This design implements a torque limiter that allows a motor to decouple from the rotating part it is driving part once a certain torque is reached. The purpose is twofold: to prevent the motor from burning out, and to avoid the rotating part from damaging things (e.g., fingers!).

A set of four ball plungers (usually used for catches on small parts) are key to the torque limiting mechanism. The balls extend into the mounting holes of a flange coupler. When too much torque is applied, the balls are forced to retract, allowing the motor and the rotating part to turn independently. This is known as a "ball detent" mechanism (see Wikipedia for further details).

To build the torque limiter, print the lower and upper parts from the provided STL files; source the additional parts; and then follow the assembly and installation instructions.

Bambu Lab

A1

No

Yes

0.4 nozzle, 0.2 layer height

15%

Supports are required so that screw/nut holes print properly.

To assemble the torque limiter, you will need the following additional parts:

• TWO 6mm-barrel flange couplers. I sourced mine from here. You can use other bore sizes (up to 8mm), as long as the other dimensions of the couplers comply with the following specs: barrel outer diameter 10mm, flange diameter 22mm, flange thickness 2mm, four unthreaded M3-sized screw holes offset by 8mm from the axis.
• FOUR 5mm stainless steel ball plungers. I sourced mine from here.
• FOUR M3 x 6mm hex button-head screws.
• SIX M3 x 12mm hex button-head screws.
• TEN M3 nuts.
• SIX M3 washers.
• ONE* 22mm (outer diamter) x 10mm (inner diameter) x 1mm (thickness) stainless steel washer. You can use other sizes (e.g., these), as long as the outer diameter is less than 22mm, and the other dimensions match. A nylon washer may also work well, although I haven't tried it.

The M3 screws/nuts/washers can in principle be sourced from anywhere, since they are standardized. I got mine from here.

To assemble the torque limiter, follow these steps (illustrated in the accompanying pictures below):

1. Insert FOUR M3 nuts into the lower body, ensuring that they are pushed all the way down.
2. Insert FOUR ball plungers into the lower body, ensuring that they are pushed all the way down.
3. Insert the stainless steel washer into the upper body.
4. Insert ONE flange coupler into the upper body.
5. Attach the upper and lower bodies using SIX M3 screws/nuts/washers.
6. Attach ONE flange coupler to the lower body using FOUR M3 screws (which should couple to the M3 nuts installed in step 1).

To install the torque limiter, push drive rods (appropriately sized to fit the couplers) into each of the flange couplers, and secure with set screws. For best results, try to ensure that the drive rod pushed into the upper coupler extends a little way into the lower flange coupler; this will ensure that the two rods are co-axial (but will only work if both couplers have the same bore size).

To increase the threshold torque at which the limiter kicks in, try widening the holes in the upper flange coupler (e.g., using a cone grinder on a dremel) -- but be aware that once the diameter of the holes matches the diameter of the plunger balls, no further increase will be possible.

To reduce the threshold torque, try inserting washers onto the M3 screws, between the lower and upper bodies.