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I already have a hand cranked sit stand desk, which I would like to motorize without too much damage to the existing mechanism and (for not too much money). As it is hand cranked, there is an existing lifting mechanism.

Details on the desk: Manufacture's website

The crank requires 14.7N (1.5Kg) at 16cm away from the center of axle: Crank front view Top View of crank

And the connection in to the lift mechanism is a 6mm hex hole: Mechanism

I was thinking of using a stepper motor to replace the crank as it would give good acuracy and was easy to control with an arduino (or something like that).
Up til now I haven't found anything with the correct amount of torque suitable for this application (I was going off ~2.5Nm).

The mechanism does not lock

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    \$\begingroup\$ Your design parameters are wrong. What you have missed is time dimension. The motor capable of that torque with 10000 RPM would break your crank in a fraction of a second. However if you use low speed motor with suitable gearbox you'll find out that your torque requirements are nowhere that high \$\endgroup\$ – Maple Aug 19 '18 at 19:56
  • \$\begingroup\$ Would you be able to point me in the right direction? \$\endgroup\$ – Benjamin Brown Aug 19 '18 at 19:58
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    \$\begingroup\$ I think I would just figure out how to attach a cordless drill. \$\endgroup\$ – mkeith Aug 20 '18 at 4:47
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Let's say it takes 20 turns to complete the transition, and you want it done in 10 seconds. Then required crank speed is 120 RPM.

Now, if you use 1:60 gearbox then the motor you are looking for is 7200 RPM with only 500 g-cm torque. There are many DC motors in this category, although this is rather on high side. If slightly longer time is OK with you then finding motor will be easier.

So, the design parameters for geared motor are: 80-150 RPM, 2.5 N-m (25 kg-cm)
For non-geared: 7000-15000 RPM, 0.04 N-m (0.42 kg-cm) + 1:60-1:150 gearbox

With all of the above start browsing. Some examples of what you can find:
24V, 30 kg-cm, 60 RPM, 1.6A
24V, 27 kg-cm, 90 RPM, 3.5A
12V, 30 kg-cm, 85 RPM, 5A - "window motor"

Automotive 12V power window motors seems to be great choice for this project.

Also note the following:

  • You don't need stepper motor. Because of high gear ratio it is easy to achieve precise positioning of the desk with simple DC motor.
  • It seems you don't need uni-directional (like worm gear) transmission either, because cranks like that usually have worm gear already, no risk of desk falling down when voltage removed. UPDATE: from your comment it is clear you need a worm gear after all.
  • You do not need Arduino or any kind of computer control. A simple sircuit with either two DPDT buttons or one non-locking rocker switch is sufficient.
  • You do need two end switches to stop motor at the ends of travel range.

Here is an example of a circuit with two "up" and "down" buttons and two end switches. When I see somebody trying to use Arduino for something as simple as this I am losing my faith in humanity :)

enter image description here

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  • \$\begingroup\$ The mechanism does not have a worm gear, if you take out the crank an apply pressure to the top of the desk, the mechanism unwinds and the desk goes down. \$\endgroup\$ – Benjamin Brown Aug 19 '18 at 22:17
  • \$\begingroup\$ Then look for worm gearbox or worm-geared motor, like in the first link. \$\endgroup\$ – Maple Aug 19 '18 at 22:19
  • \$\begingroup\$ Oops.. that is a brushless motor in the link, I missed that. You can use it, of course, but DC motor would be much easier and cheaper to control. Not everything needs Arduino. I have added another link. \$\endgroup\$ – Maple Aug 19 '18 at 22:32

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