The tractive effort rating for steam is actually quite simple - the laws of physics dictate how much tractive effort can be developed based on boiler pressure, cylinder bore and stroke, and wheel diameter, plus some corrections for efficiency. Steam was typically designed not to require more than 25% adhesion, but in reality the available adhesion conditions can easily vary from 20% to 30% based on track conditions, weather, weight distribution on drivers and the engineer's skill. What isn't so obvious, is at what point the demand of steam for the cylinders exceeds the boiler's capacity, requiring the cutoff to be changed (johnson bar towards center) and resulting decrease in tractive effort. The "Super Power" locomotive designs of the late North American steam era did have some improvements such as roller bearings, lubrication, combustion and feedwater heating, but there biggest improvement was to increase the proportion of their firebox size (and direct heating surface) in relation to cylinder hp resulting in the ability to maintain full tractive effort at a higher speed. Steam, as long as it can maintain adhesion and momentum, really doesn't have a "minimum speed".
However conventional DC drive diesel-electrics are very different from steam. For starting a train and at low speeds for a limited amount of time, a diesel-electric's tractive effort is primarily limited only by the available adhesion. So the starting tractive effort is generally taken at 25% of its working weight on drivers, though GE did promote many of its smaller designs at 30% adhesion (generally considered to be dry sanded rail). In reality, a diesel-electric's starting tractive effort will vary widely based on rail conditions, the engineer's skill and on newer designs the adhesion correction system. The SP diagram for No 1 shows it as weighing 102,000 pounds (51 tons) and having 25,500 pounds of starting tractive effort - or 25% of its weight on drivers.
Diesel-electrics obviously use electric motors to to propel themselves. And as anyone knows, electric motors have limits as to how much power they can produce before they overheat and destroy themselves. But, electric motors do have the advantage in that they are capable of operating in an overload condition for a limited amount of time. Diesel-electric locomotives use this to their advantage, and almost every one of them (at least DC drive) depends on this limited overload capability to develop their full starting tractive effort.
There is a continuous rating for the traction motors, and as tractive effort is inverse to speed for the same horsepower input, the continuous rating of the traction motors (and resulting continuous tractive effort) can also be expressed in minimum continuous speed. In addition, some diesel-electrics will have a badge plate or ammeter scale that might indicate the "short time rating" of 5 minute, 15 minute, 30 minute or 1 hour - or something similar. So for starting trains, short ascending grades and other limited heavy pull conditions, the short time rating can be used - but it really doesn't add that much more capacity on a regular basis.
Traction motors do have a maximum rotational speed before they simply disintegrate due to centrifugal force. As such, any diesel-electric design has to take this into account and match the gear ratio and wheel diameter to the desired maximum speed. This in turn affects the minimum continuous speed/continuous tractive effort as those with lower maximum speeds generally are capable of a lower MCS and resulting higher CTE. Most of the 50 ton end-cabs being discussed here were arranged for a maximum speed of 40 mph (including SP No 1 and the former United Fruit units), so the 15,800 pound continuous tractive effort at 7.5 mph (MCS) likely applied to all the 40 mph units. There were a few built for mining that had different gear ratios, likely 25 or 30 mph maximum with a resulting difference in CTE/MCS.
As to the Costa Rica Northern 52 tons - I can only guess. These had turbocharged D397D engines at 500/450 hp (50 hp higher), but I just saw them listed as 400 hp and Costa Rica had a habit of derating Cat engines. I did find a figure for the Manila 52 tons that claims 16,200 which could account for the 50 hp higher rating or perhaps improvements in traction motor design and rating. I would guess the Costa Rica Northern engines were around 16,000 unless they had 50 mph gearing, in which case they might be closer to 12,500 pounds.
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>For the record - the "47-ton Drop Cabs" on the C&TS do not have double reduction gearboxes between the traction motors and drive axles. Also the articulated truck frames allow it to be used on gauges up to 42".
>Traction motors are direct geared to the axle - just like an SD40. That is one on the flaws with this design, in order to get the motors to fit between the wheels, they use rather small, fragile traction motors.
>But there is inherent reduction between the traction motor nose gear and the axle bull gear in the 4:1 range. If you go back to my confusion on the "C " trucked
unit on the D&S and Earl's comments, note that all standard gauge "C" trucked units of that era had offset center axles to allow the fit of the traction motors. The
motors on the narrow gauge units as Earl mentions are so small that the offfset isn't required to clear the motor cases.
>Could be worse. I know where to find one with two traction motors and chain drive on the other two axles.
I call "BS" on all the above comments (found later in this thread), and that is why Steamies should stay very far away from diesels.
CATS 15 & 19 and all "47-tons" I am aware of DO have double reduction gearing.
The articulated design was very common on GE narrow gauge designs, but it was not in any way limited to 42" gauge. There were some steel companies that had GE center cabs in the 65-80+ ton range in standard gauge and maybe even broad gauge.
There is nothing "small and fragile" about traction motors. It is true that the smaller diesel-electric designs, especially narrow gauge, had limited space for the motors to be. But more importantly, there was no economic or operational advantage to try to shoehorn an oversized traction motor that was to only have limited horsepower input into it. GE built a wide range of traction motors, and by using double reduction on smaller designs was able to use smaller motors that fit the available space better, and was more cost effective for the customer. And they don't necessarily have to fit between the wheels.
The uneven three axle truck spacing generally does indicate three traction motors. But even axle spacing indicating A-1-A has so many known exceptions that it really is a mistaken conclusion made and promoted years ago. Off the top of my head, I can think of the Porter 1203 (Loop>D&S), the GE C+C and C+B+C articulateds, EMD Flexicoils and later designs and GE Floating Bolster and later designs as all three motored designs with even or near even axle spacing.
CATS 15 & 19, the Loop's 21, presumably the 1943 and many others do have double reduction gear cases that require the traction motor to be offest and just clear the wheel, subsequently limiting wheel diameter. For narrow gauge, a common situation unless they were an axle hung type or a hypoid drive (also double reduction).
As far as one motor driving more than one axle through chain drive or side-rods, an extremely common design by GE and its competitors on four wheel and smaller B-B designs. Again, if you have a 150hp diesel engine driving a 150 hp traction motor, driving the second axle through a chain or side-rod is far more cost effective than installing a second traction motor with negligable operating differences.
The biggest problem is not with the locomotives, but with their operators. A diesel-electric from a steel mill was never intended to be capable of hauling trains up and down a 4% mountain grade, yet that is what they are expected to do. You have to understand the relationship between tractive effort, horsepower, weight and the electrical system, and enforce a resonable tonnage limit or they are doomed to failure. There is a way to modify them into mountain light road units - if you can find the parts, know what you're doing, and have plenty of dough-rey-mi.
Dan
