LenNH said: (quoted from post at 22:41:32 02/05/10)
This looks a lot like some of the laws of various forces in physics, in which (IF I remember this correctly) forces go up--or down--as the square of the change in speed.
What COULD be at work in the Zimmerman research is that the shape of the early plow moldboards is responsible for at least some of the extra resistance at higher speeds.
You are correct in your assumptions. Draft force for tillage tools is a function of speed squared (In general). And horsepower then is a function of force multiplied by speed. I would encourage you to find ASAE standard D496.3 FEB2006 and D497.5 FEB2006. Go to asabe.org and try to search their publications. I would post a link, but I am afraid it won't work. I am at a university and sometimes the university logs into professional organization websites to allow me to view publications without me knowing it. Anyway, I said "in general" because a moldboard plow uses speed squared to determine draft but a chisel plow for example only uses speed multiplied by a machine specific parameter. It must be something in the nature of how the two different tillage tools fail the soil.
As far as moldboard shape: If you are interested in this stuff, you should purchase, Principles of Farm Machinery, 3rd edition by Kepner, Bainer, and Barger. The 3rd edition was published in 1978 and it is an ag engineering text book but it is a fairly easy read. You don't get bogged down in too much theory. What I like about the book is that it has tons of references to look up. You can pick the book up pretty cheap and it covers every piece of farm equipment. I read the whole book, it was so interesting.
Anyway, the book mentions research from 1959 that a high speed plow bottom needs to be taller, longer, more sharply twisted, and have less share angle. The plow bottom must at higher speeds produce the same amount of inversion and soil break-up as the older bottom at lower speeds.
Moldboard plow research was hot in the early 30's but unfortunately came to a stand still once the dust bowl hit. One interesting research article that I fould thanks to the above mentioned book was from 31 or 32 and compared draft of moldboard shapes in different soil types. One would assume that a heavy clay soil should turned over slowly and gently to create the least draft. But, the research found one particular stubble bottom beat the other general purpose and scotch bottoms in draft. I would only assume in that case, draft had less to do with the nature of the soil turning, and more to do with the soil friction against the moldboard. Interesting stuff. Most of that research is found in ASAE Transactions which should be found in a good land grant university library, or in an Ag Engineering building.
As far as steel wheel and rubber tire traction efficiency: There were research articles published in ASAE transactions in the 30's showing that rubber tires outperformed steel wheels without showing the how. One phenomena of tires and soil failure is that some slip has to occur to create the most draft force and the best traction efficiency. This creates the chicken and the egg problem: does draft create slip or does slip create draft? From what I have seen, steel wheels bite too well and don't allow any slip. That is my guess at how rubber outperforms steel wheels. There are probably other reasons.
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