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Friday, 21 January 2011

Big Boy's Toys...

Turntable Ladders - Aerial Ladders in the US - are wonderful bits of kit. The ladders are extended by means of an hydraulic ram that pushes the first extension out and then a system of fixed wires and pulleys draw out the upper extensions. Safety 'cut outs' prevent you from pushing the ladder into a dangerous extension at the wrong elevation and other sensors sound off alarms if you try to push it into a building or other obstruction.

The ladder itself is mounted on a 'turret' and can rotate through a full 360° using a small motor and geared 'training ring.' Elevation is done using hefty hydraulic rams mounted in the 'turret' and attached to the 'swinging frame' and the ladder at maximum elevation is angled upwards at 85° to the vertical. At can also be lowered to around 15° below the horizontal, but the full extension can really only be safely achieved at angles above 60° elevation. The reason is that dratted Greek fellow - Archimedes, and his Law of Levers. A 30 metre long lever has a lot of leverage... So the system has a number of safety features which are designed to prevent the operator from pushing the ladder out too far at too low an elevation. And if he chooses to ignore the first warning klaxon - it will lock his controls and not allow him to go any further. Any reader familiar with a naval gun turret would recognise the mechanisms instantly, but the irony is that Turntable Ladders were invented first and the technology then found a use at sea ...

Naturally you can't play around with a thing like this on an unstable surface or on a normal vehicle suspension - so the chassis is fitted with jacks and axle locks that prevent it moving while the ladder is in use. In fact the system won't allow you to put power to the turret until the jacks are down and the axles locked. As you can see from the photograph, the jacks extend outwards some distance and now you run into the fact that you are going to be standing 15 tons of vehicle on four steel plates each about 750 mm square... That's a lot of pressure on the road surface or ground it's standing on.

If you put the jacks down on a manhole cover, or a buried drain, or a soft patch of ground, the crashing noises as the whole appliance falls over can be very expensive and fatal for the men in the cage at the top of the ladder. So it's a tricky task operating one of these, it carries a lot of responsibility and no one actually notices if you get it right!

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