How (or should I) to connect a D-shaft to a load, through a bearing?

Question

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.

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.

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Update regarding your proposal:

Figure 1. Gearbox mounting, standoffs and bearing support.

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.

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.