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 Post subject: Question about large locomotives
PostPosted: Sun Apr 19, 2015 1:59 pm 

Joined: Sun Dec 28, 2014 11:12 pm
Posts: 182
With all the 4-8-4's being restored to operation or are operating I was wondering; what are the differences between them? I can look at 844, 4449, 611, 261, 765, and 2100 and the only real differences I can see are the decorations. Dont they all have around the same pulling power and weight/size? How can one or the other claim to be the most advanced steam locomotive or the most powerful?


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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 2:14 pm 

Joined: Fri Nov 07, 2008 11:21 am
Posts: 442
Did 765 gain an axle? I missed something........


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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 2:32 pm 

Joined: Wed Aug 25, 2004 11:16 am
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Here is a comparision put togehter by the 3751 group. Thanks to them

Robby

http://sbrhs.org/portfolio-items/comparison/


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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 3:51 pm 

Joined: Sun Sep 05, 2004 1:45 pm
Posts: 203
Location: Northern Virginia
The figures at SBRHS do as good a job comparing recent 4-8-4's as you’ll find in one place. As you can see there are large variations in all of the principal statistics such as weight, size, and tractive effort. However, I have to pick a couple of nits with the TE of the ATSF 2900 class.

First, here’s how tractive effort is calculated. It’s not complicated:

Tractive effort = ( K x P x B^2 x S)/D

where
K = constant reflecting pressure losses from the boiler to the steam chest, commonly but not always 0.85
P = boiler pressure, PSIG
B = cylinder bore, inches
S = cylinder stroke, inches
D = driver diameter, inches

The explanation on their website makes the comparison of the 2900 Class inconsistent with others on the list, and this is where things get more complicated. Although 3751, and the 3776 and 2900 classes all had a rated starting tractive effort (STE) of 66,000 lbs, different boiler pressure coefficients are used to calculate the figure. 3751 uses 0.85xP for 66,000 lbs. Both the 3776 and 2900 classes 0.70xP to compute 66,000 lbs. This lower factor reflects the railroad’s use of limited cutoff in the valve gear for the newer 4-8-4s, which supposedly reduced the STE. ATSF found at least on the 5011 class 2-10-4s that this factor was too conservative and that something closer to the traditional 0.85xP gave a better indication of actual TE.(See Lloyd Stagner’s article in Aug 1975 Trains). A factor of 0.85xP for the 2900s produces a STE figure of 79,960 lbs.

Some railroads used what appears to be a higher adjustment factor because during tests, they discovered that the use of 0.85xBP was also too conservative. Pressure losses in the steam circuit could have been less than anticipated, and the valve timing may not have restricted the steam flow at low speeds as much as was initially thought. As a result, steam pressure at the cylinders was higher than anticipated and so was TE. This is what happened on the 5001 and 5011 Class 2-10-4s, and very likely on the 2900s.

However, throwing the argument of roller bearings into the fray is inappropriate, at least here in the US. TE is computed in the cylinders using the geometry of the machinery, but no frictional forces. They are included in a separate calculation when estimating drawbar pull. Baldwin’s Chief Engineer, Ralph Johnson is very specific about the TE components (The Steam Locomotive, Second Edition, pg 139, partial quote): “In the United States, the constant K, is universally taken as 85 percent of the boiler pressure, and does not include any allowances for losses due to machine friction, as these are separately allowed for in estimating resistance.....” Generally accepted railroad textbooks of the late steam era (e.g., W. W. Hay's Railroad Engineering, 1953) also use a similar explanation.

Timken resurrected the separate resistance adjustment in its sales pitch for roller bearings. It was probably an easier sell because most railroads focused on TE, not DB Pull for comparative purposes. If an adjustment factor of approximately 0.92xP is used for the 2900s because of roller bearings (which produces a result of 86,554 lbs, very close to 86,922 lbs quoted by SBRHS), then it must equally be applied to all other roller bearing equipped 4-8-4s, absent any actual test data. Otherwise the SBRHS comparisons are not valid. You can’t calculate TE one way for your favorite and some other way for all the rest, particularly if many of them are technologically similar.

For example, the N&W J would have a calculated STE of 91,979 lbs using the 0.92xP method.. Based on actual test data, they were capable of developing maximum drawbar pulls of about 83,000 lbs at 2 mph, which indicates an actual TE of something like 85,000 to 86,000 lbs. at very low speeds when rail conditions were favorable.

Comparisons like this are always subject to a lot of what if’s, whereas’s and wherefore’s. As you can see, basically simple TE calculations are not so simple, and don’t even think about horsepower! It really gets crazy.


