When first built, Great Britain's network of canals formed a revolutionary new transport system — moving people, goods, and services. The canals served the country well for about 200 years, until railroad cars and trucks took over their functions. Canals fell into a steep decline just before World War II. Lately however, the canal and river network is being revitalized across the U. K. — fueled by nostalgia, recreational boating, tourism, and a sense of historic preservation.
One example: the Millennium Link project in Scotland has reconnected the Forth and Clyde Canal with the Union Canal, running between Glasgow and Edinburgh. Where the two canals meet, at Falkirk, their water levels differ by 115 ft. During their working years — up until the 1930s — a series of 11 locks linked the canals. However, during the demise of the canals, the locks fell into disrepair. They were filled in, and new roads were built on top of some of them. Rather than laboriously and expensively reconstructing the locks, the project contracted to design and build a more innovative solution for transferring boats between the two levels. The result is the Falkirk Wheel, the world's first rotating boat lift; a revolutionary — and aesthetically pleasing — engineering feat that has become an immediate tourist attraction for the area.
Technology takes its turn
The Falkirk Wheel might be compared to an amusement park Ferris wheel. It consists of two massive vertical steel arms that stand 35-m high and revolve at the ends of a 28-m long central axle. Two diametrically opposed gondolas are mounted between the ends of the arms. The gondolas are semicircular vessels partially filled with water that can hold as many as four 20- m long boats each. When one gondola is at top dead center, it is aligned with a reinforced concrete aqueduct that connects to the Union Canal. Simultaneously, the second, lower gondola is in a basin at the end of the Forth and Clyde Canal. Hydraulically actuated piston-like pins extend from the support structure to lock the wheel in position and prevent it from moving during boat loading.
Boats access the gondolas from the aqueduct above or circular basin below while the Wheel is stationary. Steel gates at both ends of the gondolas tilt open and shut hydraulically, allowing a boat to enter from one end and leave through the other. Additional gates, where the gondolas dock with the aqueduct and lower basin, open to let boats pass and shut to minimize the loss of water after boats are loaded. All these gates lie horizontally in their stowed position.
Incorporating technology for air-lock doors from the tunneling industry, seals at each end of the gondolas and on the canal gates of the aqueduct and the basin below it are completely watertight. The pressure of water against the vertical gates ensures an extremely tight seal after the gates are stroked into position.
Hydraulics as gatekeeper
Opening and closing the gates and releasing the seals requires a powerful hydraulic ram. This situation is complicated by the fact that the gondolas themselves carry no power units. The solution to this particular conundrum was developed by MG Bennett & Associates Ltd., Rotherham, based on their experience with sub-sea pipeline systems.
The hydraulic connection to the ram is made (and broken) by a hot stab, an external self-aligning hydraulic link that automatically extends into a port in the gondola base and mates with its hydraulic circuit each time a gondola docks with the aqueduct or canal basin. An accumulator in this circuit helps power the ram.
With the boats loaded and the gondola end gates closed, a computer-controlled water-pumping system equalizes the water levels in the two gondolas to establish near perfect weight balance before the Wheel turns. Fairfield Control Systems, Nottinghamshire, developed the software for the computer system that monitors and controls all Wheel functions. Using I/O from more than 600 points, the system meets all safety standards. For instance, it will not allow the Wheel to rotate until the safety locks on all the gates are confirmed operational.
The rotating drive system, designed by M G Bennett & Associates, incorporates a pair of 4-m diameter, three-row, slewing bearings with special seals (suppliedby SKF) to support the 1800-tonne Wheel. Positioned at either end of the axle, the bearings' outer rings are bolted to the fixed, upright support structure, and the inner rings are bolted to the tubular axle. The inside diameter of the slewing bearing's inner ring is cut with gear teeth that mesh with a reduction gearbox mounted within the central axle.
Although the bearings and the balancing arrangement result in surprisingly little power required to rotate the wheel, safety and redundancy considerations produced a much beefier hydraulic drive. Ten Bosch Rexroth bent-axis hydraulic motors arranged around a circle drive the gearbox. The multiple motors meet the redundancy requirement and, in addition, the individual tooth loads on the ring gear are reduced.
The motor mounting pattern and the critical alignment for gear contact was established via detailed finite-element analysis. The openloop hydraulic circuit for rotation meters fluid out of the motors, providing a stiff hydraulic system that prevents low-frequency oscillation of the Wheel. The motors run at only 900 rpm (about 25% of their rated speed) to provide another safety factor. The axle turns at a rate of around 0.125 rpm, which sees it lift and lower boats at an average speed of 4-m/min. With time allowed for loading and unloading boats, the Wheel completes a halfturn cycle about once every 15 minutes. The motors are fitted with hydraulic parking brakes that are engaged at the end of the cycle.
A series of synchronous gears maintains both gondolas in the horizontal plane during rotation. The gears have the very shallow involute angle found in old clocks. This profile enables the gears to accept structural deflection as the gondolas move.
In a room located inside the support structure and below the gearbox, two 45-kW electric motors drive a pair of bent-axis pumps in the main power unit that supplies the hydraulic motors. (This redundant arrangement is sized so that only one pump can turn the lift if necessary. The balancing arrangement dramatically reduces the amount of power required to rotate the wheel.) Four fluid lines are routed to each motor: flow and return to the motor as well as to its control valve. Piping from the power unit to the drive motors is electrically heated to suit coldweather operation. Overheating in hot weather is not a problem because of the short cycle time; the system never runs for more than five to seven minutes continuously under normal operating conditions. Pumps and hydraulic motors are destroked the rest of the time.
Two additional power units with 9.2-kW electric motors handle the hydraulic requirements of the gates, water-pump drives, and stop pins. These units are located in the support structures near the equipment they operate.
Click here to see a video explaining and showing the Falkirk Wheel in operation.
Nick Cooper, managing director of MG Bennett & Associates Ltd., who led the company's team on the Falkirk Wheel project, provided many details for this report.
