Posted by LenNH on December 07, 2009 at 13:46:27 from (71.192.137.159):
If any of you are armchair engineers and like to calculate what horsepower a tractor might have at the drawbar, here are some formulas. I have Wendel"s Nebraska Tractor Tests, but this book sometimes manages to leave out the some of the basic information that was in the original NBTT reports. Horsepower of any kind is based on force exerted in a certain length of time. With a tractor, using our common measurements, drawbar pull in pounds is the force, and speed is in miles-per-hour. If you have pounds pulled and speed, you can figure out the rest with these formulas (which also allow you to work backwards from the results).
Horsepower = force pulled, in pounds, times speed in miles-per-hour, divided by 300 (a "constant")
Example: The tractor pulls 2200 lbs. at 3 mph: 2200 x 3 = 6600. Divide by the constant 300. 6600/300 = 22 drawbar horsepower.
If you know force and horsepower, but not speed: Speed = Horsepower x constant 300, divided by force. Example: 22 hp x constant 300, divided by force 2200 lbs = 3 mph.
If you know horsepower and force, but not speed:
Horsepower times constant 300, divided by speed = force.
Example: 22 hp x 300 =6600, divided by 3 mph = 2200 pounds of force
I used to love to compare tractors by reading the Nebraska Tractor Tests. All of them from 1920 on were available in a nearby college library. They are absolute models of precision. Everything is in tables, so it is extremely easy to find all the info you need, including slippage (reduces h.p.), fuel consumption, rated h.p. versus maximum h.p., etc. In the 20s and 30s, all tractors were given a "rated h.p.," which just meant a h.p. that could be used all the time and which would leave some left over for the hard spots. A tractor "rated" as 10-20 would actually put out a maximum of maybe 14 or 15 on the drawbar, and 25 or a little more on the belt. The big difference between the two horsepowers in those days was the use of steel wheels, which robbed huge amounts of power from the drawbar. Compare modern Nebraska tests with rubber-tired tractors, FWD and so on to see the difference. Even in the 30s, you could do a few comparisons. The F-14 was tested on rubber, the F-12 on steel. With a little simple arithmetic, you can calculate the percentage of power lost on steel versus the percentage lost on rubber. If you like to fool around with these armchair-engineering things (I confess, I confess), you can apply your percentages to the horsepower at the drawbar a steel-wheeled tractor would have when converted to rubber. I can speak from experience on that. I spent a good deal of time on a rubber-tired F-20 and a steel-wheeled 10-20. Horsepower at the pulley is similar. The 10-20 would pull two 14's at about 3 mph--all day long, anywhere, any conditions, but it would not pull in third gear, at least not up to governed speed. The converted F-20 pulled the same plows at 4 mph (I estimate speed on 36" rubber, in second gear), and sometimes in third or even fourth, if the soil was light enough. Never had the pleasure of plowing with an F-20 on steel, but I imagine it would have performed about like the 10-20, with about a 3 mph top speed. IHC advertised the F-20 as having an "extra plowing speed" compared with the Regular, and that third speed was around 3 mph, as I remember. Can anybody with experience describe the performance of an F-20 on steel wheels?
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