Keep Those Shafts Straight With Runout!
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
2.7.1.3, 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.
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