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What is the cross-sectional shape in blue called? The shape, used on the shafts, is used for torque transfer, gears or similar. The three arcs are same diameter as the ball bearing ID, so bearings rest centered perfectly, but flat parts are used for engagement of sliding gears. Because of the shape and surface of engagement is much stronger than standard cut out keyway. cross-sectional shape

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    $\begingroup$ Annulus with flats? Or are you hoping for a single word that encompasses the three flats? $\endgroup$ – Jonathan R Swift Jul 8 '18 at 5:48
  • $\begingroup$ Can you tell us why it matters? I'd refer to it as "Callout Number X" as indicated on the Mech drawing. $\endgroup$ – Carl Witthoft Jul 9 '18 at 17:12
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I would call it a “Tri-Lobe”. It comes in many forms, but that’s the general concept.

Here’s a similar object which uses a tri-lobe shape which is a “capto” style tool holder tri-lobe tool holder

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  • $\begingroup$ Downvoted because incorrect information: A trilobe (something trilobular), is a specific "curve-of-constant width", and as such the blue shape does not match this description. The shape shown has flat faces, which cannot exist on a trilobular cross section. en.wikipedia.org/wiki/Trilobular $\endgroup$ – Jonathan R Swift Jul 8 '18 at 14:47
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    $\begingroup$ I am not familiar with the shape, but there are lots of shapes and designs that are not named. Tri-Lobe (three lobes) sounds like a good name to me. I think it encompasses the subset of Trilobular without an issue. Like Rectangle is to Square as Tri-Lobe is to Trilobular. $\endgroup$ – ericnutsch Jul 11 '18 at 22:27

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