A mildly satisfying micro project: increasing the run time of a pull-string cat toy by making the string longer

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Disemboweled cat toy.
The pull-string cat toy, disemboweled.

Some recently-bought cat toys came with spring-powered vibration motors. The springs (likely spiral torsion springs, i.e. watch springs) are loaded by drawing back a length of string attached to the motor’s axle. When you draw back the pullstring and release it, the eccentric mass on the motor’s axle causes the toy to vibrate and move slowly around on the floor. We noticed that they didn’t run for very long and I wondered whether it would be possible to extend the running time by lengthening the pull string.

Closer look at the toy's gearbox.
A closer look at the toy’s gearbox, showing the pulley and string as well as the motor axle and its eccentric rotating mass (the round metal disc with the off-center hole).

I snipped the stitching at the rear of the toy and removed the motor, which was swaddled in polyester cushion filling. The black plastic motor case was secured by four posts protruding from one half that slotted into corresponding channels on the other half and was easy to pry open.

Closer look at the toy's gearbox.
Motor with the original string swapped out for a a longer, thinner string.

Replacing the pull cord was simple. The existing cord was knotted, with the knot slid into a notch cut into the outer rim of the plastic pulley, and then tied to the inside groove of the pulley. I cut about twice as long a piece of some 0.6mm-diameter candy-cane-striped polyethylene kite string, knotted one end, and fastened it the same way the original had been attached. Afterwards, I reinserted the motor into the toy and stitched it back up. Run time is indeed longer.