The original system employed the use of the Greartisan 12V brushed motor rated at 100rpm. The system was designed to have an input crank link with a D-shaft cutout matching the motor shaft's cross section profile itself.
The motor is rated at 12V and 1.1A, and so the theoretical max power is given by P = VI, or about 13.2W. At 100rpm, by using the relation of P = Tω and rearranging, the no load motor torque at 100rpm is 1.2605 Nm. The images below describe the motor visually and by specification..
Greartisan 12V brushed motor image and specifications
However, high RPM places the system under high levels of fatigue loading experienced by the link and shaft. Macroscopically, connecting the linkages caused massive vibrations and the joints themselves (especially the nuts would unfasten) would be jostled. As for the chute slider, when using an older resin prototype, the front end would be plastically deformed with a small crack at the front lip under constant fatigue loading. Rubber bands themselves can also undergo permanent deformation and snap easily under fatigue (especially when ones with a smaller spring constant is used). High torque by the system can cause shear stresses to develop in the slots, tensile stresses in the links, bearing stress formation, and permanent plastic deformation and failure of weaker rubber bands (despite the benefit of more easily elongating them).
Therefore, it would be mechanically preferable to lower the angular velocity by at least half or more of the rated RPM. 30RPM would be optimal. The reduction of torque can also improve mitigate damage to the system. The simplest way to do is with the L298N H-Bridge motor driver. The working principle relies on Pulse Width Modulation, which reduces the average power by switching on and off an electrical signal (thus creating a discretized and modulated sequence of pulses). The L298N modulated voltage signals to reduce the average power. Its versatility lies in ease of implementation and adjustability to find a stable RPM and power figure rather than implementing a potentiometer and turning an analog dial for a rough approximation. In addition, the L298N interfaces easily with the provided Arduino UNO microcontroller and the H-bridge enables the motor to be spun clockwise or counterclockwise with a simple change in code.
High to
L298N H bridge Motor Driver
Circuit Schematic of DC motor, 9V battery, L298N and Arduino UNO microcontroller, adapted from
https://www.instructables.com/How-to-Use-L298n-to-Control-Dc-Motor-With-Arduino/
Completed circuit
The following code allows for speed control by PWM (pin 9) and direction control (pins 2 and 3, simply swap HIGH and LOW) to flip direction
Arduino Motor Control Code
Caveat & Issues:
PWM reduces both torque and speed, and in doing so, no combination was safe for the system's structural integrity or optimal to push the slider against the tension in the rubber bands. The motor kept stalling when attempting to pull back the slider and the system pauses and remains stationary. Therefore, a switch to a hand driven layout would enable more testing and operation until a more viable solution is obtained (see Conclusions and future works).
GIF depicting Motor Stall due to massive load.