Key takeaways
- Circular runout checks each surface element separately. Total runout checks the whole surface in one traverse.
- Runout controls location, orientation and form together relative to a datum axis. It does not control size.
- Concentricity controls derived median points and is difficult to inspect. ASME Y14.5-2018 removed it.
- Use position at MMC for a clearance hole, runout for a bearing or sealing surface, and coaxiality for an axis that must align.
What runout measures
Runout is measured while the part rotates about a datum axis. A dial indicator or a probe touches the surface, and the total movement of the indicator is the runout value. For a cylindrical surface, the reading is the radial variation. For a face, it is the axial variation.
The datum axis is established from the datum feature, not from the centres the machinist used. If the drawing names a bore as the datum, the inspection setup has to simulate that bore. A part that runs true between centres but not about the drawing datum has not passed the callout.
Runout is a combined control. It responds to location error, orientation error and form error at the same time. That is useful when the function is rotation, because all three affect the result. It is less useful when you need to know which of the three caused the failure.
Circular runout and total runout
Circular runout is checked at individual cross-sections. The part rotates one full turn at each section, and the indicator movement at that section must stay within the tolerance. Each section is judged on its own, so circular runout does not control how the sections line up along the length.
Total runout is checked while the indicator traverses along the surface as the part rotates. The whole surface must stay within the zone. This catches a taper, a barrel shape or a bend that circular runout can miss because each section is locally round and centred.
Total runout is therefore the tighter and more complete control for a cylindrical surface. It is also more demanding to inspect, because the traverse has to be smooth and the probe has to follow the surface without losing contact.
| Control | What it controls | Zone | Datum | Typical use |
|---|---|---|---|---|
| Circular runout | Each surface element | Two coaxial circles at each section | Axis | A seal land or a short bearing seat |
| Total runout | The entire surface | A cylinder coaxial with the datum | Axis | A long bearing journal or a pulley face |
| Coaxiality | The extracted axis | A cylinder about the datum axis | Axis | An axis that must align with a bore |
| Position at MMC | The axis at a defined location | A cylinder at true position | Datum frame | A clearance hole or a bolted joint |
Runout applies to surfaces. Coaxiality and position apply to axes. They are not interchangeable even when the tolerance values look similar.
Reading the callout and the datum
A runout frame shows the runout symbol, the tolerance and the datum axis. The frame is placed on the surface or on a leader that points to it. The datum is usually a bore, a journal or a pair of centres established as a common datum.
The datum order matters. If the part is located by a bore and a face, the bore may be the primary datum that establishes the axis, and the face may be the secondary datum that stops axial movement. Reversing them changes the setup and can change the reading.
For a part supported between centres, the drawing should say so if the centres are the datum. If the drawing names a bore, a between-centres setup is not a valid substitute, even if it is the way the part was machined.
Concentricity and coaxiality
Concentricity is a location control for the axis of a feature of size. In the ASME definition, it controls the derived median points of diametrically opposed surface elements. The points must lie within a cylinder centred on the datum axis. It does not control the size or the roundness of the surface.
That definition is difficult to inspect. The inspector has to find many opposed pairs of points, derive their midpoints and compare them with the datum axis. A lobed or oval bore can pass a concentricity call while a simple diameter measurement looks acceptable. The result is sensitive to the number of points and the software method.
ASME Y14.5-2018 removed the concentricity and symmetry symbols. The replacement is usually position or total runout, depending on the function. In ISO, coaxiality controls the extracted axis and is the more direct call for an axis that must align. Legacy drawings may still use concentricity, and those calls need a clear inspection method.
Concentricity is not the same as roundness
A concentricity call says nothing about the size or the form of the surface. If the bore must also be round and the right size, those need their own controls.
Choosing the right control
Start with the function. If the surface is a bearing journal, a seal land or a pulley face, runout is usually the best match because the function is rotation. If the feature is a clearance hole that must accept a pin, position at MMC is usually better because it models the clearance directly.
If the axis must align with another axis and the surface form is not the issue, coaxiality is a reasonable call. If the part must be balanced, runout at the bearing surfaces is a practical proxy for the eccentricity that drives the imbalance.
Avoid concentricity for new drawings unless a customer specification requires it. It is hard to measure, hard to defend and rarely describes the function better than runout or position. The inspection cost is real, and the result can depend on the analysis method.
Inspection setup
A runout check needs a stable rotation about the datum axis. Common setups are a precision spindle, a pair of V-blocks on a datum journal, a chuck on a datum bore or centres when the drawing names centres. The setup must not add its own eccentricity to the reading.
The indicator should be perpendicular to the surface and set to zero at a convenient point. For circular runout, rotate one full turn at each section and record the maximum minus minimum. For total runout, traverse the surface while the part rotates and record the maximum minus minimum over the whole path.
A CMM can simulate runout by scanning the surface and evaluating the radial distance from the datum axis. The result is only as good as the alignment and the point density. For a tight runout call, a dedicated spindle and indicator is often more repeatable than a CMM.
- Establish the datum exactly as the drawing states.
- Check the setup for its own eccentricity before measuring the part.
- Use enough sections for circular runout and a full traverse for total runout.
- Record the maximum and minimum readings, not just the difference.
- State the rotation speed and the instrument where it affects the result.
Mistakes that produce false results
The first mistake is measuring about the machining centres when the drawing names a bore. The second is using circular runout and assuming it controls the whole length. The third is applying runout to a feature that has no datum axis, which leaves the callout incomplete.
The fourth is using concentricity when position at MMC would model the function. The fifth is measuring a flexible part in a clamped state that is different from service. The sixth is comparing a runout result with a diameter tolerance and treating them as the same requirement.
A seventh is a datum axis established from two features that are not coaxial. If the drawing intends a common datum, say so. If it does not, the inspector may pick one feature and ignore the other, which changes the result.
Specifying runout on a drawing
Put the runout frame on the surface that rotates, name the datum axis and state whether the call is circular or total. If the surface is interrupted by a keyway or a hole, say how the interruption is handled. A probe that drops into a keyway can give a false reading.
Set the tolerance from the function. A bearing seat has a clearance and a required film thickness. A seal has a lip contact and a permissible eccentricity. A pulley has a belt tracking requirement. The runout value should come from one of those, not from a habit.
Finally, agree the inspection setup at first article. Runout is one of the controls where the setup is part of the measurement. A supplier and a customer who use different setups can disagree about a part that has not changed.
Frequently asked questions
What is the difference between circular runout and total runout?
Circular runout checks each cross-section separately while the part rotates. Total runout checks the whole surface while the indicator traverses along it. Total runout also catches taper and bend between sections.
Does runout control size?
No. Runout controls location, orientation and form relative to a datum axis. The size of the feature needs its own limit dimensions.
Why was concentricity removed from ASME Y14.5?
ASME Y14.5-2018 removed the concentricity and symmetry symbols because they were difficult to inspect consistently and other controls described the function more directly. Position and total runout are common replacements.
When should I use runout instead of position?
Use runout when the function is rotation, such as a bearing journal, a seal land or a pulley face. Use position at MMC when the function is clearance for a fastener or alignment of a hole pattern.
Sources and further reading
- ISO 1101:2017, Geometrical tolerancing, tolerances of form, orientation, location and run-out
- ASME Y14.5-2018, Dimensioning and tolerancing
- Qumek first article inspection and rotating part measurement notes
Qumek Quality Desk
Quality team
The quality team responsible for first article inspection, PPAP packs, supplier audits and the records shipped with production parts.
Specifying runout on a rotating part?
Send the drawing and the bearing or seal interface. The quality desk will check the datum axis, the runout type and the inspection setup.
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