In model railroading and in some prototype areas, there appears to be confusion in truck loading classification. Often we see 50 ton, 70 ton or heavy duty. These refer to the nominal capacity of the freight car which rides on them. These names may or may not be correct and do not apply to passenger, tender or traction trucks. Heavy duty is nebulous and yields absolutely nothing pertaining to type or class. The term, at best, would be era dependent and could refer to any number of classes.
Note to all manufacturers and cataloguers: Using AAR classes in listings is far more indicative and accurate than the "nominal" terms.
DETERMINING THE CLASS OF A FREIGHT CAR TRUCK
Usually under the car number in freight car reporting marks, there are three weight references in pounds:
CAPY The nominal weight capacity for quick reference.
LD LMT The maximum weight the car can safely handle.
LT WT The total empty car or tare weight for weighing purposes including the wheel assemblies.
When CAPY = 100,000 lbs. or 50 tons, often the trucks are called 50 ton. But the relationship may not be true. There are many cases, among specialized and flat cars, that are exceptions.
By adding the LD LMT + LT WT, the maximum permissible total weight is found.
An example box car has 127400 + 41800 respectively for a total of 169200 including wheel assemblies. The closest standard AAR trucks are the class 2D (commonly called 50 ton) with two axles and an axle loading of 42250 lbs for a 2 truck (4 axle) loading of 169000 lbs.
Note the car appears to be over weight by 200 lbs. Did the Standard Railroad of the World (PRR) make a mistake? No. the weight of the wheel assemblies is not on the journals.
The journal on friction bearing trucks is the axle surface that turns inside the bushing in the side frame. Axle loading is determined from the journal diameter and length. With the exception of wheel treads, the remaining fabrication of the trucks is free-lance following AAR specifications. The following table shows standard AAR classes with common truck arrangements and the total weight permissible in each case. It covers almost all friction bearing freight, most passenger and some tender trucks. Roller bearing journals apparently follow the same classes.
COMPARISON OF CLASSES AND TRUCK COMBINATIONS
The table below contains the seven standard ARR classes along with various in use, truck combinations found in car builders dictionaries, Cyclopedia and other sources. It may not include every possibility, but others can be derived easily since each row is derived from the total number of axles under the car.
The columns are the AAR classes. The rows, starting with class and axle loading, contain the truck arrangements. Thus 4X2 = 4 two axle trucks and 2x(2+3) = 2 groups of 1 two axle and 1 three axle trucks. By performing the indicated algebraic operation, the total number of axles under the car is found. Thus 2x(2+3) = 10 axles. A rare three truck flat car with one truck in the center would have been listed as 3x2, but this is the same number of axles as 2x3 and thusly omitted.
The two 3D bar graphs above, with X and Y axes interchanged, physically display the data in the table. The Z axis is the total axle loading. The highest one in the upper right represents the Pennsylvania "Queen Mary" flat car using four 4F5T1, T-1 tender trucks.
The basic AAR classification was simply:
2D designates a truck with two class D axles of 44,250 lb loading.
From the table, a standard two truck hopper car would have an axle loading of 169,000.lb and a CAPY of 140,000 lb or 70 tons. Thus 2D trucks are commonly called 70 ton trucks,
Various railroads used different truck classifications. The PRR class designation expanded on the basic AAR. In its simplest form:
|N||number of axles|
|A||AAR axle loading|
|NA||is usually the standard AAR class|
|HYPHEN||may be replaced with numbers on MU cars to indicate horse power|
|D||deviation from AAR standards is like a footnote pointing to the specifications containing the deviation. (Only used when necessary, often with hyphen omitted.)|
|S||service: passenger, freight, tender etc.|
|V||variation in casting or other features such as brake gear or spring arrangement|
|TYPICAL PRR TRUCK CLASSES|
|2D-F15||15th model 4 wheel D freight truck.|
|2C-P1||First model 4 wheel C passenger MU trailer truck|
|3D-P1||First model 6 wheel D passenger truck--standard 5.5 "x 10" journal|
|3D-5P1||First model 6 wheel D passenger truck--variant 5.5" x 11" journal.|
|4E-T1||First model 8 wheel tender truck used on Coast to Coast tenders.|
|2D13P1||First model 130 HP MU power truck.|
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