TANK CARS

[JAMES F CRONE]

I am not an engineer of any type and am aware this topic probably has been discussed before.Why cant iank cars be built like coal cars or covered hoppers ? the tank would start from first wheel set then drop down between wheel sets resulting in a lower center of gravity.Yes i know this would be more costly but would this increase safety factor?

[MidlandMike]

They did build some fish belly tank cars.  Apparently not a popular car.

http://www.krunk.org/~joeshaw/pics/pvt-tank/acfx/acfx18847-1-medium.jpg

[Euclid]

I noticed that in Russia they run a lot of tank car trains.  Their tank cars have underframes, which I assume have center sills from one end to the other.  I wonder why they prefer that design.  U.S. tank cars used to have underframes, but they abandoned that practice. 

Fish belly or “pregnant whale” tank cars have a lower center of gravity, but I have no idea what, if any, issues are present in relation to the center of gravity of current tank cars.  But with an underframe and center sill, the CofG will be relatively lower.

I once saw a tank car that I have never seen before or after.  It was a tank about the diameter of a 1232 car, but considerably shorter.  Around the outside, it had massive stiffener rings.  The rings must have been 2” thick and 4-5” wide.  The car was completely rusty without any paint at all, apparently new and not yet painted.  It must have been on its way to a paint shop somewhere. 

[Semper Vaporo]

A fishbelly shape would have seams and/or bends that weaken the structure without a lot more thickness and the addition of strengthening ribs etc.  Straight sides are much stronger, as well as cheaper in materials and construction costs.

[BroadwayLion]

MidlandMike

They did build some fish belly tank cars.  Apparently not a popular car.

http://www.krunk.org/~joeshaw/pics/pvt-tank/acfx/acfx18847-1-medium.jpg

 

 

LION thinks, you are not looking at the tank, you are looking at the insulated lagging around the tank. Absent any valves on the bottom, the lading is drawn out from the top. It is probably a gas.

ROAR

[MidlandMike]

Euclid

 

Fish belly or “pregnant whale” tank cars have a lower center of gravity, but I have no idea what, if any, issues are present in relation to the center of gravity of current tank cars.  But with an underframe and center sill, the CofG will be relatively lower.

...

While the center sill is low, the rest of the tank seems to sit higher on the old tank cars.  The newer tanks seem to sit lower over the wheels, which would lower the COG.

[Euclid]

That is a great photo of the fishbelly tank car.  It appears that the design intent was to lower the center of gravity, but it required a more complex tank.  The simplest tank is a cylinder with two end caps or “heads”.   Those fishbelly tanks have to have more diameters and a transition between the diameter changes.  This requires more scrap material, more forming operations, and more welding.  All of that drives up the cost.

The fishbelly design appears to be in response to a belief that the CofG needed to be relatively low.  There must have been a strong expectation that the added tank cost of achieving that would be worthwhile.  I wonder what rationale in either theory or practical experience led to this belief; and why it has since been dismissed.

Today, the practice has reverted back to the simple cylinder, but without any underframe.  If you look at this in side elevation view, you can see the tank seemingly riding high on each truck.  In other words, you can see where the underframe and center sill would be if they were still in use. 

The tank and load is still at the same elevation, so that does not change the CofG, but the earlier underframe was dead weight that amounted to ballast in lowering the CofG, and adding stability.  The issue of stability might not reach a point where it could tip a train over, but it might cause the rocking of loads thus inducing high stress to the rails and track structure.  

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure.  In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.   

[dehusman]

Euclid

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure. In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.

Do you have any evidence of this EVER happening?  The only impact loadings that I have ever heard of breaking a rail were from a broken wheel or a huge flat spot.

Imagine a solution to this problem and that will fix it, because the cause is in your imagination too.

[BaltACD]

Euclid

 

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure.  In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.   

 

Rock Off derailments are not caused by the rail breaking - they are cause by the rocking couple increasing to the point that the car(s) physically have wheels climb on top of the rail and come down outside the gauge or rocking with sufficient force to physically turn the car over.  Rock off derailments are not a function of cars breaking rails.

[Murphy Siding]

Euclid

The fishbelly design appears to be in response to a belief that the CofG needed to be relatively low.  There must have been a strong expectation that the added tank cost of achieving that would be worthwhile.  I wonder what rationale in either theory or practical experience led to this belief; and why it has since been dismissed.

 

  .... or perhaps this was built by someone trying to fix something that wasn't broken.  The demand for those types of products is very thin.

[Euclid]

I doubt that the industry is inclined to fix things that aren’t broken; or to believe something is broken when it is not. 

[Euclid]

BaltACD

 

Euclid

 

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure.  In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.   

 

 

 

 

Rock Off derailments are not caused by the rail breaking - they are cause by the rocking couple increasing to the point that the car(s) physically have wheels climb on top of the rail and come down outside the gauge or rocking with sufficient force to physically turn the car over.  Rock off derailments are not a function of cars breaking rails.

 

 

I realize what a rock-off is, and I would include that too as a possible outcome of instability but I was intentionally excluding rock-off when I said this:

“The issue of stability might not reach a point where it could tip a train over [i.e. “rock off], but it might cause the rocking of loads thus inducing high stress to the rails and track structure.

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure. In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.”

Are you and Dave saying that rocking cannot possibly have any effect on track?  I have heard of rocking causing rails to tip over.  I don’t know if it could break a rail or if it has ever happened.  But if rocking can develop to the point of lifting the wheels off of one rail, and tipping a car over; and if it can tip a rail over, I suspect there is a possibility of breaking a rail.  But if breaking a rail is impossible, I will cheerfully remove it from consideration.

