Can anyone explain why almost all common diameters of steam locomotive drivers are odd sizes? I know there are reasons for different wheel diameters, but why not have drivers 48, 54, 60, 66, 72 and 78 inches in diameter rather than the almost always odd diameters such as used by the Southern Pacific which were mostly as follows:
0-6-0s: 51 and 57 inches
2-8-0s: 57 inches
4-4-0s: 69 and 73 inches
2-6-0s: 63 inches
4-6-0s: 63 and 69 inches
4-4-2s: 81 inches
4-6-2s: 73 and 77 inches
2-8-2s: 63 inches
2-10-2s: 63 inches
Articulateds: 57 and 63 inches
4-8-2s: 73 inches
4-8-4s: 73 and 80 inches
The actual driver diameters most often appear 3 inches larger than the "even" size except for the largest sizes. I have a guess why, but I don't want a "guess" answer. Thanks.
Mark
Jeff But it's a dry heat!
On the other hand when the vast majority of a railroad's engines (the Reading) have wheels that are 50, 55.5 and 61.5 inch diameter they aren't "odd" sizes since most engines have those drivers.
Its all relative. Why do automobiles have 13, 14 and 15 in tires? Why is paper 8.5x11? There are lots of "odd" dimensions out there.
Better question, why doesn't a manufacturer make a model with 54-56 in drivers? a consolidation would be nice.
Dave H.
Dave H. Painted side goes up. My website : wnbranch.com
selector wrote:I will hazard a guess. Clearances and grade varied, as did the ability of the parent company to purchase engines with a useful hauling capacity...or build them. So, for a given steam production capacity in a given boiler, on given grades with a range of tonnage hauling requirements over time and conditions (weather), the chief engineer for a railroad had to pay close attention to the best combination of steam production and tractive effort and curvatures and grade....oy, it must have been quite a project to build up specifications for a railroad.
Add to that different boiler sizes, different firebox and grate sizes, different technological advances over the years, different fuel, and different job duties.
None of us appears to know. I'll ask someone else.
So far, none of your guesses have been like mine. Hint -- my guess is based on the assumption that the driving wheel is made up of two major parts affecting driver diameter: the spoke-center and the rim.
Au contraire, mon ami. All the things I mentioned are interrelated. If an engine has a sufficient weight and steaming capacity to move 60 seventy-ton coal hoppers, but has 80" drivers on it, it won't work. The leverage that the rods deliver to the offset crank would be far too little for that huge diamater. Reduce the diameter to 55" and now you have a real coal hauler. Similarly, the distance of the crank to the centre of rotation for the hole mass is going to have an effect. If you shorten that distance, you reduce the leverage. If you take the crank out toward the rim until it just clears the tie tops (if such a setup would ever be countenanced) you'd have a very capable drag engine...although a light touch on the trottle would be required to keep it from spinning in place.
Otherwise, I am not following you. Are you saying there was something about axle or hub diameter, or that spokes had to be confined to a very narrow length range....I don't see what you mean.
selector wrote: Au contraire, mon ami. All the things I mentioned are interrelated. If an engine has a sufficient weight and steaming capacity to move 60 seventy-ton coal hoppers, but has 80" drivers on it, it won't work. The leverage that the rods deliver to the offset crank would be far too little for that huge diamater. Reduce the diameter to 55" and now you have a real coal hauler. Similarly, the distance of the crank to the centre of rotation for the hole mass is going to have an effect. If you shorten that distance, you reduce the leverage. If you take the crank out toward the rim until it just clears the tie tops (if such a setup would ever be countenanced) you'd have a very capable drag engine...although a light touch on the trottle would be required to keep it from spinning in place.Otherwise, I am not following you. Are you saying there was something about axle or hub diameter, or that spokes had to be confined to a very narrow length range....I don't see what you mean.
Selector, you totally missed the question as well as my original acknowledgement that there were valid reasons for different sizes of drivers. Further, I'm not about to reveal my utter ignorance about the relationship between a drive wheel's rim and the center/spoke section. (Like on a wagon wheel, there was a center spoke section and rim made of wood, and an outer rim of metal.)
markpierce wrote:The actual driver diameters most often appear 3 inches larger than the "even" size...
marknewton wrote:Wood spokes? On steam locos? No. Originally cast iron, and later cast steel. Or are we at cross purposes?
You missed the analogy here, but your later subsequent message shows you came to understand what I said. (Or are you pulling my chain?)
this (the other) Mark
Maybe Mark Newton, or someone else more knowledgeable than myself, can confirm or correct my understanding of this principle.
