Do standard keyway width tolerances account for offset error from the center of the shaft?

[renasis]

Hello,

I have a question about keyway tolerances as specified by ANSI B17.1-1967 (R1998). For a 1/8" square key, for a class 1 fit, the width and height need to be .125 +.000/-.002. The key seat dimensions need to be .125+.002/-.000. That would give you a line fit in the worst case, but this does not account for the symmetry(or offset) of keyway about the shaft center. It states that this could be .010 max. This value is likely for a large diameter shaft, such a high number would not be tolerated on a 1/2" diameter shaft, but lets say that the keyway is offset .002 from the center. In a worst case, you would have .002 interference. What if the key slot of the hub was offset .002 in the opposite direction. In a worst case you would have .004 interference. This would make things difficult to assemble, I would think. Am I reading this correct or am I off base?

Thanks,

-ren

 

[MikeHalloran]

If there's just one key, there is no angular reference other than the key, so the concept of an 'offset' is inoperative. The key will be more or less centered in both keyseats. If there's any interference, well, that's why keys are made soft, and fitters have files and hammers.



Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

The bigger the file and hammer are tends to makes key-fitting easier , although I would think the "ART" of key fitting is a dying art now-a-days.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

Files? I'm used to seeing people use a belt sander.

 

[MikeHalloran]

Keystock will clog a sanding belt. ... and a file that has been used properly will be faster, anyway. In a shop where the files have been used improperly, yeah, the belt sander will be faster, and nobody will think of doing it any other way.

Now, renasis, if there are reference features other than the key and seats, of course then there may be asymmetries, that you typically correct with an 'offset key'. ... or you may wish to insert an intentionaly asymmetry.

An easy to find example is engine timing gears or sprockets; tuners often wish to advance or retard the cam by a fraction of a degree, much less than the tooth pitch. So speed shops stock offset keys, and sprockets with multiple keyseats, each offset by an integral multiple of the angular tooth pitch plus a degree or fraction.



Mike Halloran
Pembroke Pines, FL, USA