It does not say why 540 but according to the American Society of Agriculture Engineers, ASAE Standard. The the rear pto was "Originally developed in 1926 by a conference of engineers representing tractor manufacturers; adopted by ASAE April 1927; revised July 1928" along with revisions in 1931, 1941, 1952, 1958, 1961, 1964, 1966, 1967, 1968, 1969,1973 and 1974. It does not state what the revisions were. I believe the early years were the size of the pto shafts.
In 1976 the 540 and 1000 RPM PTO's were combined into ASAE S204.6.
If someone could come up with a copy of the 1927 ASAE Standards it might have the exact information. Mine is the 1998 edition.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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