Motor Drive for Marble Machine 2

Here is a motorized lift for Tulio Laanen's excellent and popular Marble Machine 2 (Thing #795952). Derived from KreAture's Improved lift (Thing #820323)
Powered by 2 AA batteries, the lift directly retrofits and replaces the original manual lift.
https://youtu.be/Gfk6TAG3ZFM
https://youtu.be/EiT330rlnDw
edit 9Mar16: fixed lift screw, now spirals in correct direction