I found this info very interesting at Steamlocomotive.com Maybe someone could explain why a smaller and lighter engine is more powerful.
RF&P 4-8-4's
Specifications
Wheel Arrangement:
4-8-4
Length:
110' - 3"
Drivers:
77" dia.
Weight on Drivers:
277,245 lbs
Total Locomotive Weight:
466,040 lbs
Locomotive & Tender Weight:
842,940 lbs
Grate Area:
96.3 sq ft
Cylinders (dia. x stroke):
(2) 27" x 30"
Boiler Pressure:
275 psi
Tractive Effort:
66,500 lbs
Tender Capacity:
Water 20,000 gals and Coal 22 tons
B&M and L&HR 4-8-2's
Specifications for Class R-1d
Wheel Arrangement:
4-8-2
Length:
105' - 8"
Drivers:
73" dia
Weight on Drivers:
269,300 lbs
Total Locomotive Weight:
416,100 lbs
Locomotive & Tender Weight:
788,800 lbs
Grate Area:
79 sq ft
Cylinders:
(2) 28" dia. x 31" stroke
Boiler Pressure:
240 psi
Tractive Effort:
67,900 lbs
Tender Capacity:
20,000 gals. of water and 21 tons of coal.
The only thing on the R1D's bigger were the cylinders and tractive effort this needs an explanation.
The smaller diameter drivers alone will give the engine more torque, the extra 1" in diameter on the cylinders will also produce more power with the same psi of steam applied. as far as traction goes there are a lot of variables and questions about how the weight of the loco and diameter of the wheels effect the tread/contact patch in regards to the rail. for instance the 4-8-4 has its weight divided by more wheels then the 4-8-2 so that also can effect the traction. adding a little weight to the smaller weight loco may even increase its pulling power but would lower efficiency.
I imagine there is a nice balance to be found between all the variables but to sum it up it's the wheels and cylinders diameters.
Something else I just noticed is the cylinder stoke is longer on the smaller loco this would indicate to me that the rods are attached futher out from center of the driver, which would also add leverage / torque.
2ยข
NM
Anybody know what pressure was sent to the cylinders for general running? or how the tractive effort is determined?
I did some rough math based on assumptions and ended up with a inch/pounds of torque that when divided by the number of drivers is very close to the numbers listed above. I would like to know the way it is really determined.
NM
Tractive effort and adhesion rates are a study all to themselves. I'm just surprised that Jim or Yampa Bob hasn't piped in and given me a major headache.
pdlethbridge,
In this particular case, the whole difference is in the driver size.
Remember, the smaller, lighter loco can start and pull more, but at a much slower speed.
Just an educated guess without running numbers, but the Mountain likely could only maintain about 45-55 MPH with a 5,000 ton train.
The Northern on the other hand could likely maintain 70 with a 4,600 ton train.
In all steam locomotive design you are trying to balance power and speed to the needs of a specific set of operational conditions.
Example - in the eastern mountains of the US rail lines are continiously curvy and have steep grades. Steam locos used there are designed to pull heavy trains at maintained moderate speeds (30-40 mph), with no need to ever go 70 mph. But slowing down for sharp curves would cost too much time because every curve is a sharp curve. So drivers are smaller and rigid wheel bases kept shorter.
In the west, sharp curves and steep grades are found in the mountain passes, but these locations are connected by hundreds of miles of open flat land with gentle curves and moderate grades. So speed is important, slowing down to 20 MPH at a few sharp curves is no problem if you can be going 70 MPH the rest of the time.
Compare the specs of a UP FEF and N&W class J - you will see what I mean.
Another different example - many of the biggest 2-8-0's could pull as much or more than the avarage 2-8-2, but again, they could not maintain that load at the same higher speed.
It's not always about pulling more, sometimes its about pulling nearly as much, but much faster.
I have avoided any math to help offset any headaches.
