Falkirk Wheel, Scotland, United Kingdom

Each end of each caisson is supported on small wheels which run on the inside face of the eight-metre-diameter holes at the ends of the arms, allowing the caissons to rotate. The rotation is controlled by means of a train of gears: an alternating pattern of three eight-metre-diameter ring gears and two smaller idler gears, all with external teeth. The central large gear acts as a stationary sun gear. It is fitted loosely over the axle at its machine-room end and fixed to a plinth to prevent it from rotating. The two, smaller, idler gears are fixed to each of the arms of the wheel at its machine-room end and act as planetary gears. When the motors rotate the wheel, the arms swing and the planetary gears engage the sun gear, which results in the planet gears rotating at a higher speed than the wheel but in the same direction. The planetary gears engage the large ring gears at the end of the caissons, driving them at the same speed as the wheel but in the opposite direction. This cancels the rotation due to the arms and keeps the caissons stable and perfectly level.
• Docking-pit
The docking-pit is a drydock-like port which is isolated from the lower canal basin by means of watertight gates and kept dry by means of water pumps. When the wheel rotates and stops with its arms in the vertical position it is possible for boats to enter and exit the lower caisson when the gates are open without flooding the docking-pit. The space below the caisson is empty.
If it were not for inclusion of the docking-pit the caissons and extremities of the arms of the wheel would be immersed in water at the lower canal basin each time the wheel rotates. This would result in a number of undesirable situations developing, such as providing buoyancy to the bottom caisson and the viscosity of the water causing an increase in the required power.