Freight Car Load Capacity

[Anonymous]

How are the capacities of freight cars, such as 4-bay hoppers, determined? For instance, a "100 ton" hopper shows its capacity to be 208,000 pounds while another "100 ton" hopper next to it shows its capacity to be 215,000 pounds.

Who determines the capacities - the manufacturer of the car or the railroad whose reporting marks are on the car?

[CShaveRR]

There are two determining factors in a car's capacity.

First of all, the trucks. "70-ton" trucks are smaller than 100-ton (or now 110-ton) trucks, and so on. Trucks with a given nominal capacity have a specific bearing size (Ed might be able to come up with the sizes) and spring configuration. They are given a precise gross rail load, such as 220,000 pounds for a pair of 70-ton trucks, 286,000 pounds for today's standard 110-ton trucks, and so on.

Once you have the gross rail load, the determining factor becomes the weight of the car. Subtract the weight of the car (the "LT WT") from the gross rail load, and you get the load limit ("LD LMT"). These days that's the top line under the car number, and as close to a nominal capacity as you'll get.

[Anonymous]

I understand.

According to the article in the December 2003 Model Railroader 70 ton freight truck will only support 70 tons for the entire car, not just for that particular truck. So you couldn't put a 70 ton truck on a 220,000 gr. wt. car, it wouldn't be rated high enough. You'd have to put 110 ton trucks on them, or else, put 100 ton trucks and limit the load to 200,000 pounds.

However, I'm not sure you got the focus of my question. I understand the weight figures on the sides of the cars, but what I'm asking is how do they determine that car # 12345 has a lt.wt. of 70,000 pounds, while car # 12346 has a lt.wt. of 68,000 pounds, even though they're both identical in appearance.

Who makes that determination, the manufacturer or the railroad? If the manufacturer makes the determination, what happens if the railroad takes it in for service and has to refurbish it - how do they know its lt. wt. or is it an estimate with a fairly accurate guess?

[edblysard]

Bearings, 70 ton trucks, 61/2" X 12", needle bearings.
Sure the cars are the from the same lot?
They may look the same, even be the "same" car, but from different lots.
Internal design differences, and material differences , along with age, may make a difference.

Short of getting the lot number off the car, (on hoppers, it is usually stamped into the sill, at the brake wheel end, behind the bottom side hand rung) the only bet on cars being from the same lot is if their reporting numbers are very, very close, and they are the same, exact car.

Will get exact bearing sizes when I go back to work, sat.

Ed

[Anonymous]

Rather intresting topic, I do indeed like it.

I bet those cars have design differences, and were most likely made in different years..

they may even look the same, but be different versions..

[DSchmitt]

QUOTE: Originally posted by joekc6nlx

I understand.

.

However, I'm not sure you got the focus of my question. I understand the weight figures on the sides of the cars, but what I'm asking is how do they determine that car # 12345 has a lt.wt. of 70,000 pounds, while car # 12346 has a lt.wt. of 68,000 pounds, even though they're both identical in appearance.

They weigh the car when it is empty. New cars are weighed before being put into service. Two "identicical" new cars may have different light weights, but they will be close. Cars are reweighed after work is performed on them and will also be reweighed periodically, especially if shipping charges are based on weight.

One RR operation rarely modeled is the weighing of cars.

[MP173]

Cars are weighted after being painted during the manufacturing process. That is the light weight.

If the car is manufactured to a 263,000 specification then that weight less the light weight is the capacity.

Ditto for 286,000 pounds.

Each car will have slightly different weights during the same production run, based on the exact weight of the components during the manufacture.

ed

[CShaveRR]

Joe, the quote in the December Model Railroader[/i] is,

"The rating isn't the load a single truck can carry, rather it is the total load of the car. A 50-ton truck is designed for a 50-ton-[i]capacity car, and so on. {The italics are mine.]

They're basically saying exactly what I said. Trust me on this...I deal with car weights every working day.

[Semper Vaporo]

Hate to drag this outta the past, but it is a start on a question that has been bothering me for some time.

I have seen unit coal trains with consecutive cars that have reporting numbers that are all within counts of 200 or so... (I have seen 4 cars in a row with sequential numbers), yet the "LT WT" numbers ranged from 41300 to 41900 and the LD LMT ranged from 244200 to 244900 and I could not see any relationship between the two numbers on multiple cars... i.e.: a low LT WT number did not necessarily mean a high (or low) LD LMT number.

I can see how each car can be weighed when it is empty and that value stenciled on the side of the car and I can understand how, knowing the type of truck under the car and what type and dimension of material it is consctructed from, one can calculate the capacity, but there appears (to this myopic untrained eye) to be large unaccountable variations in the numbers.

Who is slipping in an extra 600 pounds of material on some cars (or leaving out 600 pounds of rivets someplace)?  And how can someone say, "this car can hold 244900 pounds of coal, but that one over there can only hold 700 pounds less"?

