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Simultaneous Requirements Is a Rule
(In accordance with Y14.5M-1994 standard)

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Past Tips have presented the concept of simultaneous requirements. However, I still get questions about it. Keep in mind that the Y14.5 standard has many default conditions. If we didn’t have these defaults, our drawings would have a lot more notes and dimensions. Also, remember that the Standard is written mainly by people from design and they are trying to define the design intent. If you are from design and you are creating a default, I guarantee it will be conservative to protect the design. Rule #1, for instance, is a very restrictive rule defining what the limit of size means. The simultaneous requirements rule is no exception.

Let’s say this shaft rides in a bearing on datum feature A. If the components that mount on this shaft are a pulley for a V-belt and a friction drive roller, the orientation of the keyseat and the pair of flats on the other end of the shaft does not matter. However, if a timing gear is driven by the keyseat and the flats drive a cam, the orientation of these features to one another could be very important. The default due to simultaneous requirements is that the keyseat and flats must be within their position tolerances simultaneously. In other words, they are a pattern. This is more restrictive for production and therefore more costly to produce and inspect. To “unlock” this requirement for the V-belt pulley and a friction drive roller application, the abbreviation SEP REQT must be placed beneath each feature control frame. Of course, if you do not care about the cost of parts—don’t bother.

Note: The implied 90° rule defines the goal that the features are intended to be perpendicular, but it does not limit how much they can be out of perpendicular—simultaneous requirements does.

Back to Tips Tip added Feb 2008