Last edited by Dave Stephenson on Sun Apr 19, 2015 7:54 pm, edited 1 time in total.

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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 4:15 pm 

Joined: Thu May 24, 2012 1:37 pm
Posts: 1666
To add a bit to all the good things said so far: there are different aesthetics involved with the different types of locomotives, and they display different 'schools' of design (if you know what to look for and are interested). A 2-8-4 like 765 was historically a fast-freight locomotive, whereas one of the 3765 or 2900 class ATSF 4-8-4s could run either fast freight or passenger, and an engine like N&W 611, while perfectly capable of handling freight service, was predominantly used in passenger service. How the design was optimized to its region and service is also of interest, as are improvements made to a locomotive during its lifetime (one positive example being the rebalancing of T&P 610 in the late '30s, and a possibly negative one being deletion of feedwater heater from C&O 614).

There is a wonder and beauty about any large locomotive in steam, more (at least in my opinion; many think differently) than for smaller and lighter locomotives. I put B&M 3713 squarely in the big-engine category because that is how whe's proportioned. It isn't, or in my opinion ought not, to be a pissing contest about 'biggest' or 'fastest'; every locomotive has its particular beauty and style, and should be appreciated for that. Sure, we have our favorites, but we'll stop with joy to see any other...

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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 6:21 pm 

Joined: Sat Aug 28, 2004 3:25 am
Posts: 1025
The fact that ANY large steam locomotive is running in the 21st Century might be considered a triumph of preservation. It's rather amazing that the surviving Concorde SST aircraft, and NASA "Space Shuttles" are all in museums, while we can still see big steam in action, and smaller locomotives in theme parks and museums show both railfans and the general public how we "got 'er done" years ago.

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Southern California


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 Post subject: Re: Question about large locomotives
PostPosted: Sun Apr 19, 2015 9:31 pm 

Joined: Wed Oct 22, 2008 8:18 pm
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it doesnt matter what engine it is, they are designed for specific purposes, each railroad had their aesthitic design, and look, The J had to be powerful to run around them mountain grades, 261 is more a flatland engine, others needed that higher power and speed. The NKP berks were successful for their speed but shorter wheelbase for a smaller turntable, the NKP was made for the high speed volatail products such as daily produce trains. Each road had their tweaks, prods, differences in design.


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 Post subject: Re: Question about large locomotives
PostPosted: Mon Apr 20, 2015 10:00 am 

Joined: Sat Feb 09, 2013 11:26 am
Posts: 57
I started typing out a big long post about adhesive factors, but then decided that I didn't even understand what I was saying, so...yeah.

But basically what everyone else said.


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 Post subject: Re: Question about large locomotives
PostPosted: Mon Apr 20, 2015 5:37 pm 

Joined: Sat Aug 28, 2004 3:25 am
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Regarding all the variations in steam locomotive design: Then came the diesels, and although there were some rare, special purpose models, for the most part, all the railroad had to do was change the gear ratio between the traction motors and the axles to cope with changes in utilization.

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 Post subject: Re: Question about large locomotives
PostPosted: Mon Apr 20, 2015 7:35 pm 
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Posts: 2521
Location: Pac NW, via North Florida
Bob Davis wrote:
The fact that ANY large steam locomotive is running in the 21st Century might be considered a triumph of preservation. It's rather amazing that the surviving Concorde SST aircraft, and NASA "Space Shuttles" are all in museums, while we can still see big steam in action, and smaller locomotives in theme parks and museums show both railfans and the general public how we "got 'er done" years ago.

I'm not shocked at all, for the same reason that my 1944 Willys Army Jeep is still driveable, never having been fully restored, and has outlived numerous other vehicles built after it within its 70 years.
The difference is that while steam is something that takes a lot of work and requires knowledge that gets harder to obtain with each passing year, a steam locomotive is still much sturdier and durable than a Concorde or an Orbiter. For example, while there have, of course, been crown sheet failures well within the superpower era, most occured through careless operation, so many of the current airplane types have gone down through the course of normal operation without any bad attention from the head end crew (like the Air France crash in 2000, or the STS-107 Columbia loss three years later). Barring a derailment from bad track that a steam locomotive crew couldn't have foreseen or contributed to, it's far less likely that a steam locomotive would wind up in a condition like a crashed airplane (as in a field of twisted parts each no bigger than your chest) where it wouldn't be a simple matter of how much steel you'd be willing to replace or repair to get her back on the iron again...

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