[quote user="PJS1"]
BaltACD F.S. Adams Smaller wheels do lower the car’s profile as well as its carrying capacity. That’s why you don’t see them on 100 ton + capacity cars and you do on high cars. Stenciling wheel size on a car seems unnecessary as would think anyone responsible for changing or installing would know without it. In our age of graffiti much of what’s stenciled on a car is obliterated anyhow.
F.S. Adams Smaller wheels do lower the car’s profile as well as its carrying capacity. That’s why you don’t see them on 100 ton + capacity cars and you do on high cars. Stenciling wheel size on a car seems unnecessary as would think anyone responsible for changing or installing would know without it. In our age of graffiti much of what’s stenciled on a car is obliterated anyhow.
Could you repeat that, please?
erikem Dick Dawson And in shameless self-promotion, I will point out that much of this information is discussed in "The TTX Story", which is now being shipped. Based on your two replies in this thread, the book will be a good read. - Erik
Dick Dawson And in shameless self-promotion, I will point out that much of this information is discussed in "The TTX Story", which is now being shipped.
And in shameless self-promotion, I will point out that much of this information is discussed in "The TTX Story", which is now being shipped.
Based on your two replies in this thread, the book will be a good read.
- Erik
+1
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
greyhoundsOK, I've never found a reasonable answer to this question. Heck Fire, I worked for a carbuilder (Thrall) and couldn't get a response. It was too far "Out There". What's magic about the two axle truck? If the wheel bearing surface weight is a problem, why not use more wheels? As in a three axle trucks under heavy loads. The freight car's carrying capacity could increase while the number of expensive components could be reduced. For example, there would be fewer brake valves and couplers required to move the same amount of tonnage. These components all require maintenance and reducing their number will also reduce maintenance cost. Is there a logical reason heavy loads such as oil, sand, coal, ore, grain, etc., can't move more economically on three axle trucks?
What's magic about the two axle truck? If the wheel bearing surface weight is a problem, why not use more wheels? As in a three axle trucks under heavy loads. The freight car's carrying capacity could increase while the number of expensive components could be reduced. For example, there would be fewer brake valves and couplers required to move the same amount of tonnage. These components all require maintenance and reducing their number will also reduce maintenance cost.
Is there a logical reason heavy loads such as oil, sand, coal, ore, grain, etc., can't move more economically on three axle trucks?
- PDN.
erikem... the book will be a good read.
It is.
Copies are available from the PRRT&HS (PRR started TTX) at $79.95 for members and $99.95 for everyone else, with $11 shipping. Two volumes, 624 pages.
http://www.prrths.com/estore/index_estore.html
Volker,
I am not aware of anyone using the A28 wheel anymore, unless some older cars that used it are still in service. And it's a good thing that the temperature at the wheel rim generated from brake shoe heating does not extend all the way to the wheel hub. If it did, there would be fewer stresses in the wheel from differential expansion, but the expansion of the wheel hub would remove some, and perhaps all, of the interference fit that holds the wheel onto the axle. Although freight car wheels are pressed onto the axle wheel seat and not shrunk onto the axle the way steam locomotive driving wheel tires were, the effect is the same. The interference fit is the only thing holding the wheel onto the axle.
Someone had asked about the loaded weights of auto rack cars. TTX did a study of typical load weights for rack cars in 1995 to see whether we could soften the spring groups to provide a better ride for the vehicles carried. The typical weight of an ETTX enclosed tri-level car was 110,000 lbs. and load weights ranged from 30,492 to 76,125 lbs., resulting in loaded car weights of 140,492 to 186,125 lbs. The 50th percentile value was 154,115 lbs. All were therefore well within the 195,000 maximum gross weight. Even though the 33" wheels of bi-level rack cars would allow gros weights up to 220,000 lbs., the actual weights, both of the car and the loads, were lower because of one fewer deck in the rack superstructure and fewer vehicles carried. Typical lightweight was 96,000 lbs. and loaded car weights ranged from 122,950 to 167,700 lbs. The 50th percentile weight was 134,150 lbs. I suspect that, with one exception, typical weights today are not much different. The one exception is that the heaviest load on a bi-level rack was produced by a 4-door Hummer. With the Hummer no longer in production, the heaviest bi-level load may be closer to 155,000 lbs. than to 167,700 lbs. Auto rack cars already used lighter spring groups than would be used for the full axle (or wheel) rating, but based on the weight study we decided to reduce the spring groups further for bi-levels. We did not change the spring groups for tri-levels. And in shameless self-promotion, I will point out that much of this information is discussed in "The TTX Story", which is now being shipped.
