Runout vs Total Runout: Circular and Total Runout Explained

Runout tolerances are the go-to GD&T controls for rotating parts — but the difference between circular runout and total runout determines whether you are checking one cross-section at a time or the entire surface simultaneously, a distinction with major consequences for bearing journals, cam surfaces, and rotating shafts.

When a shaft rotates in a bearing or a disk spins on an axis, the geometry of the rotating surface directly affects vibration, balance, seal performance, and wear. Runout tolerances quantify how much a surface deviates from its ideal position as the part rotates about the datum axis. Both circular runout (↗) and total runout (⇗) are measured with a dial indicator while the part rotates — but the measurement procedures, the geometric errors they detect, and the functional applications differ significantly. This article explains both controls in depth.

The Datum Axis: Runout’s Foundation

Unlike form tolerances, runout tolerances always require a datum axis. This axis represents the part’s actual rotational centerline in service — the axis about which the part spins in assembly. The datum axis may be established from:

• A single cylindrical datum feature (e.g., a bearing journal): the axis of the circumscribed or inscribed cylinder. Symbol: | A |
• Two cylindrical features (coaxial bearing journals at each end of a shaft): datum axis A-B, established as the line connecting the two feature axes. This is common for long shafts.
• A flat face + cylinder combination: the flat face constrains axial position while the cylinder establishes the axis.

The quality of datum feature selection is critical for runout measurements. Using a worn or out-of-round journal as the datum introduces simulation error — the measured “runout” includes error from the datum feature itself. For high-precision work, the datum journal is specified with its own tight cylindricity tolerance to minimize this effect.

Circular Runout (↗): Per-Cross-Section Control

Circular runout controls the variation of a surface at any single circular cross-section during one complete rotation of the part about the datum axis. The measurement is taken with the indicator at a fixed axial position; the part rotates 360°; the total indicator reading (TIR) is recorded. The indicator is then moved to another axial position and the process repeats. Each cross-section is evaluated independently.

The tolerance zone for circular runout at any cross-section is an annular area defined by two concentric circles centered on the datum axis. The radial separation between the circles equals the tolerance value. The controlled surface must lie within this annular zone in each cross-section.

Circular runout simultaneously controls:

• Circularity (out-of-roundness) of each cross-section
• Coaxiality of each cross-section’s center relative to the datum axis

It does NOT control straightness of the axis, taper, or profile variations that are consistent across cross-sections (like a uniform barrel or waist). A surface could pass circular runout at every section but still be tapered, barreled, or bowed.

Total Runout (⇗): Full Surface Control

Total runout controls the variation of the entire surface simultaneously — all cross-sections and all axial positions at once — during rotation about the datum axis. The tolerance zone is the annular space between two coaxial cylinders (for a cylindrical surface) or two parallel planes perpendicular to the datum axis (for a face surface), with the separation equal to the tolerance value.

Measurement of total runout: the indicator is traversed along the entire surface axially while the part rotates. The total accumulation of all indicator readings — from any angular position and any axial position across the entire surface — must not exceed the tolerance value. This is significantly harder to achieve than circular runout because it requires the surface to be geometrically perfect in all modes simultaneously.

Total runout simultaneously controls:

• Circularity at every cross-section
• Coaxiality of every cross-section
• Cylindricity of the entire surface
• Straightness of the surface line elements
• Taper

A single total runout callout replaces individual circularity, cylindricity, straightness, and coaxiality callouts for a rotating cylindrical surface. It is the most comprehensive rotating-surface control available in GD&T.

Circular vs Total Runout: Side-by-Side Comparison

Property	Circular Runout ↗	Total Runout ⇗
Tolerance zone shape	Annular (2D, per cross-section)	Annular cylinder or parallel planes (3D, full surface)
Measurement motion	Rotation only (indicator fixed axially)	Rotation + axial traverse simultaneously
Controls circularity	Yes (at each section)	Yes (at all sections)
Controls coaxiality	Yes (at each section independently)	Yes (entire surface simultaneously)
Controls cylindricity	No	Yes
Controls taper	No	Yes
Controls axis straightness	No	Yes
Measurement complexity	Low–medium	Medium–high
Typical tolerance	0.01–0.1 mm	0.005–0.05 mm

Runout on Face Surfaces

Both runout controls can be applied to faces (surfaces perpendicular to the datum axis), not just cylindrical surfaces:

Circular runout on a face: The indicator is placed perpendicular to the face at a fixed radial position; the part rotates. TIR is the circular runout at that radius. A separate measurement at a different radius gives an independent result. This controls the wobble of the face at any given radial distance.

