13.3 Runout vs Position, Profile, and Form
Key Takeaways
Runout controls indicated surface variation relative to a datum axis; position controls a derived axis or center plane; profile controls a surface relative to true profile; form controls shape without a datum.
Runout is often economical for rotating cylindrical surfaces because one reading combines surface form effects with relationship to the datum axis.
Runout may also control a face perpendicular to a datum axis; circular face runout limits wobble by circular element, while total face runout controls the whole face.
Choose perpendicularity for a pure face-orientation requirement, and choose face runout when rotation and indicator variation are the functional concern.
For this Y14.5-2009 guide, two journals acting together are described as multiple datum features establishing one datum A, not as an ordered two-datum sequence.
13.3 Runout vs Position, Profile, and Form
Quick Answer: Select the control from what function must be protected. Runout limits indicator variation of a surface during rotation about a datum axis. Position locates a derived axis or center plane. Profile controls a surface relative to true profile. Form controls shape without a datum.
What Each Control Constrains
- Runout is datum-axis related and acts on the indicated surface. Its result combines applicable surface-form error with the surface's relationship to the datum axis.
- Position constrains the derived axis or center plane of a feature of size in a basically located zone; it does not directly control every surface point.
- Profile constrains surface points relative to true profile and can combine form, orientation, location, and sometimes size effects.
- Form controls a feature's shape without locating or orienting it to a datum.
Cylindrical and Face Applications
For a bearing journal, seal diameter, or pulley surface that must run true to an assembly axis, runout directly matches the rotational function. Circular runout checks individual sections; total runout traverses the entire cylindrical surface and adds longitudinal form control.
A flat shoulder face perpendicular to the datum axis is also a valid runout application. Circular face runout limits wobble at each circular element. Total face runout evaluates the full face during rotation and radial traverse. If the only requirement is abstract orientation, perpendicularity is usually clearer; if operational wobble during rotation is the concern, face runout may be the direct functional control.
Decision Table
| Functional requirement | Usually choose | Why |
|---|---|---|
| Cylindrical surface must run true to datum axis | Runout | Direct indicator-based composite surface requirement |
| Face wobble during rotation must be limited | Circular or total face runout | Validly relates the face to the datum axis |
| Face needs only orientation to an axis | Perpendicularity | States pure orientation without a rotational composite reading |
| Feature axis must locate for assembly | Position | Controls the derived axis or center plane |
| Complex contour must match true geometry | Profile | Controls surface points relative to true profile |
| Shape alone matters and no datum relationship is needed | Form | Datum-free control |
Worked Scenario
A shaft's two bearing journals are identified as multiple datum features that jointly establish datum A. A finished cylindrical hub surface must run true to the bearing axis and must not taper along its width. Total runout of the hub surface to A is the direct choice: both journals establish the datum axis together, and the indicator traverses the full hub surface while the part rotates.
Position of the hub's derived axis would locate the axis but would not directly limit all surface form. Cylindricity would control form but would not relate the hub to datum A. Profile could express a surface relationship, but total runout is the purpose-built rotational control for this simple cylindrical function.
For an involute gear-tooth flank, by contrast, profile is appropriate because the controlled surface follows a defined noncircular true contour. The decision turns on the actual controlled surface and function, not merely on whether the overall part rotates.
Senior Traps
- Saying every flat-face runout callout is invalid. Face runout is expressly possible.
- Applying runout without a datum reference that establishes an axis.
- Choosing position when the requirement includes surface form during rotation.
- Choosing form when relationship to the datum axis also matters.
- Treating two multiple datum features as an ordered A|B datum sequence.
A shaft hub must run true to datum A, where two bearing journals are identified as multiple datum features establishing A; surface roundness and longitudinal straightness must be included. Which control is most direct?
Position of the hub axis to A
Total runout of the hub surface to A
Cylindricity of the hub with no datum
Perpendicularity of the hub end face to A
A flat shoulder face must have limited wobble as a shaft rotates about datum axis A. The designer applies circular runout to the face relative to A. What is the correct assessment?
Valid; circular runout limits indicator variation at each circular element of the face
Invalid; runout can never apply to a planar face
Invalid unless the face is assigned as datum B
Valid only when no perpendicularity relationship exists
Which scenario is the clearest case where profile of a surface is the correct control instead of runout?
A cylindrical pump shaft seal journal that must run true to the bearing seats
A pulley belt-running surface that must run true to the bore datum
An involute gear tooth flank that must match a defined true contour relative to the gear axis
A motor shaft output diameter that must run true to the bearing journals
Why is a position tolerance generally a poor substitute for runout when the service requirement is 'the surface must run true to the datum axis'?
Position cannot reference a datum axis at all under Y14.5-2009
Position controls only the axis or center of the feature of size and says nothing directly about the surface's form, so it under-controls the composite run-true requirement
Position is always more restrictive than runout at the same value
Position is only valid for flat surfaces, not cylinders
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