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Originally Posted by Gdeaver
We're considering the Fisher price gear box for the arm. It needs additional reduction. Heat is only a problem if you over load it. The back drive problem is my main concern. We're looking at different brake designs.
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We have had good luck in re-using the plastic gears from the F-P gearbox in a custom made gearbox. We basically made our own side plates. (You'll need a mill for this) Don't use the last set of gears on the F-P gearbox. Instead, make a shaft with a hex shaped outside to fit into the second-to-last gear from the F-P assembly. This shaft will then rotate with the second-to-last gear and it can be used as your output. You'll need more reduction after that. It the past we used a big sprocket (about 10") attached to the arm and put a small sprocket on the gearbox output.
To calculate the final reduction you will need to figure out how much moment (torque) you will have at the base of the arm using the length and weight of a fully loaded arm. A good rule of thumb that I like to use is to keep the motor load less than 1/3 of the stall torque. So, if you divide the required toque at the base of the arm by 1/3 the stall torque of the F-P motor you will get a target for the overall reduction.
Matt B