But if you go back and read what I said, my point was more to the issue of stability as it relates to the center of gravity of tank cars in general.  I did not offer any conclusions that require a refutation.

Stability of tank cars also relates to the issue of sloshing potential, which has been brought up several times lately.      

 

[BaltACD]

Euclid

 

 

BaltACD

 

Euclid

 

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure.  In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.   

 

 

 

 

Rock Off derailments are not caused by the rail breaking - they are cause by the rocking couple increasing to the point that the car(s) physically have wheels climb on top of the rail and come down outside the gauge or rocking with sufficient force to physically turn the car over.  Rock off derailments are not a function of cars breaking rails.

 

 

 

 

 

I realize what a rock-off is, and I would include that too as a possible outcome of instability but I was intentionally excluding rock-off when I said this:

“The issue of stability might not reach a point where it could tip a train over [i.e. “rock off], but it might cause the rocking of loads thus inducing high stress to the rails and track structure.

This hard rocking with a high CofG could break a rail or cause some other abrupt track failure. In other words, cars suddenly developing extreme rocking harmonics could derail the train as the rocking increases.”

Are you and Dave saying that rocking cannot possibly have any effect on track?  I have heard of rocking causing rails to tip over.  I don’t know if it could break a rail or if it has ever happened.  But if rocking can develop to the point of lifting the wheels off of one rail, and tipping a car over; and if it can tip a rail over, I suspect there is a possibility of breaking a rail.  But if breaking a rail is impossible, I will cheerfully remove it from consideration.

But if you go back and read what I said, my point was more to the issue of stability as it relates to the center of gravity of tank cars in general.  I did not offer any conclusions that require a refutation.

Stability of tank cars also relates to the issue of sloshing potential, which has been brought up several times lately.      

To date, in all the derailment reports I have read, there has never been any question to the 'ride stability' of tank car or their center of gravity.  You are trying to make a problem area where none exists.  Tank cars are sized to the specific gravity of their intended contents, as such, when loaded there is very minimal 'slosh area'. Were the normal transportation situation to be handling 'part loads' then sloshing could be a problem.  Part loads are only handled in switching situations.

[Euclid]

BaltACD

You are trying to make a problem area where none exists.  

I never said there was a problem.

[Paul_D_North_Jr]

From what I recall from back then, the primary reason for the fishbelly tank car was to get more payload cargo tons 'per car' (and to a much lesser extent, perhaps more payload per ft. of car, or train).  I don't recall ever seeing a lower Center Of Gravity being cited as a reason for this kind of design (though perhaps it was an added or 'free' side benefit).  That resulted from several factors - which were also seen to some extent in the "Big John" covered hopper cars, the 80-ft. long Hy-Cube box cars for auto parts, some long auto racks and TTX cars, etc.:

• The rail industry's attempts to become more efficient by handling and billing a little more revenue cargo in each carload with the same cost structure and events, such as number of switching moves, etc. - it's still only 1 car;
• The cargo might have been one that 'cubed out' (reached the volumetric capacity of the car) before it 'weighed out' (reached the weight limit of the car), which would have been a lost opportunity for added revenue; 
• Railroad cost accounting at the time was primitive and sometimes distorted or skewed, so some costs were treated to be the same per carload or unit, regardless of the capacity or actual costs incurred (e.g., (e.g., Union Pacific's "double-engined" diesels, "station painting", signal maintenance, joint facility charges, etc.);
• Rates - as then controlled by the ICC - might be able to be reduced indirectly 'by the back door' if quoted and billed on a "per carload" basis versus  a "per ton" basis.  This was one way to disguise becoming more competitive - 10% more payload per car would be the equivalent of a 10% rate decrease.  It wasn't usually that dramatic or obvious, but it served the perceived purpose at the time.
• The added cost of the more complicated fish belly car was mainly for the car owner/ lessor/ lessee, not the railroad.  That division of costs and effects led to some distorted and now seemingly odd decisions and designs - but were rational in terms of each party's position at the time - as well as muddied the accounting and rate calculations.  

On the "rocking effect": For steel bridge design, AREMA (2008) specifies that it is +/- 20% of the design wheel load - add to 1 side, subtract from the other.  20% isn't trivial, but it isn't going to break rails, either. 

- Paul North. 

[Paul_D_North_Jr]

With the philosophy of these cars carried to its (il)logical extreme, what you get are monsters such as GATX 96500 (96+ ft. long) and UTLX 83699 - both on 4 ea. 4-wheel trucks, so effectively 2 cars in 1 (not my photos or websites):

http://ogrforum.ogaugerr.com/topic/gatx-96500-94-tanker?reply=40004357915595435#40004357915595435 

http://www.trainsim.com/vbts/Attachments/up1/122230.jpg 

http://s681.photobucket.com/user/Zorronov/media/GATX.jpg.html 

- Paul North. 

[MidlandMike]

Euclid

...

Today, the practice has reverted back to the simple cylinder, but without any underframe.  If you look at this in side elevation view, you can see the tank seemingly riding high on each truck.  In other words, you can see where the underframe and center sill would be if they were still in use. 

The tank and load is still at the same elevation, so that does not change the CofG, but the earlier underframe was dead weight that amounted to ballast in lowering the CofG, and adding stability...

I am guessing you are referring to the photo of the fishbelly tank, but that is not an example of a modern tank car.  Here is a photo of a modern tank car.  Notice how low the tank sits on the trucks

http://en.wikipedia.org/wiki/Tank_car#/media/File:TILX290344.JPG

Now compare to an old style tank car.  See how the center sill raises the whole tank up higher above the trucks.

http://www.protocraft.com/category.cfm?Itemid=254&Categoryid=22

The center sill only adds minimally to the center of gravity.  The lower tank on the modern car brings the CofG for the entire tank down.