Doug
May your flanges always stay BETWEEN the rails
Slightly off topic but still related- why have 4'8" rail spacing and not 4'6" or an even 5'?
I'm aware of a rather tongue in cheek explanation but wonder if there is a more sensible reason.
concretelackey wrote: Slightly off topic but still related- why have 4'8" rail spacing and not 4'6" or an even 5'?I'm aware of a rather tongue in cheek explanation but wonder if there is a more sensible reason.
I don't know that the old tongue-in-cheek explanation is less than sensible. After all, as I recall from my old 2nd year Latin class, rarely taught these days, we get the "mile" from "millum passuum", one thousand Roman soldiers standard steps, with the step measured from when a foot hits the ground until the same foot hits the ground again, equivalent to TWO of our modern "paces" from one foot to the other.
So why couldn't the railroad gauge be based on Roman roads, which are based on Roman chariots, which are based on a certain equine dimension?
concretelackey wrote: Slightly off topic but still related- why have 4'8" rail spacing and not 4'6" or an even 5'?
Actually its 4 ft 8 1/2 inches, not 4 ft 8 inches. So its is even less "standard". 8-)
And the urban legend version is pretty close.
dehusman wrote: concretelackey wrote: Slightly off topic but still related- why have 4'8" rail spacing and not 4'6" or an even 5'?Actually its 4 ft 8 1/2 inches, not 4 ft 8 inches. So its is even less "standard". 8-)And the urban legend version is pretty close.Dave H.
The most reasonable-sounding excuse for the 56.5 inch "standard" gauge is that Stephenson, having had experience with 56 inch gauge mine carts, decided to ease the gauge by half an inch. Note that Sir Isambard Brunel, chief engineer of the seven foot 1/2 inch gauge Great Western Railway (UK) was known to refer to Stephenson's gauge, rather derisively, as, "Mine Cart Gauge."
Common carrier rails have been gauged at everything from 15 inches (Romney, Hythe and Dymchurch) to 84.5 inches (as mentioned.) If you want to spend some time and seven or so sheets of paper, Google Railroad Track Gauge. There is one comprehensive list that covers everything from garden railways to the rails under ship elevators.
Chuck (modeling Central Japan in September, 1964 - prototype 3'6" and 2'6" gauges)
No, it's not true.
http://www.snopes.com/history/american/gauge.asp
http://www.dnaco.net/~vogelke/articles/railroad.txt
The wheel sizes on railroads were a function of the best judgment of their engineering staffs, or those of the manufacturers. But there was something to be said for selecting a variety of sizes and sticking to them. I am relying on my memory here, but I recollect reading that the Pennsylvania Railroad had strict rules about introducing a "new" driver or tire size into the company. Standardization and inventory control had quite a bit to do with it but they had strict rules.
As I recall, someone wanted to introduce a new wheel size, perhaps for an electric, or the 4-8-2? -- here my memory gets really fuzzy -- and was told no. Then they learned that way back when, decades and decades earlier, the trailing truck of an obsolete 4-4-2 or 4-6-2 had used that very diameter of wheel or tire-- and suddenly it was OK! Even though none of that size was still in stock, once they could prove that it had been a diameter used by the PRR at some time the rules said it was OK. I may be garbling the story --but the basics are I believe accurate.
Dave Nelson
markpierce wrote:my guess is based on the assumption that the driving wheel is made up of two major parts affecting driver diameter: the spoke-center and the rim.
I would say the engineers had this calculated into their design. I don't see any advantage to manufacturing a wheel or wheel/tire to specific sizes. I hold to the theory that the design engineer specified these sizes to match cylinder pressures, push rod strength, desired speed, desired drawbar pull etc. All a complicated compromise determine ultimately by - calculus. Probably taking the second derivative of all the combined variables to zero for an "optimal" solution.
Whatever the reason, it worked, at least on SP. Those little C-9 Class 2-8-0's could pull a house down; in the Bay Area 75 to 100 cars was nothing for them as drag engines. The SP's MT Class Mountains were generally praised as some of the smoothest, best running locomotives on the system, while the balance of SP's fleet were acknowledged as some of the best locomotives on the planet, by a group of men who were intimately familiar with them all, although generaly biased....SP Enginemen on the Coast, Western, Sacramento, Los Angeles and San Joaquin Divisions.
In that regard, SP's Mechanical Engineering Department was second to none.
