What are the advantages and disadvantages of using circular section ("Scotch") keys, keyseats and keyways? I am not referring to Woodruff style, I am only interested in the type where the key itself if a full cylindrical piece and the shear force is diametric. For instance...

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Woodruff style and straight cut style seem overwhelmingly more common than circular section types. What is the reason for this?

It seems to me that it would be very convenient to be able to simply drill a keyway/keyseat into a parts before turning the ID/OD to it's final diameter.

In addition I would think that a circular feature such as this would eliminate stress concentrations that would be present on broached and milled square section cuts. Key joint cutting is in any case a precision operation so I doubt there's any major loss in tolerance quality over other types of keys.

I don't have any particular application in mind for this, I'm merely curious. However I would like to know some situations where circular keys are advantageous or not.

Edit: Further thoughts: Wouldn't a circular key actually experience more of a compression force than a shear force, especially under high torque applications? Given the many non-parallel shear forces all converging at the centre of the key, this would manifest as compression would it not?

  • $\begingroup$ Try making that configuration and I think you'll quickly see what some of the disadvantages are. Without support a drill bit will wander. Likewise, the tool cutting out the inside of the shaft will skip around if the keyhole has already been cut. $\endgroup$
    – user16
    Sep 27, 2017 at 12:05
  • $\begingroup$ @GlenH7, Lathes and mills can easily perform interrupted cuts if properly configured. If drilling the keyseats simply do so before the bore is cut. $\endgroup$
    – user6335
    Sep 27, 2017 at 12:06
  • $\begingroup$ one way to do that would be to cut the bore and make a cylindrical plug to fit, then drill the slots with the plug keeping the bit from wandering. Seems like an approach one might take if you had to add the keyway in the field or for whatever reason the part made it difficult to mill a square slot. You can also imagine situations where you would first assemble bore and shaft and drill them together. $\endgroup$
    – agentp
    Sep 27, 2017 at 13:24
  • $\begingroup$ I seem to have misplaced my formula collection but i seem to remeber that the shape has a significantly lower ability to carry torque. But yes its really easy to make with a mill $\endgroup$
    – joojaa
    Sep 27, 2017 at 17:53

1 Answer 1


The only time I have seem them used is on overload clutches where a spring allows the dowel to pop out of the inner member when a torque is exceeded.


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