# key - oversized or undersized?



## Karter2 (Nov 17, 2011)

PAK65 said:


> I have an impulse 9 motor, which has a keyway described on the spec sheet as 0.252/0.250 wide. The to differnt keys available from Grainger are listed as either oversized +0.002 or undersized -0.002.
> 
> Does the 0.252 on the spec sheet mean that I should use the oversized key?
> 
> They are only $2 each, so maybe I'll just get both to make sure I get a tight fit.


Get oversize and lap it to fit correctly.


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## electro wrks (Mar 5, 2012)

This is Impulse's way of calling out tolerances. What they say as a keyway width of 0.252/0.250, is the same as a nominal width of 0.250" between(over) +0.002", (under) -0.000" width. Notice on the Impulse spec. sheet drawing the main output shaft is shown as 1.1250/ 1.1245 diameter, which is 1.1250" between(over) +0.0000", (under) -0.0005". Or, in other words, it is undersized from a nominal 1.1250" diameter by up to 0.0005", or half a thousands of an inch. Note that within these tolerances, which are typical for an electric motor, the width of the keyway or the diameter of the shaft could vary along their lengths. This could make fitting a key or shaft hub with closer (more precise) tolerances or over/undersized a difficult task.


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