Sheldon
pdlethbridge,
And, with the two locos you sighted, consider their uses, the B&M had mountains and grades.
The RF&P is almost dead flat in its short run from Washington to Richmond.
One loco built for power, the other built for speed.
Sheldon
2 locos mentioned were not the norm. The J could take curves but the way it was designed, it was built for speed. The balancing and suspension design of the J was awesome. The Pennsy in a test was afraid to run it over 115 mph.
The R1D of the B&M could run at 70mph all day, it was considered one of the most powerful 8 drivered engines made. Including some 4-8-4's.
pdlethbridge,
Yes both those locos could go fast, but those larger driver locos could go even faster. No one is contesting the excelent design of the J here. I was simply pointing out why it had smaller drivers than locos like the FEF, Big Boy, or Challenger.
The R1D is also a great design, and could run all day under load at 70, but 70 was about tops, and it could also pull a heavy load up a grade at 40-50. The RF&P loco could not do that, BUT, the R&P loco could easily top 70, with 90 to 100 likely being its upper limit.
Mountain types where infact the first well balanced design combining good power and speed. Many designs that came later with the same goals (Berksire's, Northern's) where only marginaly better.
But again, each road had its own understanding of its needs and designed/purchased power that it felt was best for the jobs - not every such decission was perfect.
Again, the RF&P 4-8-4, or the even larger drivered UP FEF or SP GS4, would have a much larger radius requirement for any given speed in its operational range, but would have had a higher top speed - all other conditions being equal.
It is also more valid to look at how each railroad actually used the power in question rather than to talk in terms of top speed or maximum load.
You don't run your car to the limits of its performance all day every day and railroad locomotives are no different in that regard.
What a machine will do flat out, and what it will do day in and day out with a problem are two very different things.
If you are going to run at 75 MPH, you want a loco that can easily do more. If you only going to run at 50, one that will go 70 easily will last a long time and give good service at 50.
Railroad engineering departments are well known for their conservative use of the company resources for money and safety sake.
Sheldon
Also, a different example of a "smaller" loco being "more powerful".
The Great Northern O-8 Mikado - 69" drivers, 77,300 lbs of TE
The C&O Kanawha 2-8-4 - 69" drivers, 69,350 lbs of TE
The NKP Berkshire 2-8-4 - 69" drivers, 64,135 lbs of TE
I don't have the numbers handy, but an O-8 weighs less total but has more weight on the drivers than either of these 2-8-4's.
Sound like the those 2-8-4's could have just as well been Mikados, had more weight on the drivers and thereby been more powerful?
Well here on the Atlantic Central that's just what we did. Lima built us two groups of super power Mikes with 69" drivers, boosters, 98 sq ft of grate, and 77,000 lbs of TE (all fictional of course, but it could have been) Lima did in fact build super power mikes for the DT&I with 63" drivers that look exactly like a C&O Kanawha slightly shrunk down.
I will post a picture of my model of these super power mikes in a day or two.
Sheldon
what I found interesting is how the locos looked. except for the 4 wheel trailing truck and tender, the RF&P northern looked just like the R1D.
A few things of note about the R1D:
It had no booster, even though many other B&M engines did ( 3700's pacifics with 80" drivers had one on the trailing truck, 2-10-2 with 1 tender booster and the 0-8-0's used for hump service had 2 tender boosters.)
It had lower steam pressure (240 PSI) than many other super power steamers.
The bearings were standard not roller.
Any of these could have helped this all ready powerful engine.
Remember that all the B&M Mountains other than the "d" subclass, went to the B&O and worked into the late 50's. The B&O was so happy with them and their own shop built 4-8-2's, that they never owned a Berkshire or a Northern.
Sheldon
yes they did and they ran the wheels off them because they loved them so much.
Tractive effort is usually 25% of total design weight that are on the drivers themselves. Not including the rest of the axles if any. That will make the coefficient about 4.0 or so. I give myself a headache as to why they do it this way.
