Thread: Push-Pull
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Old 22nd February 2006, 01:09   #11
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Join Date: 7 Sep 2005
Location: Loughborough
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Just a few comments on Bill EWS's post. Firstly, the original sets operated on the Glasgow to Edinburgh run were top and tail using Class 27s. They didn't use radios, but the blue star multiple control system was wired thorugh the train, and infact was very similar to the system on the HSTs. Recently, the same system was used on Wexssex Trains and Arriva Trains Northern services using 31s and 37s respectivly, top and tailing through wired air conditioned Mk2s. In the case of the original Scotrail service, a big problem was jerking and snatching caused by the delay in the control signals getting to the rear loco. With the HSTs experiance with this type of operating allowed the problem to be reduced, but it is still there, and is very noticable if the train is badly driven. Interestingly, the HSTs are set up so that on notch 1, the rear powercar is pushing the train more than the front is pulling, since the engine is running a notch higher. From notch 2 though the engines are at the same speed, and since the rear powercar is providing the train supply, the front is pulling more than the back. With the more recent top and tail trains, further leasons were learned and they supposedly managed to deal with the problem of jerking quite well. When it comes to push-pull, jerking is again quite a problem. It is true that the ECML sets arn't as bad on starting, and I think one TN is right in saying that the 91s are set up to work well with the Mk4s. They still suffer from jerks when power is shut off, in particular when previously accelerating hard. The 91s and 90s seem worst for this, which makes me thing it is the automatic speed control that is to blame often. A particular problem is that the ASC is set to apply the loco's rheostatic brake if the speed is above that set by the driver (to hold speed downhill). If speed is set low, and full power is used, by the time the system reacts the speed can be above the set speed. This then causes the train to go from full power to fairly hard rheostatic braking quickly. Since the train will be buffered up, this causes the train to snatch as the couplings suddenly go from compression to tension. Another problem can be if the driver sets a lower speed whilst power is on. This again causes the rheostatic brakes to apply. Then again, jerking is noticable on many units, most of which work on a sort of push pull system with driving trailers at each end.
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