Dick Dawson
@Dick Dawson: Thanks for the explanation.
But there arises a question. The linked AAR standard contains an A-28 wheel with an allowed wheel load of 32,875 lbs. So for 8 wheels that is 263,000 lbs. Allowed but not used?Regards, Volker
Dick,
Thanks for jumping in, very informative post. Also kudos to Volker for posting the link to the AAR specs for wheels.
My question on the load limit for 28 inch wheels was focused on rail fatigue, which was based on a Trains news note from when the UP was first trying out 125 ton coal cars. The report indicated that the UP was experiencing more rail wear than what would be expected for the cumulative gross tonnage carroed by the rail.
Interesting that thermal stresses are the limiting factor for wheel loading, I was guessing the thermal limitation would be brake fade, i.e. the coefficient of friction for the brake shoe dropping significantly above a certain temperature. I can also believe that the hub and inner portions of the wheel plate are significantly cooler than the rims. Some simplistic analysis of heat transfer was suggesting that the wheel plate was radiating more heat than the plate was conducting towards the center with radiated flux proportional to the 4th power of absolute temperature.
So many aspects of this question; so little space to address them. Let's look at them one at a time:
1. Why are 28" wheels used? Their purpose is to lower the floor of a car. At a time when virtually all freight cars used 33" wheels, they were originally used on depressed-center and heavy-duty flatcars using specially designed (A28) wheels that were rated for 100-ton loads (251,000 gross weight of four-axle cars at the time). In 1960, lighter 28" wheels were used in combination with other car design techniques to lower the floors of TOFC flatcars by 10" so that higher (13' 6") trailers could be carried on the PRR through the Baltimore and Washington tunnels. The same cars were later used with auto rack superstructures. Virtually their only use today is with auto rack cars. They were limited to 179,200 lbs. gross weight for four-axle cars.
2. Why is the gross weight lower when the same 6"x11" axle is allowed 220,000 lbs. with 33" wheels? Even though the axle is good for more, the wheel itself is the limiting factor. The critical loading condition for the wheels is not the mechanical load of supporting the weight of the car, but the thermal load from braking. When brakes are applied for long periods of time (like descending a long grade), the whhel rim heats up. The heat causes the rim to expand, but the hub and inner portions of the wheel plate remain cooler. The difference in expansion generates significant stresses in the wheel. Because of their smaller mass, 28" wheels cannot handle as much heat as larger wheels and the heat input is proportional to the braking force applied, which in turn is based on the weight of the car. Improved 28" wheel designs now allow 195,000 lbs. gross weight, but not the 220,000 lbs. of 33" wheels.
3. Yes, the PRR used to have a limit of gross weight per inch of wheel diameter (which was also related to contact stresses at the wheel/rail interface), but that limit is not part of present AAR requirements.
4. Why are 28" wheels stenciled on the car? When 33" were used on almost all cars, it was assumed that a car would carry 33" wheels unless otherwise indicated. Therefore, all cars with wheels other than 33" are required to have the wheel size marked on the car. That includes 36" and 38" as well as 28". The considerable majority of cars today use 36" wheels, but the stenciling requirement remains. With the current trend to replace much physical marking on cars with entries in the electronic record for each car, I would not be surprised to see wheel size markings disappear.
5. What is magical about two-axle trucks? The truck frame and springing of three-axle (or four-axle) trucks is much heavier than that of conventional two-axle trucks. As a rule of thumb, three-axle trucks are twice as heavy and expensive as two-axle trucks for a 50% increase in capacity. They also have track negotiation problems. When TTX was establishing its QTTX heavy-duty flatcar fleet, we agreed not to acquire cars with two three-axle trucks, using cars with double two-axle trucks at each end. When we needed cars with more capacity than could be provided by four two-axle 125-ton trucks, we reluctantly accepted cars with double three-axle trucks at each end.
Retired Chief Engineer, TTX Company
Paul_D_North_Jr Erik - I recall seeing the metric expressed as something like tons of axle load per inch of diameter, someplace or another quite a few years ago. Now that I think about it, probably by John Kneiling - he often discussed wheel & axle sizes for the lighter loads of his integral train systems. Maintaining that ratio - though probably not quite so linear for the contact area size - would go a long ways towards adjusting the axle load to match the wheel diameter, since the circumference is linear with diameter (C = D x pi, 3.14) [I know you know this - I'm just going into the details for those who might not]. For example, 33 tons on a 36" wheel = 0.92 tons/ inch diameter; 28" x 0.92 = 25.7 tons < 15 tons hypothetical. - PDN.
