Uboats dropping load with throttle changes

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Uboats dropping load with throttle changes

Post by Spur44 »

So the title says it all. I've read about this phenomenon several times, on various forums, but I don't think I've ever seen it fully explained or described. So was this something related to the U series, across the board. Was it with every single throttle change and when it did drop load how much of a change was it? Did it drop load completely or just a little?

Thanks in advance for the info!

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Re: Uboats dropping load with throttle changes

Post by Wayside »

In my experience (operating freight trains for PC/CR for roughly 15 years) this was a phenomenon that plagued the U series units. Compared to the EMD products (35 series and later) the load control of the U-boats was atrocious. Early U25/28 series were the worst, as I recall, in that they would sometimes load tremendously in the lower throttle positions, especially as speed was reduced such as when attempting to glide to a smooth stop when power (stretch) braking. This power surge could be enough to break a knuckle if one wasn't careful.

And yes, when accelerating, every throttle notch would often cause a dramatic drop in load. What ensued next was a slow spool-up of the turbo (and a cloud of black smoke) and redevelopment of power for the notch selected. EMD had developed a system that produced a specific horsepower output for each throttle position. It took years for GE to figure out how to do the same thing.

You might get the impression that I didn't like early GEs. And you would be right in that regard.
We don't know what we don't know.

Engineer Spike
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Re: Uboats dropping load with throttle changes

Post by Engineer Spike »

I have seen this as late as -9. I work for D&H, and we had a intermodal train which was NS haulage. We used to spot this train in the stub ended tracks in Kenwood Yard, in Albany, NY. Spotting these trains was tricky because at that time there was still a good mix of 89' flats and five pack wells and spines. The trick was to shove back with just enough air so the cars would stop on spot, yet not have too much air and too much buff force. The -9 were hard to regulate because of the total drop if the throttle was changed. It was always a much easier task if a SD60 or 70 was leading. Additionally sometimes dragging a little independent was needed. This was tougher on the GE power because they had fly by wire CCB brake, which wasn't too easy to make a 1-2 psi change. The ENDs had real air which added to their user friendliness.

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