Doug, yes, that is what I meant. The unbelievable tractive effort would probably just be wasted as spinning under a loco too light. So, with cylinder size and piston travel and all those things interrelated, the designers had to figure out what was the optimum working dimension for all those pieces to make the locomotive effective/utile for the work they had in mind.
I, too am no engineer, as God knows, but I was envisioning all this having to work together. I was having difficulty following/understanding Mark's question, it seems, and not appreciating what he wanted to know. Why would some wheels be 3" in diamter greater than their larger, perhaps more modern descendants on the larger/faster/stronger locomotives?
So, Mark, what do you think?
-Crandell
selector wrote: Doug, yes, that is what I meant. The unbelievable tractive effort would probably just be wasted as spinning under a loco too light. So, with cylinder size and piston travel and all those things interrelated, the designers had to figure out what was the optimum working dimension for all those pieces to make the locomotive effective/utile for the work they had in mind.I, too am no engineer, as God knows, but I was envisioning all this having to work together. I was having difficulty following/understanding Mark's question, it seems, and not appreciating what he wanted to know. Why would some wheels be 3" in diamter greater than their larger, perhaps more modern descendants on the larger/faster/stronger locomotives?So, Mark, what do you think?-Crandell
Lacking input from any expert, I'd say that the center portion of most all drivers for American-manufactured locomotives was based most often on some multiple of a foot, such as 48" (4 feet), 54" (4.5 feet), 60" (5 feet), 66" (5.5 feet) inches, and so on. Rims for the most part were 1.5 inches thick, resulting in the common 51, 57, 63, and 69 inch diameter driver.
markpierce wrote:Lacking input from any expert, I'd say that the center portion of most all drivers for American-manufactured locomotives was based most often on some multiple of a foot, such as 48" (4 feet), 54" (4.5 feet), 60" (5 feet), 66" (5.5 feet) inches, and so on. Rims for the most part were 1.5 inches thick, resulting in the common 51, 57, 63, and 69 inch diameter driver.
Not at all. I don't understand the need to have driver diameters in even fractions of a foot. It serves no benefit that anybody has identified. Parts can be "standard" without being an even fraction of a foot.
People have already explained why drivers are the size they are. The boiler pressure, steam production and cylinder size and stroke determine the power and "RPM" available. That coupled with the driver diameter and rod position on the driver serve to optimize the speed or power of the engine.
Also remember that the steam engines were designed by dozens of different makers over decades. Steam pressures, engine sizes, speeds required, boiler design evolved over time. The equation I described in the paragraph above was constantly changing. So the driver size evolved with it. A steam engine's wheels lasted the life of the engine (unless there was some catastrophic failure). The tires had to be custom made for each wheel. So there was no need for 'standard' wheels like on a railroad car or diesel (which by the way have had 28-33-36-38-40-41-42 in diameters). Likewise bearing and axle sizes have also evolved. And not at even fractions either.
Why are steam drivers odd sizes and not even fractions of a foot? Because there is not mechinical or engineering advantage to making them even fractions and there is mechanical and engineering advantage to sizing them to the mechanical situation.
Besides, everyone knows that it was to ensure a locomotive of a given weight could lift itself over acorns and pennies left on the tracks by squirrels and kids respectively.
Sheesh!
The trainmaster tells the new hire brakeman to pay close attention to what the old head brakeman has to say, because he has all sorts of information gleaned through years of service that can prove invaluable.
For instance, the trainmaster says, the conductor can tell you how to determine if the black speck on the rail in the distance is a bird or a rock that might derail your train.
The new brakeman turns to the conductor and ask, "How do you tell the difference?"
The old conductors say, "If its a bird, it'll fly away."
Baadum dum.
The young fella ponders a moment, squinting at the hogger.
"Okkkaayyyyy........ But.......what if the speck doesn't move and you can't stop in time?"
Hogger lifts an eybrow and bores his eyes into the young fellers.
"Ooohhhhhh....." Gulp!
One word.....circumference
Pi x D = circumference of a circle (or wheel).
4 chuffs to 1 rotation gives you distance moved.
i.e. 69 inches x 3.141 = 216.7 inches or about 18 feet.
Backup 8 chuffs....thats 36 - 38 feet or one 'old' boxcar length. Nice to guess when you can't see the brakeman around a curve. (Counting ties helps too).
Not sure, but something from deep within my railroading ancestry seems to know this.
Try it with the other diameters.
Just a guess & I may be wrong, but I do use sound on my steam locomotives to spot cars on a long consist on my layout both in the yard and at drop-off & pick-up points.
Happy Railroading!
Mark Wallace
Collegeville PA