I noticed on a video of the UP challenger that on a slow speed move that the rear set of drivers slipped followed later by slippage of the front set. On simple articulateds was it more common for one set of drivers to slip over the other set? On Mallets would slipping of the high pressure set of drivers build up pressure to cause the low pressure set to slip also? Would the low pressure set slip when there would not be adequate steam pressure from the high pressure set? Also would the resistance of the low pressure drivers help the high pressure from slipping? Not earth shattering questions, just curious. Any ideas or knowledge?
John
Pretty deep stuff there John…
I’m guessing here, but let me throw out a couple of considerations -
One factor that could influence drivers slipping would be relative weight on the drivers. Logic would dictate that a set of drivers with less weight would tend to slip. This wouldn’t account for first one set slipping, then the other, however.
Assuming equal weight on drivers, and steam feed issues also being equal, the other factor that could affect slippage would be the coefficient of friction between the drivers and the rail. It would make sense that a slowly moving locomotive passing over a section of slippery rail would slip first the leading drivers, then the trailing. With the long wheelbase, this could also be a factor of the track profile.
Slipping the trailing, then the leading drivers would probably involve steam flow…
As the for compound/Mallet cylinder pressure thing, that’s way beyond what I’ve been able to figure out for myself about those things.
Correct me if I’m wrong but weren’t the UP Challengers(in fact all 4–6-6-4’s built in the US) simple articulateds? Thus there is no compound use of steam so all 4 cyinders are high pressure…
You’re right about the challenger being simple articulated. I was curious about the simple and the compound. It sure would be fun to go run a test on this issue!!
John
I would think compounds would be much less likely to slip for long. The high-pressure side must exhaust through the low-pressure cylinders and the low-pressure cylinders would soon consume the available steam from the high-pressure cylinder. The back pressure from the low-pressure pistons would prevent the high-pressure drivers from slipping very long.
One factor not mentioned is the weight on each set of drivers. According to the books I have, on UP’s first series of Challengers the front drivers had a tendency to slip. This was fixed by evening out the weight distribution by changing the hinge to the front drivers for the second series. I guess after the fix the Challengers slipped less and when they they did slip they slipped with equal probability.
The Pennsy had the same problem with their rigid frame T-1’s. They supposedly reduced the problem as time went on. I knew a mechanical engineer from the Pennsy who rode the cab of a T-1 on a damp evening through Indiana. He said that at 90MPH the locomotive would start severe vibration from wheel spin and the engineer would have to back off the throttle. This happened many times. Sounds like a real thrill ride.!
I have a video of N&W 2-6-6-4 with front driver slip. My guess is that this was more likely. One reason for wheel slip was wet rail and the forward drivers were the first to encounter the wetness. They left in their wake drier rail for the rear drivers. Not only that, but sand laid down for the front probably had some effect for the rear as well. In the case of a compound, again this is only a guess, but it makes sense to imagine some tendency for the front to slip because of rear slipping. At least in theory. I cannot imagine low pressure slippage due to inadequate steam from the high pressure cylinders.
One other thought is that the front engine probably supported less weight than the rear because it had to move side to side under the rigid boiler. That lateral motion was quite wide, judging from photos of mallets rounding sharp curves. In a turn the center of support would not be under the center of the boiler which might demand some offset of the weight to the rear. I hope someone with some real knowledge of steam construction can chime in here with some authority.
Another reason for slippage is the water in the boiler surging to the rear anding weight to the rears unoading the fronts then when the water heads the other way unloading the rears that could also cause it. Imagine a boiler as a tank of water which it basicly is shove it forward the water comes to the back then goes forward and remember on an articulated the firebox was huge. Big enough to cause surge issuses.
Compounds got their high starting TE ratings by starting in simple. The large front cylinders added weight to the front engine, but some like the later N&W Y’s had lead ballast added to the front engine to help control slipping. They switched over to compound operation by around 10 mph.