I have been looking at the size of the drivers of some German steam locos. It looks like it is possible to use their mechanisms for models of American steam locomotives. I know the then editor of Continental Modeller (a PECO publication), David Lloyd, modelled some Western Maryland steamers in the late 80’s by using German models as a basis. Has anybody done that too and what prototypes?
What are common prototype driver sizes? And what locomotives used them?
Some well known German freight steam locomotives have driver sizes of 1400 mm (baureihe 44, a 2-10-0) or 1600 mm (baureihe 41, 2-8-2).
Wow I don’t think there is a common size. 55", 60", and 79" come to mind, but I’ve heard of everything from 40" to 80". I think it would it would depend on the type, roadname, and class of the locomotives one might be interesting in modeling.
Yeah, I just checked a couple of books that were sitting here handy.
0-6-0 NP L-9 class 51"
2-8-0 NP Y-2 class - 63" ← I believe these are extra large for a consolidated
2-8-2 NP W-5 class - 63"
2-10-4 AT&SF 5000 - 69"
2-10-4 AT&SF 5001 class - 74"
2-10-4 AT&SF 5011 class - 74"
2-10-4 AT&SF 3800 class - 63"
4-8-2 AT&SF 3700 class - 69"
4-8-4 AT&SF 3751 class - 73"
4-8-4 AT&SF 2900 class - 80"
4-8-4 UP 820 class - 80"
4-8-4 NP A-3 class - 77"
4-6-2 AT&SF 3400 class - 73"
4-6-4 AT&SF 3450 class - 73"
4-6-4 AT&SF 3460 class - 84"
2-6-6-2 NP Z class - 55"
2-8-8-2 NP Z-3 class - 57"
First, what sizes are represented? I am assuming the US prototype model and the German mechanism are to be modeled in the same scale. That makes the 1400mm driver just undersize for 56 inches and the 1600mm within a few scale millimeters of 63 inches.
In the spectrum of US driver sizes, 56 inches was not too common. The best known examples were older 2-8-0’s and true Mallets. The earlier N&W 2-8-8-2 classes were built with 56 inch drivers, but the later classes went to 57 and 58 inch. All of the locos with this size driver would be classified as low speed power.
On the other hand, 63 inches was very common. Many 2-8-2’s, including the USRA designs, came with 63" drivers. Pennsylvania Railroad locos from 2-6-2 to 2-10-2 ran on 62" drivers. The same basic driver sizes were used all over the United States for drag freight locos. Even early 2-8-4 and 2-10-4 types were built with 63" drivers. 63 inch drivers were largely confined to medium speeds, primarily because of counterbalancing issues.
Looking at the entire field of driver size yields the following:
50-51 inches. Switching locos and some very early low speed road power.
52 inches. “Russian” 2-10-0’s, and a few early drag freight locos.
55-58 inches. Slow freight locos, including most Mallet-system articulateds.
61-64 inches. “Standard” freight locomotives, medium speed.
68-70 inches. Fast freight locos, including the most modern semi-articulateds (UP Big Boy, N&W Class A, 4-6-6-4s owned by a number of railroads) and most types with four wheel trailing trucks. Also subur
1300 mm drivers would be the equivalent of 51/52" drivers. These were used in switch engines, most notably USRA 0-6-0 and 0-8-0
1400 mm is roughly equivalent to 55 inches (could be used for 56/57 inch driver locos as well). These would mostly be 2-8-0’s (and some really early 2-8-2’s).
1600 mm are the rough equivalent of 63-64" drivers (generally speaking, larger 2-8-0’s, 2-8-2’s, early 2-8-4’s and 2-10-4’s)
1750 mm drivers are the equivalent of 69/70" drivers. USRA light and heavy 4-8-2’s used 69" drivers as did a number of other 4-8-2’s (NYC L-2, for example)
1800 mm is roughly the equivalent of 72" drivers. USRA light 4-6-2’s had 73" drivers as did many other 4-6-2’s. Pennsy M-1 4-8-2’s used 72" drivers, while Southern Pacific 4-8-2’s used 73" drivers.
2000 mm drivers are about 79/80". Some examples: NYC 4-6-4’s, USRA heavy 4-6-2’s, Pennsy K-4. This was also a common size for 4-8-4’s ( New York Central, UP, Santa Fe, Southern Pacific, Atlantic Coast Line, etc.)
It seems I will have to do some more searching, always enjoyable.
I model the midwest in N-scale (interested specifically in Minneapolis & St. Louis and Wabash with secondary interest in C&NW, Burlington and Rock Island) and I noticed the C&NW had some older steam locomotives to the end so it should be possible to use German mechanisms to get there. I am not yet a detail freak and rivet counter so I will first try to make a reasonable model (currently I do only scenery) to give the impression of the prototype and them refine them as I learn.
Alfred W. Bruce in “The Steam Locomotive in America” (published in 1952) notes that for many years the maximum safe speed in miles per hour was roughly the same number as the driving wheel diameter in inches, although often exceeded in practice, frequently by 25% by modern high speed steam loco’s in passenger service. He notes that maximum horsepower outputs were reached at about 300 rpm for modern heavy freight locos and at about 360 rpm for passenger-service locos. As determined by the geometry of the conventional rod loco the driving-wheel diameter speed is 336 rpm i.e. when the loco speed number in mph equals the number of the driver diameters in inches the drivers are turning at 336 rpm. He also notes that at driving-wheel diameter speed the piston and allied parts make 672 strokes per minute.
Looking at those numbers it’s easy to understand why counterbalancing all those heavy masses of metal in high speed motion was so important and why the small-driver-diameter locos were speed limited.
That doesn’t make sense to me, Isambard (my understanding of locomotive valve gear and piston stroke may need some tuning [%-)]).
Each driver revolution requires one cycle of the piston and one cycle of the valve since the driving energy is ported as steam. Are you saying the pistons and valve cycle back and forth twice per driver revolution? Otherwise, please explain the process based on this gentleman’s information (above).
Crandell, it does make sense, if you think about it. We’re counting each reversal of the piston as a stroke. For each revolution of the coupled wheels the piston must make one forward stroke and one rearward stroke, so the figure of 672 piston strokes for 336rpm is correct. Cheers, Mark.
He is quite correct but that does not mean that the pistons move cycle back and forth twice per driver revolution. Since the pistons are double acting, and since the cranks are located at 90 degrees from each other there are four power strokes per driver revolution for a two cylinder locomotive. If the pistons were set up at 0 degrees from each other or 180 degrees from each other the locomotive would not always be able to begin moving from a full stop. With the pistons 90 degrees apart one of them is always on a power stroke and can start the locomotive moving.
Thanks, Mark and Jonathan. Perhaps I had auto-engine on my mind, but I do see your point. Auto only has one power stroke, but steam locomotive induces power with each change of direction of the valve and piston. Got it.
My thanks also to Mark and Jonathan. They got there before me with clearer explanations than this non-mechanically-inclined body could have provided. I did manage the arithmetic to demonstrate to myself that the “driving-wheel diameter speed” is 336 rpm. The more I read of Bruce’s book the more I appreciate that modern steam locomotives were/are complicated beasts.
One thing that’s also worth remembering - driver sizes were a nominal figure. The wheels would be turned on a lathe once the drivers had worn out of shape, to ensure that the wheel retained the correct profile. I believe the size could vary as much as 3" or so.