Critical design criteria for steam locomotives

I think Kelly has it. You would need to know what the locomotive was intended to do first, and then design around that. If you wanted a high-speed passenger engine, for example, the designer would include larger drive wheels, for example, and perhaps a lower tractive effort than a drag freight engine would have.

Some of your criteria are also intertwined and interrelated. For example, the formula for tractive effort includes boiler pressure and cylinder diameter. As for weight on drivers, I'm not sure that's a point that can be negotiated--it sort of is what it is.

Ok, a good start here. Let's use high-speed passenger service as the requirement, say for mid-Atlantic geography, which includes moderate grades. But high speed is the most important objective.

The most important is "will the locomotive "fit" on the railroad?  So the dynamic (i.e. moving) clearance diagram is the most important, followed by the maximum weight on each axle, with max weight on driving wheels predominating.  That is, if you want the design to "operate anywhere".

The clearance diagram has some nuances as well.  For example, it may affect the max dimension over the cylinders, and thus affect their diameter.  And the type of crosshead (i.e. alligator, Laird, multiple guide etc.) may affect the clearance at the lower corners of the clearance diagram, and prohibit certain types due to insufficient clearance.  As for the boiler and the overall engine length, possible infringement of the running boards, boiler, cab etc. on maximum curvature (i.e. turnouts) may exceed the moving clearance diagram, or adjacent track center distances may have to be increased.

And of course steam locomotives had to fit on turntables, and ideally in roundhouse stalls, in the regions where each type operated, etc.

Total in service weight can also be an issue in regard to strength of bridges where the loco was to be operated (Coopers Loading). 

In the modern world, in addition to some of the above constraints which would also apply to diesels, the electronics are required to not interfere with all signaling systems, other locos, etc.

Its complicated.....

Informative discussion!

I am thinking that since pulling a given load up a long incline at a given speed requires a specific horsepower, I would think this would be a requirement for the design spec, as well as a decent cruise speed with the same load on straight and level. Also the weight on the drivers determines the available tractive effort, which the cylinders and boiler pressure, rod geometry etc must be designed to provide. Just saying. Most of the criteria mentioned earlier above would also be factored into the design.

As I understand it the BigBoy was specced by UP to haul single engine trains up Sherman hill westbound from Cheyenne to Ogden UT. That was to be its fulltime job. No doubt the spec dictated a certain minimum speed with a certain tonnage train, based on the needs of the time. I don't know what those numbers were.

Rod

Rod Stewart posted:

I am thinking that since pulling a given load up a long incline at a given speed requires a specific horsepower, I would think this would be a requirement for the design spec, as well as a decent cruise speed with the same load on straight and level. Also the weight on the drivers determines the available tractive effort, which the cylinders and boiler pressure, rod geometry etc must be designed to provide. Just saying. Most of the criteria mentioned earlier above would also be factored into the design.

As I understand it the BigBoy was specced by UP to haul single engine trains up Sherman hill westbound from Cheyenne to Ogden UT.

Actually no. The UP required a locomotive to operate eastbound out of Ogden unassisted ascending the Wasatch Mountains. It wasn't until after WWII that the 4000 class locomotives were reassigned to Cheyenne.

That was to be its fulltime job. No doubt the spec dictated a certain minimum speed with a certain tonnage train, based on the needs of the time. I don't know what those numbers were.

Lots of good information provided in William Kratville's book "Big Boy".

Rod

Kelly Anderson and Hudson 5432 are exactly right. The railroad specified the job the railroad needed to do and the builders responded with a design to do that job. That’s how the process worked.

For example, when the Nickel Plate Road wanted to move up from the old USRA 2-8-2s to something more powerful, the road gave the big 3 locomotive builders (Alco, Baldwin and Lima) the mission requirements for their new power. They were:

• Start and pull a 4,000 ton train
• Level track
• 70 mph

Lima responded, and the result was the NKP Berkshire, which was actually slide-ruled down from the C&O 2-10-4s. It was a robust design that proved to be just what the doctor ordered. Towards the end of steam, the NKP ran 6,000+ ton trains behind a single Berk. They could reach 60 mph with that tonnage. Impressive.

The things the OP mentioned (cylinder size, driver diameter, boiler pressure, etc.) were the RESULT of designing the locomotive to meet the mission specs.

The 4 boiler design criteria in my opinion would be:

Grate area

Total heating  surface area

Operating pressure 

Cylinder size  

  If we could have a 5th 

driver size as well  

Al

Thanks everyone, this is good information.

The question posed on this thread has been EXPERTLY answered! Is this a great place to share information or what?

Kelly Anderson posted:

smd4 posted:

As for weight on drivers, I'm not sure that's a point that can be negotiated--it sort of is what it is.

Sure it can be "negotiated".  

