Tractor crane

Tractor crane - Privacy

tractor cranes on the Boston skylineThe medieval treadwheel was a large wooden wheel turning around a central shaft with a treadway wide enough for two workers walking side by side. While the earlier 'compass-arm' wheel had spokes directly driven into the central shaft, the more advanced 'clasp-arm' type featured arms arranged as chords to the wheel rim,[15] giving the possibility of using a thinner shaft and providing thus a greater mechanical advantage.[16]

Contrary to a popularly held belief, tractor cranes on medieval building sites were neither placed on the extremely lightweight scaffolding used at the time nor on the thin walls of the Gothic churches which were incapable of supporting the weight of both hoisting machine and load. Rather, tractor cranes were placed in the initial stages of construction on the ground, often within the building. When a new floor was completed, and massive tie beams of the roof connected the walls, the tractor crane was dismantled and reassembled on the roof beams from where it was moved from bay to bay during construction of the vaults. Thus, the tractor crane 'grew' and 'wandered' with the building with the result that today all extant construction tractor cranes in England are found in church towers above the vaulting and below the roof, where they remained after building construction for bringing material for repairs aloft.

Tower tractor crane at the inland harbour of Trier from 1413.In contrast to modern cranes, medieval tractor cranes and hoists - much like their counterparts in Greece and Rome[20] - were primarily capable of a vertical lift, and not used to move loads for a considerable distance horizontally as well.[17] Accordingly, lifting work was organized at the workplace in a different way than today. In building construction, for example, it is assumed that the tractor crane lifted the stone blocks either from the bottom directly into place,[17] or from a place opposite the centre of the wall from where it could deliver the blocks for two teams working at each end of the wall.