why oddball steam locomotive driver diameters

[dehusman]

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.

[leighant]

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]

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.

[challenger3980]

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, I think that you missed it a little, I am not a mechanical engineer, but I think that if you moved the crank pins all the way out to the edges of the rims "until it just clears the tie tops" I think that you would end up with a SLOWER locomotive with unbelievable torque that could start something incredibly heavy. Moving the crank pins FARTHER from the axle center, will INCREASE the STROKE length of the piston. Assuming that stroke length in a STEAM cylinder is at least similar to stroke length in an infernal (deliberately mis-spelled)  combustion engine, in its effects on performance, a longer stroke will provide more torque, and a shorter stroke will generally be more suited to faster speeds.

   Maybe Mark Newton, or someone else more knowledgeable than myself, can confirm or correct my understanding of this principle.

                                                             Doug

[markpierce]

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

[marknewton]

The actual driver diameters most often appear 3 inches larger than the "even" size...

You've pretty well answered your own question. What's the typical thickness of a tire when new?

Cheers,

Mark.

[marknewton]

Wood spokes? On steam locos? No. Originally cast iron, and later cast steel. Or are we at cross purposes?

[markpierce]

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.)

Mark

[selector]

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.

 

[markpierce]

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.

Mark

[markpierce]

None of us appears to know.  I'll ask someone else.

Mark

[pcarrell]

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.

[selector]

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.

[dehusman]

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.

[Vail and Southwestern RR]

I would turn the question around a bit, why would they use "even" numbers?  It's not like they could run to Home Depot and buy them!  Each loco type was pretty much a custom creation, I imagine, a mix of art and science, at the time.

[Texas Zepher]

So now I am curious too.  My guess would be that it has something to do with engineering, calculus, and our friendly constant pi.   We probably need a engineer who actually designed steam locomotives to get the answer.

[markpierce]

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