Total runout on a face: The indicator traverses radially while the part rotates. TIR over the entire face and all rotations is the total runout. For a face, this controls flatness + perpendicularity to the datum axis simultaneously — a very comprehensive control for sealing faces, bearing thrust washers, and encoder disks.

Practical Setup for Runout Measurement

Accurate runout measurement requires careful setup:

• Support the datum features properly: For a shaft with datum axis A-B (two journals), support the part in V-blocks or between centers at both journals. For a single datum bore or journal, use a mandrel or chuck aligned to the datum.
• Eliminate axial float: Axial play during rotation will cause false indicator readings. Use a light axial preload (push pin at one end) or locate against a shoulder.
• Use the correct indicator type: Dial indicators (graduation 0.001 mm) are standard. For total runout traversal, an electronic indicator connected to a data recorder gives the best traceability. The indicator contact force should be light to avoid deflecting thin features.
• Rock test the datum: Before measuring, confirm the datum features are stable. For V-block support, lift the part slightly and re-seat — repeat 3 times and verify the indicator returns to the same zero. Rocking datum features invalidate all runout measurements.
• Record all cross-sections: For circular runout, document the TIR at each axial position and report the worst case. The drawing callout applies to every cross-section simultaneously.

When to Use Circular Runout

Circular runout is the appropriate choice when:

• Cam profiles: Each cross-section of a cam must follow its programmed lift curve — the profile at any given axial position must be within spec, but the relationship between adjacent cross-sections (which would be controlled by total runout) is not a functional requirement.
• Stepped shafts: Short bearing journals with different diameters benefit from per-section circular runout to allow manufacturing flexibility in the transition zones.
• Cost-effective specification: Circular runout is easier to achieve in manufacturing and easier to verify than total runout. If total cylindricity of the full surface is not functionally required, circular runout prevents over-tolerancing.
• Moderate-speed rotating parts: Where vibration from non-cylindricity (taper, bow) is not critical, circular runout on key cross-sections is sufficient.

When to Use Total Runout

Total runout is appropriate when:

• High-speed bearing journals: Turbine shafts, motor spindles, and machine tool spindles require total runout to control all sources of eccentricity and form error that cause vibration and bearing wear at high speed.
• Precision gear and pulley bores: The bore that seats on a shaft must be geometrically precise over its full length for uniform contact and torque transmission.
• Encoder disks and optical elements: Wobble at any point on the surface affects measurement accuracy; total runout on the mounting face is required.
• Sealing grooves: An O-ring or lip seal groove requires total runout to ensure the seal contacts the surface uniformly around the entire circumference and along the contact length.

Runout vs Concentricity and Cylindricity

Engineers sometimes debate whether to use concentricity, cylindricity, or runout for a given rotating surface. The practical guidance from ASME Y14.5-2018:

• Concentricity was designed to control axis location based on median points — it is measurement-intensive and has been removed from ASME Y14.5-2018. For coaxiality of rotating surfaces, circular or total runout is preferred because it is directly measurable with a dial indicator.
• Cylindricity is a form-only control — it does not reference a datum axis. A perfectly cylindrical surface could still be severely eccentric relative to the rotation axis and cause severe runout. Cylindricity and runout control different things.
• Circular or total runout captures the functional behavior during rotation: both coaxiality AND form error contribute to the indicator reading. This is exactly what matters for a rotating part.

Conclusion

Circular runout and total runout are both measured with a rotating part and a dial indicator, but they answer different functional questions. Circular runout asks: “At this cross-section, how much does the surface deviate during rotation?” Total runout asks: “Over the entire surface, what is the worst-case deviation during rotation?” Use circular runout for cam profiles, stepped shafts, and cost-sensitive specifications. Use total runout for precision journals, spindles, and any application where the cumulative form and location error across the entire surface length must be controlled. Specifying the correct runout tolerance ensures measurability, manufacturability, and true functional compliance for rotating parts.