Nile Barrage Lifting Bridge at Assuit
Read an in-depth article about this model.
This is a model of the bridge featured on the front cover of the June 1938 Meccano Magazine. It is a lifting bridge at Assuit, in Egypt, forming part of the Nile barrage system.
Its unique feature is the rolling counterweight, running on a variable sloped track, shaped to maintain constant balance in the cables that connect the counterweight to the bascule section and which pass over pulleys at the top of the tower.
The basis of its operation is that when raised, the hoist cable tension is minimal, as the hinge points carry the bascule weight. Only a small out-of-balance component remains due to the hinge offset, this being necessary to overcome friction and ensure the bascule is biased to lower naturally. In this position, the counterweight is on the minimum slope of the track, imposing minimal hoist cable tension and maximum load to the track. To raise the bascule from the horizontal position, the hoist cable tension is at maximum, due to the weight transfer of its centre of gravity and the angle posed by the hoist cables. This is when the counterweight is virtually vertical applying maximum hoist effect and minimum load to the track. The Meccano Magazine article gives a very good account of the bridge engineering, structure and proportions, with pictures. I used 18½” angle girders for the 60-foot span of the original, giving a scale of 40:1.
I set up a spreadsheet with dimensional and physical variables for the purpose of calculating the unknown properties of weights and track geometry.
The model has an electric motor in a housing 6” above the roadway. This drives a vertical rod via pinion and contrate gearing to the top platform. A rubber rod connector allows slip as overload protection in the vertical drive.
I used a mechanical differential to maintain equal tension in the hoisting cords.
Pairs of cords each side, connect the counterweight to the bascule, passing over 1½” pulleys free mounted on axle rods, at the top of the tower. Between each pair of pulleys is a gear driven hoist drum.
The prototype counterweight is a steel box filled with iron and concrete, with reserve capacity for fine-tuning on testing. A scale box was constructed, with the object of filling it with axle rods.
Four oscillating bogies were designed as close to the originals as the detail in the pictures conveyed, ¾” flanged wheels forming the running gear. Arched connecting members each side of the box carry the bogies, and the box is tri-mounted to maintain wheel contact with the track to compensate for any irregularities.
Each bogie carrier is fitted with adjustable and pivoted drawbars. The track was constructed by bending (old) 12½” and 5½” strips to shape against a layout of the calculated curve. To enable automatic termination of the raise and lower movements, Elektrikit limit switches were added and an Elektrikit knife switch mounted on the side of the tower for reversing the raise/lower function.