Erik -
I recall seeing the metric expressed as something like tons of axle load per inch of diameter, someplace or another quite a few years ago. Now that I think about it, probably by John Kneiling - he often discussed wheel & axle sizes for the lighter loads of his integral train systems.
Maintaining that ratio - though probably not quite so linear for the contact area size - would go a long ways towards adjusting the axle load to match the wheel diameter, since the circumference is linear with diameter (C = D x pi, 3.14) [I know you know this - I'm just going into the details for those who might not]. For example, 33 tons on a 36" wheel = 0.92 tons/ inch diameter; 28" x 0.92 = 25.7 tons < 15 tons hypothetical.
Wheels have to comply with AAR Specification M-107/ M-208. www.wheels-world.com/UploadFiles/standard/AAR_M-107_M-208-2009%20%20Wheels,Carbon%20SteeL.pdf
The wheel loads are on page 31.
I tried to post just a Word table with just the loads, but the editor doesn't transfer it to the post.Regards, Volker
SD70Dude charlie hebdo I think Ambrose Bierce was the master of acerbic cynicism, even more so than Mencken. (from The Devil's Dictionary): Idiot, n. A member of a large and powerful tribe whose influence in human affairs has always been dominant and controlling. The Idiot's activity is not confined to any special field of thought or action, but "pervades and regulates the whole." He has the last word in everything; his decision is unappealable. He sets the fashions and opinion of taste, dictates the limitations of speech and circumscribes conduct with a dead-line. Is he orange?
charlie hebdo I think Ambrose Bierce was the master of acerbic cynicism, even more so than Mencken. (from The Devil's Dictionary): Idiot, n. A member of a large and powerful tribe whose influence in human affairs has always been dominant and controlling. The Idiot's activity is not confined to any special field of thought or action, but "pervades and regulates the whole." He has the last word in everything; his decision is unappealable. He sets the fashions and opinion of taste, dictates the limitations of speech and circumscribes conduct with a dead-line.
I think Ambrose Bierce was the master of acerbic cynicism, even more so than Mencken. (from The Devil's Dictionary):
Is he orange?
Hard to believe that was published in 1906. Apparently Bierce wasn't just a cynic; he could see into the future.
duplicate
greyhoundsWhat's magic about the two axle truck? If the wheel bearing surface weight is a problem, why not use more wheels? As in a three axle trucks under heavy loads. The freight car's carrying capacity could increase while the number of expensive components could be reduced. For example, there would be fewer brake valves and couplers required to move the same amount of tonnage. These components all require maintenance and reducing their number will also reduce maintenance cost. Is there a logical reason heavy loads such as oil, sand, coal, ore, grain, etc., can't move more economically on three axle trucks?
The magic of the three-piece truck is that its pieces can be made very strong and reliable for a minimum of components. Even a 'simple' three-axle version (which is what the Buckeye truck is) requires 'handed' sideframes, knuckles for the arrangement around the center axle, and a bolster arrangement with different side-bearing locations than three-piece trucks use.
There was, in fact, an organized effort to develop and sell a three-piece truck back around the turn of the century, when HAL to 315,000# looked as if it would produce more track damage than would be tolerable. The very fact that it is so hard to find information on this, or even who currently holds the patents to it, should tell you the perceived market for three-axle freight trucks at present.
Yes, one of the selling points was that six smaller wheels had the load-bearing capacity of four bigger ones, so the envelope of the cars using them could be larger and still fit reasonable plate limits.
OK, I've never found a reasonable answer to this question. Heck Fire, I worked for a carbuilder (Thrall) and couldn't get a response. It was too far "Out There".
Greetings from Alberta
-an Articulate Malcontent
Be careful to keep this separate from P.T. Barnum's
I conflated those two for many years because professors I trusted had said the quote was 'You'll never go broke underestimating the taste of the American public" -- which you'll confess has the proper Balimerean snarl...
H.L. Mencken's (from Baltimore) actual quip on this was: “No one in this world, so far as I know — and I have searched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby. The mistake that is made always runs the other way. Because the plain people are able to speak and understand, and even, in many cases, to read and write, it is assumed that they have ideas in their heads, and an appetite for more. This assumption is a folly."
PJS1 BaltACD F.S. Adams Smaller wheels do lower the car’s profile as well as its carrying capacity. That’s why you don’t see them on 100 ton + capacity cars and you do on high cars. Stenciling wheel size on a car seems unnecessary as would think anyone responsible for changing or installing would know without it. In our age of graffiti much of what’s stenciled on a car is obliterated anyhow.
