Suppose a 'D' shaped shaft with a 4 mm diameter like this type

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is fitted inside a module 1 (plastic) gear with a round hole with the same diameter. If the gear has a set screw, will this still be a good fit? Will all the force be transferred to the set screw or will it be distributed onto the plastic surface as well?

Should only round shafts fit inside gears with a round hole? Would connecting D shaft to a round hole gear be less preferable than connecting a D shaft to a D hole - even with a set screw? Yes, there's a set screw, but the flat part of the shaft will have a smaller contact surface, right?

  • $\begingroup$ FYI, this exact question was asked by OP on the Robotics SE site, where I have voted to close it. $\endgroup$
    – Chuck
    Commented Jan 7, 2016 at 14:08

1 Answer 1


Quick Answer

  • You can fit D shaped shaft into round hole if diameters are OK.
  • D shaft to D hole without a setscrew will be preferable to D shaft to round hole with setscrew if there is no axial movement. Best is D shaft to D hole with setscrew(s). However you can't always find D holes because they're more costly to manufacture.
  • Shear forces act on setscrew if holding torque isn't exceeded. You can check holding torque below.

Setscrew holding power

Setscrews' holding power is proportional to its diameter you can check Marks’ Standard Handbook for Mechanical Engineers, Eleventh Edition, Table 8.2.18. gives holding power for Cup-Point Setscrews. Maximum load you can get from a setscrew from that table is 7000lb axial holding power for 1 in setscrew. The torque is


where $F_{a}$ is holding power and $D_{s}$ is shaft diameter in inches. Sorry I don't have metric at hand.

D shaft

Advantage of D shaft is it prevents shaft from spinning inside the gear hub. So it is preffered in gears.

An example

If you choose No0 screw (1.524mm diameter), axial holding power is 50 lb (222N) so your holding torque is 444 Nmm. If you exceed this torque setscrew will fail but because of D shaft it will hold till material failure (on plastic shaft probably).

  • $\begingroup$ Thanks. How about round shaft into round hole with setscrew? Is that preferred over D shaft into round hole? $\endgroup$
    – John M.
    Commented Jan 7, 2016 at 15:25
  • $\begingroup$ If you won't exceed torque limits there won't be a problem with round hole round shaft and setscrew(s). However if you exceed, it'll spin. It's up to your failure criteria and cost benefit analysis. $\endgroup$ Commented Jan 7, 2016 at 15:29
  • $\begingroup$ All this theoretical stuff is nice, but set screws almost always vibrate loose sooner or later. Real World Solution: drill a little hole in the shaft as deep as the diameter of the set screw for the set screw to nest in and use a little Loctite (semi-permanent) on the set screw threads. $\endgroup$ Commented Jan 7, 2016 at 18:38
  • $\begingroup$ @WilliamHird I agree all threads get loose given enough time and vibration. $\endgroup$ Commented Jan 8, 2016 at 11:06

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