 |
UK Broadcast Transmission |
        |
|
    |
|
|
|
|
|
||||||||
|
|||||||||||
THE TROUBLE WITH TUBULAR MASTS
By Brierly Hill
Tubular masts usually incorporate an internal lift system for access (otherwise it's a highly-claustrophobic climb...). These are not the usual cable-hoisted lift: they are perhaps better described as a vertical version of the Snowdon Mountain Railway, in that they run up vertical guide-rails, climbing a vertical rack using a driven pinion. Mercifully, steam propulsion is spared us; an electric motor mounted on the roof of the lift-cab drives a gearbox and thence the pinion. Power for the motor is picked up by brushes, from exposed bus bars mounted behind the cab, running the full height of the lift's run. The motor is controlled from within the cab, which has its own lighting from the same source as the motor.
The snag with this form of propulsion is that if the pinion breaks, or becomes otherwise disengaged from the rack, your descent rate is likely to become fatal (as the proprietors of the S.M. Railway found to their cost on opening-day). In order that insurance firms might even entertain the possibility of covering the lift, some form of emergency brake system had to be provided. In brief, it worked like this: -
A light steel rope was attached to the top of the lift-cab (I'll come back to this attachment in a moment). It ran up to the top of the lift run, and thence round a drum a few times before descending again. This drum had an over-speed trip attached to it: if the cab descent speed became excessive, this trip would operate and apply a brake to the drum.
Back to the top of the lift-cab. The cable was not attached directly to it; instead, many turns were wound round another drum, which was normally static. In the event of an over-speed trip stopping the cable, the cable would naturally start to unwind from this drum, rotating it and operating a pair of screws, which wound emergency brake-skids against the rails along which the lift-cab ran. The cab would therefore (one hopes) come to a halt. Well, that was the intention. It didn't always work out like that.
In practice, this device was known to operate even in the absence of a mechanical failure. Excessive zeal on the part of the operator of the speed-control could cause it, as could a weight overload inside the cab. At this point, someone in the cab had to step outside and on to the climbing ladder mounted adjacent, climb to the top of the lift, reset the over speed trip and release the brake, climb down again and step onto the cab roof, and rewind the drum there to release the emergency skids. Once was usually enough for most folk.
The inside of these tubular masts often had their own microclimate. It was a bizarre experience, on a blazing hot summer's day, to open the door in the base of the plinth, step inside, and find that it was raining in there. This was due to the fact that the mast, being enclosed, was not ventilated and yet at the same time not quite watertight either. Previous rain would percolate in small quantities, and having nowhere to go would turn to vapour. Humidity inside those masts could be tremendous. Since the top of the mast was often quite cooler than the bottom, often by several degrees, condensation would form at the top and then rain down. Some riggers used to swear that clouds formed inside the top of these masts: I've never seen this myself, but certainly if you switch the internal lights on, it often looks very misty at the top. The other downside to this condensation can be found in the wintertime. Quite often, ice forms on the supply bus bars towards the top of the mast. You only find out about this when your ascending lift reaches this iced-up section, the brushes are pushed off the bus bar, and the motor stops. Also, of course, the cab lights go out. The only cure for this is to switch off the motor, climb on the cab roof, and laboriously wind the motor shaft, by hand, back down onto a non-iced section. One may then descend. People absolutely determined to reach the top have been known to beat the bus bars with wooden sticks (they are still live, of course) to remove the ice, but this is really only a sport for the hardy.
MAST COLLAPSES
Two tubular masts have suffered collapeses : Emley Moor Mark II, and Waltham Mark I.
Emley had lots of publicity, Waltham less so since it collapsed during construction and so didn't inconvenience any viewers. The causes of these collapses were quite different.
The Emley Moor collapse is usually stated to have been due to severe icing. Others claim it was a design flaw. Certainly, heavy icing was present on the occasion of the collapse, but there was little wind at the time (indeed, freezing fog and rain) and other masts in the British Isles have withstood far worse before and since. If you don't believe that, see these Holme Moss photos which show some pretty spectacular icing, only a few miles away, on a much more exposed site.
The Emley collapse ensured that all of its relays lost their signal. It also, and much more importantly, had the effect of putting Belmont BBC2 off the air, since that station received both main and reserve signals off-air from Emley at the time. The quick cure for this should have been, of course, to install new aerials at Belmont on, say Waltham : but since the Belmont mast was an identical design to the Emley one that had just failed, no-one seemed over keen about going on-site there until the weather eased and the cause of the Emley collapse became apparent. The cure was a good piece of lateral thinking: a spare transposer was acquired, and tuned up for Waltham BBC2 in, Emley BBC2 out: this was taken to the end of the Belmont station drive in a van, a receive aerial on a temporary pole was pointed at Waltham and fed the transposer, which then converted the signals to those from a pretend Emley, and which were fed to another aerial directed onto the receiving troughs located on the Belmont mast. Problem solved, without even going on site. And it worked reasonably well, too: not perfect pictures, but a lot better than nothing.
The Waltham collapse was quite different. This time, collapse occurred in the middle of the night, and there were no witnesses, so the causes have always been slightly conjectural. However, the evidence seems to point to the following scenario: -
At the time of the collapse, the mast structure was substantially complete: the tubular section was finished, and the lattice cantilever extension that was to carry the aerials was in place, but seemingly not completely secured. The upper section of the mast was still on temporary stays. That night, a gusty breeze seems to have sprung up which caused oscillation of the top lattice section. This became excessive, and the reduced number of securing bolts in position started to fail. Eventually the entire lattice section fell off and down the side of the mast, unfortunately shearing at least some of the temporary stays on that side. Progressive failure of the rest of the structure followed. It was quite a surprise to the construction teams arriving next morning to discover no mast. In its collapse it completely demolished the BBC transmitter hall, though left the IBA hall and common areas untouched. (If you go today into the reconstructed hall, and look at the wall behind the (analogue) BBC1 combiner frame, you will see a delightful series of scores in the brickwork, an inch deep in places, which mark the limits of the former mast's progress). It was a very good thing that no-one was on site when it collapsed: quite apart from the rain of metalwork, failing stays, under high tension, lash about catastrophically: they had to be dug out of the ground from a depth of several feet in places, and are alleged to have cut a sheep in half. The brand-new transmitters had been delivered only the previous day, but fortunately they were a late arrival and had not been moved into the transmitter hall as had been planned.
In spite of these unusual circumstances, it does seem that these tubular masts had a design problem. They were always subject to excessive lateral oscillation in winds of certain speeds. A rigger acquaintance describes them as slightly unnerving in certain winds - if you looked down the mast from the top, the whole mast could be seen to be in a snaking sort of oscillation, with the inspection platforms at each level moving in different directions at any instant in time. These oscillations were deemed to endanger the structural integrity of some of the masts, and a weird modification was designed: chains were installed; tons of them, hanging down the inside of the tube and in contact with the walls. They were heavily coated with grease, and enclosed in heavy canvas tubes (fire-hose material, I think). The rationale was that if the tube swayed outwards at one point, the chain at that position would remain hanging vertically, but when the tube moved back it would swipe the chain hard and thus transfer some of the vibrational energy to the chain, thus damping the oscillation of the tube. Incredibly, this seems to have worked, though it did make these masts very noisy in certain breezes. At Belmont, and until these chains were put in position, when the wind speed rose to a critical value it was the custom for all the on-site staff to retire to a caravan at the end of the station drive, out of harm's way. Just in case.
Down the Tubes - The Collapse at Waltham | The Rise and Fall of Emley Moor
|
