| July/August 1977

106 South Elm St., Newkirk, Okla. 74647

A good horse hitched to a plow or to a lever of a horse-power can exert a pull of 165 pounds and walk at the rate of 200 feet per minute. He can pull much more than this for a short time, but we are talking about the power that an average horse can maintain all day. This must not be confused with the load that might be hauled on a wagon. We mean the load that can actually be lifted or shown on a dynamometer connected between the horse and the load.

When power machinery came into a couple of hundred years ago or more, engineers and mathematicians adopted the 'foot-pound' as the unit of energy. It is defined as energy enough to lift one pound at the rate of, one foot in one minute. The steam engine came into use at about the same time and it came to be rated according to the number of horses it could equal. Since the engineers had already learned that a horse could put out thirty-three thousand foot-pounds continuously, they called this a 'horse-power and defined it as power enough to lift thirty-three-thousand pounds at the rate of one foot per minute or conversely, power enough to lift one pound thirty-three-thousand feet per minute.

This definition is a bit confusing as 33,000 pounds is an awfully heavy weight and one foot per minute is a very slow speed. A horse would need to be hitched to the end of a lever 200 feet long with the hitch point one foot from the fulcrum to be able to lift that much weight but the horse could walk at the rate of 200 feet per minute and the pull at the end of the lever would be 165 pounds. The problem can be stated: Force: 165 lbs. multiplied by: Speed: 200 feet per minute: Equals 33,000 foot pounds which is one Horsepower. To figure it another way: You take a larger but slower horse could be hitched at a point on the lever 165 feet from the fulcrum where he would have to exert a pull of 200 lbs. and walk at the rate of 165 feet per minute to lift 33,000 lbs. at the rate of one foot per minute. If you were plowing, a fairly fast medium weight team pulls a 12 inch plow 200 feet per minute. A heavier team could pull a 14 inch, but if they walked at the rate of 165 feet per minute you will get about the amount of ground plowed. To sum up, I am trying to say Force multiplied by Speed equals Energy.

The 'drawbar horsepower' of Steam traction engines is tested by connecting a dynamometer between the engine or other load to register the pull in pounds so that it can be multiplied by the speed. Most steam tractors were geared to travel about 2 miles per hour which is very close to 200 feet per minute, so this is a handy figure to use.

A fairly large steam engine on ground soft enough to plow can exert an average pull of 5,000 lbs. on the draw-bar and travel 200 feet per minute. So 5,000 x 200 = 1,000,000 foot-pounds. This divided by: 33,000 equals 30.3 horsepower. A Steam Engine Indicator attached to the cylinder of this engine would show it to be developing about 60 HP in the cylinder. Now what became of half our power? One of the unpleasant facts of life is loss. It begins friction between piston rings and cylinder walls, crosshead and guides, connecting rod bearings, crankshaft bearings, and the valve gear and slide valve can use a lot of power if not well lubricated. The transmission gearing and axle bearings take power but the greatest loss occurs at the contact of the wheels on ground soft enough to plow. To demonstrate; take an engine which we assume to be in good operating condition, hitch it to one of equal size fully loaded with fuel and water but with the steam shut off. You will find that you can pull it very easily on a hard level road. Now engage the clutch so that you have the full friction load of both engines and you will notice that it pulls harder, but on hard level ground you can go easily. Now pull off the road into soft plow land and you will find that you will have all that you go with. This goes to prove that it takes fuly one half of the power of a heavy steel wheeled traction engine to move itself on ground soft enough to plow. Few of the early manufacturers gave any draw-bar ratings for their engines. One leading steam engine builder says in their 1918 catalogue: - 'Our engines will develop from 50% to 75% of their brake HP. on the draw-bar.' Of course the power you can get on the draw-bar varies greatly. On a tightly packed clay and gravel road an engine might put 80% of its power on pulling a string of wagons, but in sand or mud could not even move itself.