I have a gear with a D-shaft that is intended to directly drive a turntable. Something relatively heavy at very low speeds and with not much torque, if it's important.

I really don't want the gear to take the weight of the load, so i want to put a ball bearing between them. So now i am drawing a shaft that comes out of the turntable and goes through the bearing, and has a D-shaped hole in it.

Problem 1: the D-shaped hole requires some fillets as i understand the production - but also small fillet radius is not liked by the manufacturers.

Problem 2: the shaft diameter is 8mm, the bearing bore diameter is 9mm. So that leaves me only 0.5mm thick tube for the D-hole. Seems too thin. Larger bearing is a problem because then i don't know how to mount the motor.

upd. A picture.

parts that i need to connect - not including anything to hold the red shaft or motor

  • $\begingroup$ Picture > 1000 words. $\endgroup$
    – Transistor
    Dec 30, 2023 at 4:35
  • 1
    $\begingroup$ it's actually pretty hard to me to make a clear picture. But I'll try $\endgroup$
    – TQQQ
    Dec 30, 2023 at 4:40
  • $\begingroup$ have you considered using an 8 mm bore bearing on the motor shaft? $\endgroup$
    – jsotola
    Dec 30, 2023 at 5:27
  • $\begingroup$ @jsotola I can't figure out how to do it. Which does not mean it's impossible, I am not a mechanical engineer $\endgroup$
    – TQQQ
    Dec 30, 2023 at 5:34
  • $\begingroup$ duckduckgo.com/?q=pillow+block&iax=images&ia=images $\endgroup$
    – jsotola
    Dec 30, 2023 at 5:35

1 Answer 1


From the comments (but should be in the question!):

The motor is going to be positioned vertically. So the weight of the table (up to 15kg) will be directed axially, which is why i don't trust gear/motor internal bearings.

You are right to be concerned. You need a vertical pillow bearing to take the weight and possibly a second bearing to ensure shaft alignment. See vertical+pillow+block+bearing image search.

Then you need a flexible shaft coupling to cater for any axis misalignment and to ensure that no vertical load is applied to the gearbox. In your case you want one that you can drill out to 9 mm on one end and 8 mm on the other. A clamp ring will grip the D-shaft adequately but you could use a grub screw if you wish.

Update regarding your proposal:

enter image description here

Figure 1. Gearbox mounting, standoffs and bearing support.

enter image description here

Figure 2. Cross-section view.

Instead of two standoffs, turn a hollow cylinder to go from the gearbox plate to the bearing support (and make the bearing sit into the top). Add a pair of slots to allow access to the coupling clamping screws.

enter image description here

Figure 3. A star coupling. Image source: Miki Pulley.

Another possibility is a star coupling. This allows for some misalignment of the two axes, provides some vibration absorbtion and has the advantage that the stars can be tightened on each component before assembly. During assembly the star is inserted, the shafts aligned and the two halves engaged.

  • $\begingroup$ Thank you. Sorry, but I still don't understand what to do about the pillow bearing $\endgroup$
    – TQQQ
    Dec 30, 2023 at 16:40
  • $\begingroup$ Have a look at 2:45 on youtube.com/watch?v=8q25EUszBSI. $\endgroup$
    – Transistor
    Dec 30, 2023 at 16:47
  • $\begingroup$ OK, so this part i did know. Overall, correct me i am wrong, you propose to fix the bearing in some way to hold the weight, have the table shaft go through the bore down, then have a shaft coupling fixed on it and the motor. I will try to model it, but it seems i will not have a good access to the coupling between the bearing and the gear, if any $\endgroup$
    – TQQQ
    Dec 30, 2023 at 18:13
  • $\begingroup$ Yes, the bearing can just sit in a counterbore hole or pocket so that it's supported underneath. You really need a second bearing to keep it straight. $\endgroup$
    – Transistor
    Dec 30, 2023 at 18:46
  • $\begingroup$ Got it, thank you very much $\endgroup$
    – TQQQ
    Dec 30, 2023 at 19:19

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