Can we build an arm without an idler?

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in the hardware upgrade thread I made an effort to design an arm that would not need an idler gear, which looked really good until Bar pointed out that it could not be stacked as the motor overlapped the spool (oops)

So I went digging to find other options

@bar, what is the spec for the motors (speed/torque), and what is the spec for the motor drivers (max current)?

I’ve found a couple interesting options (for apples to apples comparisons, the motor shaft is in the same place as the current motors, but that can be adjusted

The current spool has an outer diameter of 125mm and a bearing dia of 82.5mm and doesn’t quite hold 14.5ft of belt (my calcs show it should be 126 and 82.8 with the belt turned in and belt teeth on the spool)

There are 37mm motors with the gear shaft offset 7mm from the center, this would allow a 18 tooth gear to extend past the side of the motor. This could mate up with a 97 tooth spool, which would be ~148mm on the outside, and 112mm opening at the bearing surface with room for 4500mm of belt

an example of motors in this class are:
3a stall current 37mm
https://www.aliexpress.us/item/3256805242378821.html
3a stall current 37.5mm
https://www.aliexpress.us/item/3256805252915678.html

There is also a line of high torque 32mm motors with centered drives, they would allow for a 24t gear at the motor and a 91 tooth spool with an OD of 139mm and a bearing surface of just under 100mm with 4500mm of belt

an example of this motor is:
4.5a stall current
https://www.aliexpress.us/item/3256807592368250.html

and finally, with the current 37mm motors, we would have to use a 27 tooth gear (43.5mm diameter, which would make the angles worse) with a 88 tooth spool, OD 135mm bearing diameter 95mm

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I initially designed it to use those 37mm motors without an idler gear, but those motors do not use a planetary gearbox. The planetary gearbox can handle a much higher torque and lasts longer because there are three gears engaged instead of one which makes a huge difference for how long they last

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Bar wrote:

I initially designed it to use those 37mm motors without an idler gear, but those motors do not use a planetary gearbox. The planetary gearbox can handle a much higher torque and lasts longer because there are three gears engaged instead of one which makes a huge difference for how long they last

I thought about that, and with things as crowded as they are, I’m thinking the
32mm planetary motor may be better.

what is the torque/rpm of the motors we are using, and how much current can the
motor drivers on the board support?

Davig Lang

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The current motors are 50rpm at 30kg-cm. I think that the motor divers are good for 8A or something like that so it shouldn’t be a limiting factor. The strength of the belt is the limit on the system now.