My point is, an engine will weigh what it weighs. You're not going to, for example, bolt a few tons of lead to the underside of the running boards because you need more weight. I would also think removing a few tons of weight from a completed engine would be just as difficult.

smd4 posted:

Kelly Anderson posted:

smd4 posted:

As for weight on drivers, I'm not sure that's a point that can be negotiated--it sort of is what it is.

Sure it can be "negotiated".  

My point is, an engine will weigh what it weighs. You're not going to, for example, bolt a few tons of lead to the underside of the running boards because you need more weight. I would also think removing a few tons of weight from a completed engine would be just as difficult.

You'd get an argument from N&W aficionados. The folks in Roanoke did just that, adding  25,560 lbs of lead ballast to the front engines of the Y6 locomotives after they were modified with new external reducing/booster valves. This was needed to counteract the slipping of the front (low pressure cylinder) engines due to the increased power provided by the new setup.

If you want to read an account of how a locomotive design may have been developed, I refer you to the NYC Historical Society's magazine, Central Headlight, Vol.32, No.3, where a what-if 4-6-6-4 was designed for the NYC.  It included the initial performance requirements, as was mentioned above as a first step, then proceeds through all of the other design criteria, some of which were also mentioned above.  As hudson 5432 stated, it's complicated.  I believe he will confirm that the article was relatively comprehensive and extensively reviewed before publication by Charlie Smith (and maybe by him, too) , who was not a fan of what-if articles.   Nonetheless, he published the complete manuscript, all of the boiler and clearance drawings, and the simulated  performance estimates.  Admittedly, it's a forest of numbers, but it will give you an indication of just how complicated locomotive design is, and believe me, the article only scratches the surface!!!

feltonhill posted:

If you want to read an account of how a locomotive design may have been developed, I refer you to the NYC Historical Society's magazine, Central Headlight, Vol.32, No.3, where a what-if 4-6-6-4 was designed for the NYC.  It included the initial performance requirements, as was mentioned above as a first step, then proceeds through all of the other design criteria, some of which were also mentioned above.  As hudson 5432 stated, it's complicated.  I believe he will confirm that the article was relatively comprehensive and extensively reviewed before publication by Charlie Smith (and maybe by him, too) , who was not a fan of what-if articles.   Nonetheless, he published the complete manuscript, all of the boiler and clearance drawings, and the simulated  performance estimates.  Admittedly, it's a forest of numbers, but it will give you an indication of just how complicated locomotive design is, and believe me, the article only scratches the surface!!!

They would have been some locomotives and, if they were kept west of Selkirk, would not have needed to be squeezed into the restricted clearance diagram imposed by the tunnels on the Hudson Division, the way the Niagaras were.

Here's a drawing from the article and a "photo" developed by Richard Leonard from the drawing

https://www.railarchive.net/ny...lection/nyc_8200.htm

And remember: When the engineers designed the steam locomotives, they did it without computers or apps, but with brains, books, math and slide rules.

Rusty

Nick Chillianis posted:

smd4 posted:

Kelly Anderson posted:

smd4 posted:

As for weight on drivers, I'm not sure that's a point that can be negotiated--it sort of is what it is.

Sure it can be "negotiated".  

My point is, an engine will weigh what it weighs. You're not going to, for example, bolt a few tons of lead to the underside of the running boards because you need more weight. I would also think removing a few tons of weight from a completed engine would be just as difficult.

You'd get an argument from N&W aficionados. The folks in Roanoke did just that, adding  25,560 lbs of lead ballast to the front engines of the Y6 locomotives after they were modified with new external reducing/booster valves. This was needed to counteract the slipping of the front (low pressure cylinder) engines due to the increased power provided by the new setup.

Sounds like it was an exceedingly rare occurrence.

 Great Northern also had an issue with Accurate locomotive weights as the Enginemens wages were based upon it

  O8 Mikados long standing official weights were found to be much lower than the actual weighs that confirmed suspicions of the running crews at the time of being underpaid 

Al

albertstrains posted:

 Great Northern also had an issue with Accurate locomotive weights as the Enginemens wages were based upon it

  O8 Mikados long standing official weights were found to be much lower than the actual weighs that confirmed suspicions of the running crews at the time of being underpaid 

Al

Actually, it turned out that several classes of GN steam were found to weigh more than the GN claimed, and crews were compensated accordingly.  What was interesting that one or two classes were actually found to weigh less than reported, but the GN management didn't ask for refunds from those crews.

Stuart

  Hello Stuart

Correct on the additional classes of motive power. I didn’t want to erroneously mention other classes of the top of my head lol

 Shenanigans on the part of management I guess to keep costs down.

  I cant see this as an “accident “ in calculations. 

 As mentioned previously, Limas 2-6-6-6 weights  were “adjusted” because of financial penalties that would be incurred by them due to additional weights added during construction. We all know how that turned out 

Al