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I choose an AC motor (IEC) with frame number 71. The key on the shaft of the motor is $5mm,$, $4.23 mm$ and $6 mm$ in diameter. At other hand i cant't find a safety coupling (torque limiter) with exactly the same dimensions as the key way on the motor. They are all a bit smaller or bigger but they have the same diameter as the motor. for instance $4 mm$, $2.23 mm$ and $6 mm$ in diameter.

what can i do ? Is it possible to use a slightly greater safety coupling or the dimensions of the both, should exactly be the same ?

enter image description here

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  • $\begingroup$ A diagram would help, what is the shaft diameter? And woodruff keys are usually rectangular... $\endgroup$ – Solar Mike Apr 21 '18 at 21:02
  • $\begingroup$ @SolarMike i try to update the question by providing photos. The shaft of the motor is 12 mm $\endgroup$ – Sam Farjamirad Apr 21 '18 at 21:07
  • $\begingroup$ Did you notice the R6 measurement means radius of 6mm, 12 mm diameter. This will change your search options. $\endgroup$ – fred_dot_u Apr 21 '18 at 21:22
  • $\begingroup$ @fred_dot_u , i know, but as i stated, that rectangular sections causes me troubles, i can't find any safety coupling with exactly the same dimensions as this rectangular section $\endgroup$ – Sam Farjamirad Apr 21 '18 at 21:25
  • $\begingroup$ As solar mike suggests, woodruff keys are rectangular. The drawing you've provided suggests that the proper key size is as dimensioned. I expect that you'll find a slot in the motor shaft to accept a key to mate with your coupling. One then cuts the key to fit as required. Do you know if the shaft has the keyway cut and provides a key? More product information would be useful (links?) $\endgroup$ – fred_dot_u Apr 21 '18 at 21:29
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This extract from a Baldor IE Chart may help.

enter image description here

Figure 1. The Frame 71 keyway dimensions are highlighted.

As everyone else has pointed out, keyways and keys are rectangular so we can't figure out where all your diameters are coming from.

Your drawing is a little strange as it seems to have a shaft entry with keyway rather than a protruding shaft.

Is it possible to use a slightly greater safety coupling or the dimensions of the both, should exactly be the same?

The keyway should be a snug fit. Any play or slop will result in backlash during starting and stopping and this will chew up the key or shaft or both.

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Once you find a coupling that fits the shaft diameter and the other constraints such as size etc, then all you need to do is to get a machine shop to cut a matching keyway to match the one in the motor shaft.

They will also machine a key to match f necessary. This is a small easy job...

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