HORSEPOWER IN STEAM TRACTION ENGINES


| May/June 1981



115 1st Avenue, N.E., Kenmare, North Dakota 58746

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.