Granted 600 or 700 pounds is only 1.5 percent of the LT WT, and even less of the LD LMT, but how can the LD LMT be determined with such accuracy, if they can't manufacture each car with the same accuracy for LT WT?

 

[CShaveRR]

Charles, I doubt that you kept notes of load limits and light weights, but the sum of those two numbers on any given car (at least on the ones you're describing) should be 286,000--the Gross Rail Load.  In other words, the higher the light weight, the lower the load limit.

The only exception to this is when the Load Limit is "starred" for some reason--then it can't be changed by anyone except the owner, even when the weight is changed.  This usually applies in cases when the trucks have been modified for certain special car types (auto racks and auto-parts box cars come to mind), and shouldn't be seen on coal cars.

[blhanel]

Hey Charlie, to the forum!  Still over in 108?

[tdmidget]

Got to call you on that Ed. No car or locomotive has " needle" bearings. They are tapered roller bearings.

[Semper Vaporo]

Thanks... and nope!  I'm retired (and absolutely LOVING IT!  Wish I'd done it 40 years ago!)

As for the sum of the numbers.  Yes, I did write down the numbers and no they did not all sum to the same number.  That is part of what has me confused.  I thought that either the sum or the difference would always be the same number from car to car.  Wish I had also written down the car reporting marks, too.  Of course, my rapid short term memory has always been very suspect... Next time I am out watching trains I'll try to video tape a train and grab some frames to get the numbers in a form I can share yet removing my hand eye coordination with a fast unit train from the mix!    I could very well be wrong in what I have been thiMkin'!

[Anonymous]

Hate to drag this outta the past, but it is a start on a question that has been bothering me for some time.

I have seen unit coal trains with consecutive cars that have reporting numbers that are all within counts of 200 or so... (I have seen 4 cars in a row with sequential numbers), yet the "LT WT" numbers ranged from 41300 to 41900 and the LD LMT ranged from 244200 to 244900 and I could not see any relationship between the two numbers on multiple cars... i.e.: a low LT WT number did not necessarily mean a high (or low) LD LMT number.

I can see how each car can be weighed when it is empty and that value stenciled on the side of the car and I can understand how, knowing the type of truck under the car and what type and dimension of material it is consctructed from, one can calculate the capacity, but there appears (to this myopic untrained eye) to be large unaccountable variations in the numbers.

Who is slipping in an extra 600 pounds of material on some cars (or leaving out 600 pounds of rivets someplace)?  And how can someone say, "this car can hold 244900 pounds of coal, but that one over there can only hold 700 pounds less"?

Granted 600 or 700 pounds is only 1.5 percent of the LT WT, and even less of the LD LMT, but how can the LD LMT be determined with such accuracy, if they can't manufacture each car with the same accuracy for LT WT?

The maximum gross weight is the number that matters.  It's an admittedly arbitrary number within a certain range (there's no practical difference between 286,100 lbs and 286,000 lbs.) but it is the number that is established by agreement among the railroads.  The line has to be drawn somewhere and that's where it's drawn.  The car is placed on a scale, and whatever it weighs is subtracted from the desired maximum gross weight, and that difference becomes the capacity.

Steel castings and forgings (truck sideframes, wheels, etc.) have small variations in wall thickness, etc.  Steel shapes and sheet have dimensional tolerance range according to their specification.  If you want to buy 0.5000 sheet steel and not 0.5010 sheet steel or even 0.5001 sheet steel you can do that, but you will pay a tremendous amount of money for it. 

Consult your ANSI standards for sheet steel and shapes, and AAR standards for steel castings and forgings.  They will describe to you the tolerance range that is the standard for the steel manufacturing industry and the railroad industry, respectively.

The variations tend to mostly cancel each other out within a narrow range.

If you want precision you will pay for it.  No one needs precision in a freight car body.  For each order the manufacturer provides a guarantee that the cars will fall into an agreed-upon weight range, a range which is derived by the manufacturere from experience.  If you wanted every car in an order of 1000 cars to weigh within 100 pounds of each other you would pay a huge premium to the manufacturer.  Will you realize that premium in car efficiency or rate reductions?  Absolutely not.

Manufacturing has tolerances established for everything.  The tolerances are only as tight as they need to be for the function of the product, within economic reason.  At a certain point the cost of manufacturing to tighter tolerance exceeds the value realized from the tighter tolerance, and that's where you establish your tolerance range limits.

S. Hadid

[jeaton]

  The car is placed on a scale, and whatever it weighs is subtracted from its maximum gross weight, and that difference becomes the light weight.

S. Hadid

I know.  It was a long day.

[Anonymous]

  The car is placed on a scale, and whatever it weighs is subtracted from its maximum gross weight, and that difference becomes the light weight.

S. Hadid

I know.  It was a long day.

5 a.m. to 11:30 p.m. to be exact ....