Let's keep everything on a need to know basis and no one needs to know.
Never too old to have a happy childhood!
Rio Grande Valley, CFI,CFII
F.S. AdamsSmaller wheels do lower the car’s profile as well as its carrying capacity. That’s why you don’t see them on 100 ton + capacity cars and you do on high cars. Stenciling wheel size on a car seems unnecessary as would think anyone responsible for changing or installing would know without it. In our age of graffiti much of what’s stenciled on a car is obliterated anyhow.
Never underestimate the abilities of one to misunderstand what is actually required.
Smaller wheels do lower the car’s profile as well as its carrying capacity. That’s why you don’t see them on 100 ton + capacity cars and you do on high cars. Stenciling wheel size on a car seems unnecessary as would think anyone responsible for changing or installing would know without it. In our age of graffiti much of what’s stenciled on a car is obliterated anyhow.
Maintaining that ratio - though probably not quite so linear for the contact area size - would go a long ways towards adjusting the axle load to match the wheel diameter, since the circumference is linear with diameter (C = D x pi, 3.14) [I know you know this - I'm just going into the details for those who might not]. For example, 33 tons on a 36" wheel = 0.92 tons/ inch diameter; 28" x 0.92 = 25.7 tons > 15 tons hypothetical.
P.S. - Edited the last line 8-04-2018 to correct the < to >
Paul_D_North_Jr However, I suspect the axle load for a tri-level is fairly light. As a 'first approximation' (guess), 3 levels x 5 cars per level x 2 tons per car = 30 tons per car, spread over 2 trucks - plus the tare weight of the car and the rack. Even one of those articulated ocars with a common center truck - essentially 30 tons on that 1 truck - wouldn't be challenged all that much. It would be interesting to see what the actual weights are from someone who knows.
However, I suspect the axle load for a tri-level is fairly light. As a 'first approximation' (guess), 3 levels x 5 cars per level x 2 tons per car = 30 tons per car, spread over 2 trucks - plus the tare weight of the car and the rack. Even one of those articulated ocars with a common center truck - essentially 30 tons on that 1 truck - wouldn't be challenged all that much. It would be interesting to see what the actual weights are from someone who knows.
Paul,
That's close to what I would estimate for loading. While cars are still primarily made of steel by weight, they are pretty much air by volume. 30 tons per truck translates into an axle loading of 15 tons, well below the 33 - 38 tons for many RR cars. My contention was that while the axles on a 28" wheel set might be able to handle 33 tons, I wouldn't be so sure of the contact surfaces. For a 15 ton axle loading, the 28" wheels would likely have a lower peak contact pressure than a 36" wheel with a 33 ton axle load.
A 28" wheel set should be lighter than a 36" wheel set and the reduction in unsprung mass should improve ride quality.
Paul_D_North_JrHowever, I suspect the axle load for a tri-level is fairly light.
Would explain why long strings of racks never seem to have many units on the front end while they're passing at a good clip.
erikemI wouldn't be as worried about the axle as I would be about the contact surface of the rail. Seems to me that the smaller wheel would have a smaller contact patch (fore/aft) than a larger wheel and lead to higher contact pressure for a given axle load.
David1005 The nominal capacity of a wheel is determined by both its ability to carry the weight of the loaded car and its ability to dissipate heat generated in braking.
The nominal capacity of a wheel is determined by both its ability to carry the weight of the loaded car and its ability to dissipate heat generated in braking.
I've wondered about the latter in the past, particularly after reading the report on the Sand Patch runaway (due to brake fade from overheated wheels). A larger wheel would have more area to dissipate heat at the cost of being heavier than a smaller wheelset.
PJS1what would be the size of the wheels on Amtrak's Superliner cars?
The GSC-G70s have nominal 36", but that doesn't account for wear. At least one 110-mph design started with 36.5" nominal to keep the performance closer to 36" over the wear life of the wheels.
I think the MD-70s (which are European) may have been delivered with 914mm wheels (which DOT indicates are nominally equivalent to 36.0")
Since it is so infuriating to find meaningful information on Superliner wheels on the Web, I refer you directly to DOT 11695-DS1 (which probably has all the detail you'd want on either truck design).
https://rosap.ntl.bts.gov/view/dot/11695/dot_11695_DS1.pdf
WARNING if you thought the fluidized-bed boiler patent was TMI...
Thanks to all for the insightful and helpful information.
To stray a bit, what would be the size of the wheels on Amtrak's Superliner cars?
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