Terrain has a issue. Richmond is flat, Roanoke is not.
Also dynometer data as gathered for engines will help the railroad determine which one is good for the job. I think you dont want to have a puny runt engine panting on no water and hardly any coal trying to drag something like 5000 ton fast enough to clear the main for that day.
The Y6b could pull the earth itself against it's own rotation but only up to 30 mph or so. Stories I have heard where Crews were brave and had em up to 60 mph. If so, it's a testament to the excellent trackwork by the MOW.
The Pennsy never found the true top end potential of some engines like the 4-4-4-4 or even the 4-4-6-4 ( think I had it right.) there was simply not enough train you can tie onto it due to coupler Strength limitations. And not enough room to take it beyond 120.. however there were again, one off stories that will never make official. (Best they didnt)
Finally but not last. You can only boil so much water so fast with whatever size firebox you have on a engine. Bigger = heavier more headache to maintain and feed. Smaller requires more engines and thier crews to help you out on a hill.
All steam engines have a sweet spot for wherever they are suited for the work that they are asked to do.
"I think I can, I think I can, I think I can...."
B&O also had those B&M 4-8-2s! ;)
And only if the N&W Y7 had ever been built, we'd still be seeing steam today.
One thing that was mentioned peripherally, but is important is the difference in the grate area. Those extra 16 square feet of grate area in the Governor meant greater steaming capacity, especially at high speeds. Steam generating capacity is the real bottom line in over-the-road work. The Pennsy duplexes were intended to reduce the wear and tear of high-speed running by dividing the drive train into lighter, and therefore better counterbalanced mechanims. Would likely have worked out, but diesels were already arriving on the property.
Then that begs the question of why is that engine more powerful, smaller grate lower steam pressure smaller wheels but slightly larger cylinders. It doesn't add up. TE is also a way to say horsepower. But even the loco weights are such that you'ld think the heavier engine would be more powerful, but thats not the case. The driver size is not a significant difference to give the smaller engine that much more TE? Or is it?
Using a fixed pressure to run calculations shows that the smaller Engine would have more foot pounds of torque at the wheels.
I think the larger boiler pressure and grate size could have been more for volume of steam, then for pressure sent to cylinders, this would give the engine a larger margin in which to operate.
NM
Your saying that the higher boiler pressure and smaller cylinders would be more efficient than the lower boiler pressure and bigger cylinders? Then why doesn't it calculate at the axle?
The top speeds were 87mph for the RF&P and 83 mph for the B&M Not really much of a difference
But because the drivers are 4" bigger they lost in the tractive effort ( could have been 70000 )
Try this:http://www.steamlocomotive.com/misc/TractiveEffort.shtml (http://www.steamlocomotive.com/misc/TractiveEffort.shtml)
If you take the engine weight divided by the wheels it will show that the B&M engine puts more weight on the drivers
466040/16=29127.5 4-8-4
416100/14=29721.4 4-8-2
pdlethbridge,
Locomotive weight is not evenly divided amoung all wheels. It is evenly divided amoung drivers, but leading truck and trailing truck axles can support more or less weight than the drive axles, and in most cases this was true.
Example - DT&I 800 class 2-8-2, adheasive weight 113T, EWWO 168T
lead truck axle - 40,000 lb
4 drive axles - 56,500 lb each
trailing axle - 70,000 lb
And remember, when compairing old specs, each was calculated by a different engineer who would have had his own opinion about fixed factors and such.
It is not just simple math, it hinges on the experiances, beliefs and preferences of the designer and builder. So compairing specs down the lb, psi, sq ft, etc, can be very misleading.
Same is true with automobiles. I built hot rods years ago, I beat lots of guys who's spec's said their car was faster.
