13.1 Circular and Total Runout
Key Takeaways
- Runout is a datum-related surface control evaluated by rotating the part about a datum axis; both circular and total runout require a datum reference that establishes that axis.
- Circular runout limits full indicator movement at each circular element independently; on a cylinder it combines local roundness and coaxial relationship but does not control variation along the length.
- Total runout controls the entire indicated surface during rotation and sweep; on a cylinder it adds control of longitudinal form, while on a face it controls the entire face relative to the datum axis.
- Runout can validly apply to a face perpendicular to a datum axis; the tolerance-zone geometry depends on whether the controlled surface surrounds the axis or intersects it.
- Y14.5-2009 calls two or more datum features acting together multiple datum features; examples in this guide identify two journals as jointly establishing datum A instead of using a two-letter datum-precedence sequence.
13.1 Circular and Total Runout
Quick Answer: Both runout controls require a datum axis. Circular runout limits full indicator movement at each circular element independently. Total runout evaluates the entire indicated surface while the part rotates and the indicator traverses the surface. Runout is not limited to cylinders and cones: it can also control a face perpendicular to the datum axis.
The Datum Axis Is Mandatory
Runout is evaluated relative to an axis established from the referenced datum feature or features. A reference that establishes only a plane is insufficient unless another referenced feature establishes the axis needed for rotation. The setup must simulate the specified datum system; simply spinning the part about a convenient machine axis does not satisfy the drawing.
Circular Runout
For a cylindrical surface, circular runout is checked at each cross-section normal to the datum axis. The indicator remains at one axial location while the part makes a full revolution; full indicator movement at that circular element may not exceed the tolerance. Each section is evaluated independently, so circular runout does not control taper, bow, or other longitudinal variation between sections.
For a face perpendicular to the datum axis, circular runout may be checked along individual circular elements on the face. It limits face wobble at each selected radius during rotation. This is a valid face application, not an invalid callout.
Worked Cylinder Example
A shaft has two separated bearing journals identified as the multiple datum features that jointly establish datum A. A middle diameter has 0.05 circular runout to A. Both journals are simulated together to establish one datum axis, and the middle diameter is checked at several axial stations. A taper may pass if every station separately stays within 0.05; circular runout does not relate the readings along the length.
Total Runout
Total runout evaluates the entire controlled surface as the part rotates and the indicator traverses that surface. On a cylindrical surface, the tolerance zone is two coaxial cylinders separated by the tolerance value. It combines the coaxial relationship with circular and longitudinal surface variation, so taper or barrel shape that circular runout misses can fail total runout.
On a face perpendicular to the datum axis, the total-runout zone is two parallel planes perpendicular to that axis. Sweeping the indicator radially while the part rotates controls the entire face's variation relative to the datum axis, combining face flatness effects and perpendicular orientation in one requirement.
Comparison
| Aspect | Circular Runout | Total Runout |
|---|---|---|
| Evaluation | One circular element at a time | Entire controlled surface during rotation and traverse |
| Cylinder | Local roundness plus relationship to datum axis | Adds longitudinal form, including taper and generator variation |
| Face | Wobble at each circular element | Variation across the entire face |
| Datum axis | Required | Required |
| Same-value relationship | Less comprehensive | At least as restrictive for the corresponding surface application |
What Runout Does Not Do
Runout does not independently report how much error comes from form versus coaxiality or orientation; it limits their combined indicator effect. It does not replace position when the design intent is to locate a derived axis for assembly, and it does not apply to an arbitrary nonrotational contour such as a keyway wall. Select the control from the functional requirement and the surface being indicated.
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 circular runout to a flat shoulder face on a shaft, referenced to the shaft datum axis. What is the correct assessment under ASME Y14.5-2009?