I'm in the need to create a "drivetrain" for a smooth and slow rotation and need only very low torque (for a Christmas pyramid). Such a Christmas pyramid usually is driven by candles and turns very slow (magnitude of ~10min-1), has very low friction (a metal tip rotates inside a tiny, concave glass plate similar to a contact lense) and continues to rotate a couple of times for itself. Using a gearbox is no option for me.

Using a stepper motor would allow the slow rotation, but I think it will not be smooth enough for the required low speeds. Would it be possible to use an old BLDC (hard disk) motor to run at so low speeds smoothly? Are there other options of low-torque slow motors?

  • \$\begingroup\$ Why not gearbox? \$\endgroup\$ – winny Oct 21 '17 at 15:13
  • \$\begingroup\$ No chance without a gear. Use the outer radius of the lowest floor of the pyramid as the big wheel, and the axis of the motor with a rubber hose on it as the pinion. This should be small enough not to be seen. \$\endgroup\$ – Janka Oct 21 '17 at 15:23
  • \$\begingroup\$ @Janka: Unfortunately, this would put force onto the spindle and cause damage to the glass plate. Using the bottom of the lowest floor also would not work because it is not 100% perpendicular to the spindle. \$\endgroup\$ – Thomas S. Oct 21 '17 at 20:25
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    \$\begingroup\$ If you need it real flat, you could also use the drive motor of an old 3,5" floppy drive. That's about 5mm in height. You need to adjust the speed, though. \$\endgroup\$ – Janka Oct 21 '17 at 20:30

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Figure 1. A typical low-powered AC gearmotor.

Using a gearbox is no option for me.

It's not clear why this is so but if it is due to space constraints you may not be aware that miniature mains voltage AC all-in-one gearmotors exist. They are very low power (a few watts) and available in a very wide range of shaft speeds.

The only downside I can see to this solution is that the output shaft is generally off-centre and this may be an issue in your project.

Crouzet is a name that comes to mind from the distant past and they are still in business. I'm sure you will find something suitable.

  • \$\begingroup\$ +1 And cheap as dirt.. You can usually pick one up as a spare part at any hardware store that sells furnace humidifiers. \$\endgroup\$ – Trevor_G Oct 21 '17 at 15:47
  • \$\begingroup\$ I known that commercial providers use such AC gear motors (e.g. used for disco spheres), but they need to raise the pyramid to hide it below it. Of course, the space problem would be the same for a lot of other motors, e.g. most steppers. \$\endgroup\$ – Thomas S. Oct 21 '17 at 20:27
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    \$\begingroup\$ You can also look for a broken microwave oven, they have those under their plate. \$\endgroup\$ – Janka Oct 21 '17 at 20:28
  • \$\begingroup\$ A small step motor might work....just keep the excitation (current in the windings) low. That will result in less vibration. Also, do not use (switching or otherwise) current regulators. Use a plain L/R drive.That will also result in less vibration at low speed. \$\endgroup\$ – whitegreg56 Oct 26 '17 at 2:11

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