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TEXT ONLY | CREDITS | APPLY FOR SCRAPHEAP 2005 | |
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The Locomotives Special | |
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Science | |
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Science
 Overview General science The designs Pros and cons
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Overview Background It's 200 years since engineer Richard Trevithick's first locomotive steamed into the history books and now our three teams of railway-mad bodgers had just 20 hours to build a loco that was ready to race. The test Even with an electric multiple unit coach bogey as a starting point (weighing a hefty six tons) this was no easy challenge … the teams had to design a locomotive capable of drawing a 30-tonne passenger coach as well as attaining the fastest flat-out sprint. Each team had to use different fuel and technology that spanned the history of locomotive development. In the end, all the designs worked well, which was a testament to the work ethic of the teams and their experts. |
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General science Electric motors Electric motors work by using attractive and repulsive forces between magnetic poles to twist an object (the rotor) around in a circle. Both the rotor and the stationary structure (the stator) are magnetic. The poles are arranged so that the rotor must turn in a particular direction in order to bring its north poles closer to the stator's south poles. The magnets of the rotor and stator aren't all permanent magnets. At least some of the magnets are electromagnets – their magnetism can be turned on and off with the addition of current from brushes that clip them as they turn. It's one of these brushes which snapped off the Nutcrackers' motor. In a typical motor, these electromagnets are designed so that their poles change just as the rotor's north poles have reached the stator's south poles. After the poles change, the rotor finds itself having to continue turning in order to bring its north poles closer to the stator's south poles – this spinning action produces toque for the drive shaft. Diesel engines The diesel-powered internal combustion engine harnesses the power of the explosion that results when you compress air, combine it with fuel and ignite it in a small chamber. The cycle of an internal combustion engine typically involves four strokes: the intake, compression, combustion and exhaust. It does this hundreds of times a minute and uses the resulting energy to drive the vehicle. Steam engines A steam engine works by taking the energy stored in coal or some other fuel to boil water in the boiler and generate steam under pressure. This steam is directed to drive a piston down in a cylinder (and often up again in a double-acting engine). Valves are used to get this steam into the cylinder and to exhaust it out again when its expansive power is spent. This exhaust steam is directed into the chimney of the boiler creating a draught that creates an even hotter fire, generating steam even faster. The linear up down motion of the piston is converted to rotary motion to drive the wheels – on a conventional steam engine by crankshafts – but on The Clagsters' loco by using chain drive. The mechanics are such that once the engine is running, the process is automatically repeated. Chain and belt drives As the bogeys have no drive shafts and it is impossible to remove the wheels, the teams are limited in the ways they can get power to the axle. The Nutcrackers and The Clagsters both used chain drive systems while In Training went for a belt drive. A chain drive is a system where the power input is provided by a gear or sprocket, which turns and moves a chain that then moves other gears or sprockets to transfer the power. A belt drive works in a similar way and runs from a pulley at one end to an axle at the other. The crucial difference is it has to be held taut otherwise it won't turn. Getting the static tension in the belt requires precise adjustment. |
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The designs Electric motors The Nutcrackers chose an old battery-powered electric baggage tug from Heathrow Airport as the basis for their locomotive. They decided to use a chain drive from the axle of the electric tug to the axle of the passenger coach. The Nutcrackers found the requisite chain sprockets and assumed their battery had sufficient juice to turn the chain. Rather than simply mounting the baggage tug on the top of the bogey, they decided to strip out all the control mechanism and axle, which looked great but meant they had to trace a lot of wires and be extremely careful not to cut any. The lads managed to break a brush on the electric motor when removing it from the tug – that could have caused serious problems – but dexterous Deano was able to carry out the fiddly repair. Having stripped off a lot of the weight the team had some worries about traction, but the addition of the heavy battery cured that problem. The team cut into the differential and welded it up – they didn't want to end up sending power to the redundant axle. When Robert and Lisa blew the final whistle, they had finished long before the other teams with a locomotive that provided instant torque and lots of it. Diesel engine The team used a belt drive to turn the bogey axle, and a 40-year-old Massey Ferguson diesel tractor provided the power. Their first challenge was to fix a damaged clutch. They managed it but it took most of day one. The belt, which was to run from the pulley on the bogey to the axle of the tractor, could not be adjusted like a chain, by taking out or adding links, so it was critical the team lined up the tractor correctly on the bogey. Any slack in the belt would mean no friction and no friction would mean no drive. The other main problem was positioning the belt – it ran down from the axle, past the edge of the bogey to the wheel – except the bogey was in the way of the belt and massive amounts of gas axe work was needed before the belt would run cleanly. The team kept the gearbox, which meant they were able to select different gearing for the speed and haulage rounds. Unfortunately, the low top speed of the tractor meant they had trouble in the speed round. Steam power The team used a turn-of-the-century, two-cylinder steam engine to power their loco. Originally, the engine may have been used to power one of the steam launches that were very popular on Britain's waterways before the internal combustion outboard engine took over. A vertical boiler provided the steam to turn the engine over. It was critical everything was connected properly as the team wanted to get as close to the engine's pressure limit to get the maximum power (blow power) to get the loco in motion, and this put a great deal of strain on their fabrication. The Clagsters used a massive RSJ (reinforced steel joist) which they scavenged off the heap to get plenty of weight over the rear drive axel to add traction. The drive to the axle was supplied by chain drive similar to The Nutcrackers' solution. Like The Nutcrackers, The Clagsters chopped the sprocket in half and welded it around the axle. The loco had a novel way of feeding water to the boiler – a bike was pressed into service with the chain driving a feed pump – that meant the team had to peddle frantically to keep the boiler supplied with water as they steamed into the Scrapheap record books. |
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Pros and cons The Nutcrackers Pros
In Training Pros
The Clagsters Pros
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