You should AxlesJanuary 15, 2014
Axles are two six-inch-edge cubes joined perfectly on one face. They are presumably in activation and physical nature otherwise similar to Cubes, but this is not explained. When activated, Axles become a perpetual motion machine: One side rotates relative to the other, very slowly, taking 6.9 seconds per revolution (this translates to about 8.7 rpm), but have apparently infinite torque. This, combined with their complete autonomy without fuel and the use of a series of gear speed/torque gears allows them to power the mechanics and industries of entire dwarf cities, including tow-powered traffic.
The Axle used by the Ankh-Morpork grags during the events of Thud became the property of the city. Its uses in the city are still being researched by the Artificers' Guildmaster Mr. Pony, but in theory it could revolutionize the heavy industrial and municipal workings of Ankh-Morpork. Lord Vetinari appears, speculatively, to link the use of the Axle with the extensive dwarf tunnels under the city. According to Captain Carrot, only three other Axles are known to exist.
It seems (based on comments of a forthcoming 'Undertaking' in 'Making Money') that Vetinari intends to use the axle and dwarf tunnelling methods to create Discworld's first mass-transit system, seemingly based on the London Underground, a possibility reinforced in 'Thud' by Vetinari closely regarding the Dwarf sign for 'mine', which resembles the London Underground emblem.