The Falkirk Wheel is a rotating boat lift located in Scotland, UK, connecting the Forth and Clyde Canal with the Union Canal, opened in 2002. It is named after the nearby town of Falkirk which is in central Scotland. The two canals were previously connected by a series of 11 locks, but by the 1930s these had fallen into disuse, were filled in and the land built upon. The plan to regenerate the canals of central Scotland to reconnect Glasgow with Edinburgh was led by British Waterways with support and funding from seven local authorities, the Scottish Enterprise Network, the European Regional Development Fund and the Millennium Commission. It was decided early on to create a dramatic 21st century landmark structure to reconnect the canals, instead of simply recreating the historic lock flight. Designs were submitted for a boat lift to link the canals, with the Falkirk Wheel design winning. As with many Millennium Commission projects the site includes a visitors' centre containing a shop, café and exhibition centre. The difference in the levels of the two canals at the wheel is 24 metres (79 ft), roughly equivalent to the height of an eight- storey building. The Union Canal, however, is 11m higher than the aqueduct which meets the wheel, and boats must pass through a pair of locks to descend from this canal onto the aqueduct at the top of the wheel. The aqueduct could not have been positioned higher due to conflicts with the historically important Antonine Wall. The structure is located near the Rough Castle Fort and the closest village is Tamfourhill. On 24 May 2002, Queen Elizabeth II opened the Falkirk Wheel as part of her Golden Jubilee celebrations. The opening had been delayed by a month due to flooding caused by vandals who forced open the Wheel's gates.

Architectural services were supplied by Scotland-based RMJM, from initial designs by Nicoll Russell Studios and engineers Binnie Black and Veatch. The main project architect was an RMJM architect named Tony Kettle. Bachy/Soletanche and Morrison Construction Joint Venture won the contract to design the wheel and receiving basin, a new section of canal, a tunnel beneath the Antonine wall and a section of aqueduct. In turn the Joint Venture appointed Butterley Engineering to design and construct the wheel. Butterley undertook all construction work for the wheel and set up its own team to carry out the design work. This team comprised Tony Gee and Partners, to undertake the structural design responsibilities and M G Bennett & Associates to design the mechanical and electrical equipment for the wheel. The wheel has an overall diameter of 35 metres (115 ft) and consists of two opposing arms which extend 15 metres beyond the central axle and take the shape of a Celtic-inspired, double-headed axe. Two sets of these axe-shaped arms are attached about 25 metres (82 ft) apart to a 3.5 metres (11 ft) diameter axle. Two diametrically opposed water-filled caissons, each with a capacity of 80,000 imperial gallons (360,000 l; 96,000 US gal), are fitted between the ends of the arms. These caissons always weigh the same whether or not they are carrying their combined capacity of 600 tonnes (590 LT; 660 ST) of floating canal barges as, according to Archimedes' principle, floating objects displace their own weight in water, so when the boat enters, the amount of water leaving the caisson weighs exactly the same as the boat. This keeps the wheel balanced and so, despite its enormous mass, it rotates through 180° in five and a half minutes while using very little power. It takes just 22.5 kilowatts (30.2 hp) to power the electric motors, which consume just 1.5 kilowatt-hours (5.4 MJ) of energy in four minutes, roughly the same as boiling eight kettles of water. The wheel is the only rotating boat lift of its kind in the world, and is regarded as an engineering landmark for Scotland. The United Kingdom has one other boat lift: the Anderton boat lift in Cheshire. The Falkirk Wheel is an improvement on the Anderton boat lift and makes use of the same original principle: two balanced tanks, one going up and the other going down, however, the rotational mechanism is entirely unique to the Falkirk Wheel. Since 2007 the Falkirk Wheel has featured on the obverse of the new series of £50 notes issued by the Bank of Scotland. The series of notes commemorates Scottish engineering achievements with illustrations of bridges in Scotland such as the Glenfinnan Viaduct and the Forth Rail Bridge.

The wheel was constructed by Butterley Engineering at Ripley in Derbyshire under Millennium Plans to reconnect the Forth and Clyde Canal with the Union Canal, mainly for recreational use.

The wheel rotates together with the axle, which is supported by four-metre-diameter slewing bearings that are fitted to the ends of the axle and have their outer rings mounted on the plinths, which in turn are constructed on top of piled foundations. The slewing bearing at the machine-room end of the axle has an inner ring gear which is configured to act as a rotating annulus. The rotating annulus is driven by ten hydraulic motors which are assembled on a stationary bearing and motor assembly known as the planet carrier which in turn is also mounted onto a plinth similar to the one at the other end of the axle. The drive-shafts of the motors have pinion gears which act as stationary planetary gears in this train of gears and engage the rotating annulus ring gear. An electric motor drives a hydraulic pump which is connected to the hydraulic motors by means of hoses and drive the wheel at 1/8 revolution per minute.

How the caissons are kept level
The caissons need to rotate at the same speed as the wheel but in the opposite direction to keep them level and to ensure that the load of boats and water does not tip out when the wheel turns. 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.

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.

How the canal was routed through the wheel
The route chosen to take the Union Canal to the site of the wheel involved building a completely new section of canal, leading from the original terminus at Port Maxwell to link up with a new basin to the south of the wheel. The water level in this basin is the same as the aqueduct at the top section of the wheel, the two being joined by the new 150 metre long Rough Castle Tunnel with elliptical cross section. This is the most recent new canal tunnel to be built in the UK since canal excavation in Dudley, West Midlands. There are two locks to drop the canal level from that of the Union Canal to this basin. The tunnel was required because the canal had to pass underneath the route of the Antonine Wall without disturbing its archaeological remains. Just at this point the tunnel also passes below a road and the main Edinburgh to Glasgow railway line.

Costs and prices
The Falkirk Wheel cost £17.5 million, and the restoration project as a whole cost £84.5 million (of which £32 million came from National Lottery funds). The Falkirk Wheel Visitor Centre offers scheduled one-hour, round trip boat tours, called "The Falkirk Wheel Experience", that include passage on the wheel. The tours start below the wheel in the Forth & Clyde Canal, ascend via the wheel to the Union Canal, visit nearby areas on the Union Canal, and then return. As of 2010 , the boat tour costs £7.95 for adults, £4.95 for children aged 3”“15 (free for children under 3), OAP concession £6.95, student/state benefits concession £6.95, and family price of £23.24 (2 adults and 2 or more children -10% discount) with a discount of 10% for a group of 20 or more.

Future rotating boat lifts
A similar design of boat lift has been suggested for a proposed new canal that would run along Marston Vale in Bedfordshire. It would be part of a large-scale project creating an area of leisure and tourism facilities linked to the future expansion of Bedford and Milton Keynes. The lift would link the Grand Union Canal at Milton Keynes with the River Great Ouse at Bedford. A future expansion of the Forth & Clyde canal at the entrance to the River Forth has been proposed. It would use two new boat lift structures, with horses' heads around 115 feet (35 m) tall. Models of the Kelpie (mythological horse) heads planned for use in this new boat lift can be seen at the basin of the Falkirk Wheel.