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Keep those Shafts Straight with Runout!
(In accordance with Y14.5M-1994 standard)

PDF of this Tip

Designers are often faced with specifying a means to control the straightness of shafts. Straightness of an axis on a regardless of feature size (RFS) basis may satisfy the functional requirements but may cause nightmares in inspection. Some designers rely on the straightness control automatically provided by Rule #1 and the size tolerance. Since size tolerances on shafts are usually tight to assure good fit in bearings, this approach frequently results in a straightness requirement that is tighter than required. This is especially true when the length/diameter ratio is large. Also, Rule #1 does not apply for diameters left in the "as furnished" condition (ASME Y14.5M 1994 Section, paragraph (a)). A functional alternative that will usually do the job is to combine datum targets and circular runout as shown below. The datum targets are located where the shaft contacts the bearings. Circular runout controls the circularity and concentricity of each slice of the feature. If the shaft is out of straight, some slices will be eccentric to the datum axis and will show up in the circular runout inspection.

Back to Tips Tip added Nov 1999