The B&M loco breaks down like this most likely:
Lead axles - 40,900 lbs each
Drive axles - 67,325 lbs each
Trailing axle - 65,000 lbs
The pivot points and lever lengths of the locomotives suspension made it possible to load one axle more or less than the others. Trailing and lead trucks are linked to the driver suspension to keep all wheels on the rail all the time and because of the multi wheel nature of such a system, these lead and trailing trucks could suppoer more or less weight than the drive axles.
Some 2-8-0's had as little as 10,000 or 20,000 lbs on their lead truck, with drive axle loads as high or higher than any Mike, Mountain, Berk or Northern.
While other locos had large amounts of weight of lead or trailing trucks.
Yes, it is a very complex science.
Sheldon
Quote from: pdlethbridge on January 12, 2010, 05:49:18 AM
Your saying that the higher boiler pressure and smaller cylinders would be more efficient than the lower boiler pressure and bigger cylinders? Then why doesn't it calculate at the axle?
The top speeds were 87mph for the RF&P and 83 mph for the B&M Not really much of a difference
But because the drivers are 4" bigger they lost in the tractive effort ( could have been 70000 )
Try this:http://www.steamlocomotive.com/misc/TractiveEffort.shtml (http://www.steamlocomotive.com/misc/TractiveEffort.shtml)
No, I said it gives it a larger margin to operate in, nothing to do with efficiency.
Efficiency on a locomotive would be how hard the crew has to work to maintain steam and how far the loco will travel on a lump of coal and a gallon of water.
To understand why the small engine has a higher TE you have to look at the WHOLE package.
The numbers do add up, the smaller loco has more power per pound when sending the same psi to the cylinders.
200psi.X 28dia X 3.14 X 15.5 = 272552 / 12 = 22712 / 788,800 =
0.028793948613928329952670723461799
200psi.X 27dia X 3.14 X 13 = 220428 / 12 = 18369 / 842,940 =
0.021791586589792867819773649370062
. I think that, except for the discussion of different locos other than the 2 mentioned, this has been a great discussion. The fact that these 2 engines are so similar and yet so dissimilar, is amazing. The science behind the design done at the time, was all done in the head and on paper. No computers or calculators to make these work.
Over the years, locomotive design did have its failures but there were a lot of successes like these two engines
Sorry if I disrupted "your" discussion, but I sighted the information I posted to deminstrate other examples of how some of your basic assumptions were wrong - sorry to have bothered you.
Sheldon
Sheldon, you didn't disrupt it , you shed light on the subject. My thinking was involved around these two particular statistics and what made them so different and what made them so important to their particular railroads. When you cook, if you add a pinch of this and a touch of that, everything changes. How these two were changed with a pinch of this and a touch of that is totally amazing. None of the changes were that great but what a difference.
To some extent, this discussion points up the series of issues that had to be addressed when designing a locomotive. Generally, the head of the engineering department would develop a set of specs for what a new loco was to do - He had to take into account things like axle loading, clearances, type of fuel, minimum curves, grades, etc. Then, unless he worked for someone like the PRR, N&W, SP, he would send the spec out to the builder for a quote and a more detailed design. Once approved, the locos would be built. There were always trade-offs. High passenger speeds could be had with big drivers, but at the cost of tractive effort and factor of adhesion. Lugging power could be had with a lot of small drivers (think
2-10-2), but then your speed might be limited to 25 mph. One big advantage diesels had was that by making them in units that could be mated together and controlled from the head end was that the lash-up could match the need of each train.
The end of steam brought in an era of standardization and engineering productivity.. Steam engines were basically a custom build, with each being different in some way. Diesels, on the other hand, were assembly line built, all the same except for paint and numbering. A gp-7 was the same for hundreds of units, but not a 4-8-2 or a 4-8-4. A quick look at steamlocomotive. com showed at least 4 makers of 4-8-4's, many different designs, 4-8-4's that started as something else. ( Reading t-1s that were originally I-10 2-8-0's) And maybe 20 different names. ( Niagara, northerns, governors, big apples, pocono's, J's etc