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.
Dick Dawson
Retired Chief Engineer, TTX Company
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.
- Erik
@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
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 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.
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
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 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
+1
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
[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?
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