4-8-4 vs 4-8-2

[pdlethbridge]

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.

[NarrowMinded]

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.
2¢

NM

[NarrowMinded]

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

[pdlethbridge]

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.

[Atlantic Central]

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

[Atlantic Central]

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

[pdlethbridge]

 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.

[Atlantic Central]

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

[Atlantic Central]

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

[pdlethbridge]

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.

[Atlantic Central]

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

[pdlethbridge]

yes they did and they ran the wheels off them because they loved them so much.

[Heave]

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...."

[Guilford Guy]

B&O also had those B&M 4-8-2s!

[pdlethbridge]

And only if the N&W Y7 had ever been built, we'd still be seeing steam today.