GD&T Runout

Mufaddal Rasheed Author

What is the meaning of Runout in GD&T callouts? what is circular and Total runout ? How is it inspected?

Runout geometric control is generally applied to surfaces which undergo rotation like shafts, hub , wheels etc.

There are two types

• Circular runout
• Total runout

Here is a callout of a circular runout on a stepped shaft.

The primary datum feature of size is the smaller step of the shaft and the large step of the shaft is being controlled with respect to its runout within a tolerance zone of 0.1

What is the definition of the tolerance zone ?

• The tolerance zone for circular runout is 2D disc with a width of 0.1 spaced equally about the nominal surface of the shaft.
• The tolerance zone is not a cylinder but a local disc for each cross section of the shaft being inspected for circular runout.
• The part is held at the primary datum feature A which is smaller step and then rotated.
• A dial gauge indicator is in contact with the surface as shown in green
• As the part is rotated , the dial gauge deflects and gives a total indictaor reading.
• This variation is nothing but local runout or circular runout for that cross section.
• Then this is repeated for multiple cross sections of the shaft
• Each element is controlled separately with circular runout.

Here is circular runout applied on perpendicular to axis surfaces.

The tolerance zone shape changes here , it is a set of concentric circles with a width of 0.1 based on the end flat face which is being controlled.

The inspection procedure remains the same , that the dial gauge indicator is brough into contact with surface and set to zero , then the total indicator movement reading is taken if its below 0.1 mm then part is acceptable in circular runout.

Same drawing with total runout applied:

Tolerance zone for total runout is concentric cylinders spaced with a value of 0.1 .

Circular runout vs total runout:

• The major difference is that when Total runout is inspected the dial guage reading is taken for the complete surface as a whole at one instance and not individual cross section
• Total runout is more stringent, a part can pass off in terms of circular runout but may fail the test for total runout.
• Ther tolerance zone definition changes with respect to the rotating surface targeted .
• There are no material modifiers attached to Runout

Example of application of runout:

Runout vs cylindricity?

Total Runout is a composite geometric control which controls both Cylindricity, Concentricity (position) and orientation of axis.

A reason why Runout is more commonly used for rotating cylindrical surfaces as compared to fundamental form controls like cylindricity.

When is runout used?

• When symmetry and balance of part is critical around an axis
• To avoid vibrations and oscillations
• Total runout is preferred when parts require tighter form control additional to orientation and position control
• For drill bits, precision rotating shafts Total runout is preferred.

To learn more about GD&T have a look at this course

Geometric Dimensioning and Tolerancing: Basics

Categories: GD & Tolerancing