How Does a Treadwheel Crane Lift Stone? People Walking Inside a Wheel

A simple windlass turned by handles lets a worker apply force through their arms and shoulders, which is limited and tiring. A treadwheel converts the problem into walking, which uses the legs and the whole body weight and can be sustained for hours, and it multiplies force by the ratio between the radius at which the worker pushes, meaning the inside of the wheel, and the radius of the axle around which the rope winds. A wheel of four metres diameter winding onto an axle of a quarter metre gives a mechanical advantage of roughly sixteen before any additional pulleys, so two people inside can lift loads far beyond what they could otherwise move. Adding a pulley block multiplies further at the cost of rope length and speed. The trade is exactly the one every simple machine makes: the load rises slowly while the workers walk a long way, which is acceptable because time is the cheapest input available on a building site lasting decades.

The construction and operation follow from what medieval carpentry could do:

• •A timber wheel, typically three to five metres in diameter, wide enough for one or two people to walk inside on transverse rungs
• •A central axle carrying the rope, mounted in wooden bearings greased with animal fat
• •A jib or projecting beam over which the rope passes, sometimes with the whole frame able to rotate to swing a load sideways
• •A braking arrangement, which is the critical safety element, since a descending load drives the wheel backwards and the workers inside cannot stop it
• •Frequently no ratchet at all in surviving examples, which means control depended entirely on the operators and on a friction brake, and accounts of accidents are common
• •Installation inside the building, with the crane hoisted into the roof space and left there, which is why many survive: there was no reason to dismantle something that would be needed for repairs

The treadwheel crane is Roman in origin, described by Vitruvius and depicted on the Haterii relief in Rome, and the Roman versions achieved lifting capacities that were not exceeded for over a thousand years. The technology disappeared from western Europe with the end of large-scale Roman construction and reappeared in the high medieval period, coinciding with cathedral building, from around the thirteenth century. Their capabilities shaped what could be built, since the maximum liftable block and the height reachable are direct constraints on design, and the gothic preference for many smaller worked stones rather than monolithic blocks reflects that as much as it reflects aesthetics. Harbour cranes were built on the same principle, several surviving in German and Baltic ports as substantial buildings in their own right. They remained in use into the industrial period, with steam and then electric power displacing them through the nineteenth century, and a number of examples survive in cathedral roof spaces at Salisbury, Beverley, Peterborough and elsewhere, along with restored harbour cranes still operable.

The crane is a useful correction to two opposite misconceptions. It shows that medieval builders had sophisticated mechanical understanding, since the machine embodies the principles of mechanical advantage, bearing design and load control, and was built to fine tolerances in timber by carpenters working without drawings in the modern sense. It also shows that the fundamental constraint was human and animal power, since every joule lifting a cathedral came from food eaten by workers, which sets a hard limit on what could be attempted and explains why the great buildings took generations. The organisation is equally revealing: a cathedral site was a long-term enterprise with a resident lodge of masons, a master mason directing design and construction together in a role combining architect and engineer, and a workforce whose skills were transmitted within the site over decades. The crane in the roof, left in place for the next repair, is the physical evidence that these buildings were understood as permanently unfinished rather than as completed objects.

A treadwheel crane multiplies force by the ratio of wheel radius to axle radius, commonly around sixteen to one, and lets workers apply their whole body weight by walking rather than pulling, trading speed for lifting power. Braking was the weak point, since a descending load drives the wheel and many examples had no ratchet. The technology is Roman, vanished with large-scale Roman construction and returned with cathedral building, and surviving machines remain in roof spaces because they were needed for repairs.