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Cog control

Whether engineers call the wheel a moving structure or a piece of canal machinery, all are agreed that the really clever bit is a row of five differing sized cogs hidden behind the arm nearest the aqueduct.

The prime challenge in changing from free hanging gondolas - as on the original ferris wheel - to fixed units supported each end within the propeller shaped arms, was simply keeping them horizontal.

As the arms rotate, the wheeled gondolas try to remain level by running against a single curved rail track fixed to the rim of the hole in the arm supporting it. But wheel friction, and the inertia triggered by 250t of disturbed water, would result in the gondola sticking and tilting.

It is estimated that, after just 40 seconds, the resulting 4infinity tilt could cause the gondola to tip over totally. The row of cogs, between axle and gondolas, now prevents this possibility by keeping both troughs horizontal throughout the arms' rotation.

The wheel is perfectly balanced and its axle can be turned by 10, 7kw geared motors located around it for a cost of just £10 a day. As the wheel rotates, two small cogs on each arm turn against a stationary larger cog fixed to the aqueduct pier behind the axle.

These small cogs also connect with two outer cogs, the same diameter as the central one and located in each arm's gondola support hole. Outer cogs, and the gondolas rigidly fixed to them, always rotate at the same speed as the turning wheel - effectively keeping the gondolas horizontal.

This simple yet innovative mechanism, devised by RMJM architect Tony Kettle, had to be demonstrated to the client and funders. So he modelled the cog arrangement with Lego 'borrowed' from his eight year old daughter Sarah, quickly proving the technique's effectiveness.

'Sarah was impressed that I could assemble something without instructions, ' says the architect.

'But now she wants the parts back to rebuild her helicopter.'

Butterley's Colin Castledine, a civil engineer, admits that it was a 'brilliant idea', but adds with a smile: 'The gear ratios on his model were all wrong and the gondolas would still have turned over.'

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