Zipline trolley Challenge.

[NolanShepard]

I need to create a zipline trolley, that goes along a clothes line for an interval of distances and times. This can be between 1-8m in 0.5m increments, OR between 20-90 seconds at 5 second intervals. The trolley cannot use chemicals or electrical components and must be made purely out of mechanical parts, gears, elastic bands, etc... Please help if anyone has any ideas to make it function in these ranges.