The expanding pulley is a mechanism which can change its size while maintaining it's circular shape. This concept could be used for changing the gearing on a machine by dynamically increasing or reducing the pulley radius, thus changing the gear ratio. Aside from uses in gearing, this type of mechanism can also be used in multidirectional actuators, lock bolts, or some "whacky wheels".

Supplies

3D Printer
Small screw and washer

Step 1: 3D Printing

Attached are the three files needed for the expanding pulley. Print the upper frame and lower frame once each, and print six of the slides. I printed these in PLA without supports at 20% infill density. Note that the largest piece is just under 6 1/4 in. so it may be necessary to scale down the models so they fit on a smaller print bed.

Step 2: Assembly

Everything should slide together fairly smooth. Make sure that the parts have no rough edges or little plasic burs. Simply use your finger or some sandpaper to remove these before moving on. If the slides still don't move smoothly, use vegetable oil and apply it to the slides with a cotton swab. Next insert all the slides and place the upper frame on top, with each peg on the slides in the corresponding slot.

This would be a good time to test it to make sure it works before screwing the pulley together. Test it by holding the lower frame still, and rotating the upper frame.

Finally, place the washer over the center of the frame, and screw the whole assembly together. Make sure not to overtighten it as that might lock the frames together.