HORSEPOWER IN STEAM TRACTION ENGINES

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Among steam traction engine fans in this country, apparently the most misunderstood aspect is horsepower ratings. The purpose of this article is to shed some light on this subject, in what I hope will be simple, everyday language. I am sure that, in some cases, the proud owner or operator of a 28-80 Case, a 20-75 Nichols and Shepard or a 19-65 Baker engine has not fully understood those figures relating to his engine.

The confusion in this matter basically stems from the fact that the drawbar horsepower and belt (or brake) horsepower are expressed in two different terms. The smaller figure, which is drawbar horsepower is expressed as NOMINAL horsepowerwhile the belt or crankshaft horsepower is expressed as calculated BRAKE horsepower. It is not my purpose here to go into formulas for horsepower calculations, which have been well covered in other articles and which would only serve to complicate this treatment of the subject.

To separate and clarify the two terms for horsepower expression, I will explain that NOMINAL horsepower is a somewhat arbitrary term going back to the first manufacture of the portable steam engines which were moved about and pulled into position for belt work, such as threshing or corn shelling, by teams of horses.

To put 'nominal' horsepower simply and shortly: an engine that would perform the same equivalent work as 12 average work horses was designated a 12 horsepower engine. Simple enough!

For reasons that are not clear, the engine manufacturers retained this system of stating the drawbar horsepower ratings only after they began building the self-propelled traction engines and continued doing so up until the last such engine was built in about 1929. Along with that, they designated the belt (crankshaft) horsepower using actual calculated (brake) horsepower. This calculation is relatively simple, once the pertinent information on the cylinder, boiler pressure, etc are known. But, as I said, I will not go into that for our purposes here.

About the best way to clarify an explanation of anything is with an example and I will use the 28-80 HP Case engine to show the ACTUAL relationship between belt and drawbar horsepower, when expressed in the same terms.

In their 1918 catalog, in the specifications of this engine, Case says, 'drawbar horsepower will be 50% to 75% of engine (crankshaft) horsepower' when both are expressed in the same terms. In other words, when thus expressed, the engine will have a drawbar horsepower of 40 to 60 horsepower NOT the 28 horsepower as normally expressed.

This may also explain why, down through the years the steamer has usually 'pulled the tail off' of the gas tractor with the same apparent horsepower rating, for the gas and kerosene tractors were, and are, rated by still another method, which I do not fully understand myself.

For those of the newer generation who did not have opportunity to see these monster engines in action when they were new and at their best, using full boiler pressure, it is understandably impossible for them to believe the incredible brand of pulling power of which these engines were capable. Take the above example of the 28-80 Caseand it is only one example. In my school days here on the Dakota prairie, I knew a man who did custom plowing with an 80 HP Case, regularly pulling 20 stubble bottoms when burning hard coal. There, of course, was no economy of coal and water, but it illustrates the unbelievable power of these brutes!

To you experienced steam engineers, I will welcome anything you can add to this to corroborate, criticize or enlarge on my attempt to sift this horsepower business out for the benefit of all.