A-B. During inspection, the indicator is set at three planes along the mid-shaft: near each end and at the middle. At each plane the part is rotated 360 degrees and FIM is recorded.
- If the mid-shaft is perfectly round but its axis is offset 0.04 from A-B, every plane reads ~0.08 FIM (twice the offset) - the part fails.
- If the mid-shaft is concentric with A-B but has a 0.06 roundness error at one plane, that plane reads ~0.06 - the part fails.
- If the mid-shaft is round and concentric at all three planes but tapers from nominal at one end to -0.07 at the other, all three circular runout readings pass - circular runout does not see the taper.
Total Runout: The Full Surface at Once
Total runout applies the tolerance to the entire surface simultaneously. The indicator sweeps along the full length of the feature while the part rotates (or the indicator is moved axially while the part turns), and the entire surface must stay between two coaxial cylinders t apart. The FIM over the WHOLE surface cannot exceed t.
Total runout therefore bundles everything circular runout catches PLUS:
- Cylindricity along the full length (no barrel, banana, or taper)
- Straightness of the surface generators (the longitudinal lines on the cylinder)
- Coaxiality of the whole feature, not section-by-section
A part that passes total runout automatically passes circular runout at the same value - but not vice versa. Total runout is always the more restrictive callout.
Worked Example: Pulley Hub
A pulley has a bore (datum feature A) and an outer belt-running surface controlled with 0.1 total runout to A. The inspector mounts the pulley on a precision arbor simulating A, puts an indicator on the belt surface, and sweeps axially across the full belt-contact width while the part rotates. FIM across the entire surface must stay within 0.1.
If the belt surface is a perfect cylinder but its axis is offset 0.04 from the bore axis, total runout reads ~0.08 - passes. If the same surface is concentric but tapered 0.12 across the width, total runout reads 0.12 - fails, even though circular runout at every section would have passed. This is the classic Senior-exam contrast: only total runout catches the taper.
Comparison Table
| Aspect | Circular Runout | Total Runout |
|---|---|---|
| Tolerance zone | Two concentric circles per section | Two coaxial cylinders along full length |
| Inspection | One plane at a time, FIM per plane | Sweep full surface, FIM over whole feature |
| Controls circularity | Yes (per section) | Yes (whole feature) |
| Controls coaxiality | Yes (per section) | Yes (whole feature) |
| Controls taper / cylindricity | No | Yes |
| Controls straightness of generators | No | Yes |
| Datum axis required | Yes | Yes |
| Applicable to cones (tapered surfaces) | Yes (conical section) | Yes (two coaxial cones) |
| Relative restrictiveness | Less restrictive | More restrictive |
What Runout Does NOT Control
Both runout controls are surface controls tied to a datum axis. They do not:
- Control a flat surface (use orientation or profile for a face perpendicular to the axis)
- Control location of an axis independently (use position; runout's location effect is composite with form)
- Control a non-revolution surface such as a rectangular keyway side
- Apply without a datum reference - ever
The Senior exam loves to present a flat face on a shaft with a runout callout and ask what is wrong: the answer is that runout does not apply to flat surfaces; the designer should have used perpendicularity (orientation) or profile of a line/surface.
A finished shaft passes circular runout inspection at every cross-section but fails total runout at the same tolerance value. Which error did the part most likely have?
Which statement about ASME Y14.5-2009 runout controls is correct?
An inspector reports that a pulley's belt-running surface reads 0.06 FIM under total runout to datum A (the bore). The surface is a perfect cylinder but its axis is offset 0.03 from the bore axis. What is the expected reading and the pass/fail result for a 0.10 total runout tolerance?
A designer applies a circular runout tolerance to a flat shoulder face on a shaft, referenced to the shaft's datum axis. Under ASME Y14.5-2009, what is the correct assessment?