I'll give this a try! Yes! For the same through put of material a big rotor will take the same HP as a smaller rotor, maybe even a fraction less HP due to those other factors I mentioned in the previous post. But a big rotor and cage also has the extra area to have a much greater volume of material flowing through before it reaches the same material density as a smaller rotor, therefore it has a much greater capacity along with a proportional increase in HP which will be required. It also allows a wider margin for the operator to achieve that happy medium where the rotor is operating comfortably within it's limits and is not being too heavily loaded or too lightly loaded for good threshing and separation. The capacity downside of the BIG rotor is when you cannot get enough material in because of crop or seasonal conditions. Threshing and separation will become problems much easier than a smaller rotor. Try thinking the opposite way! A 12 inch diameter rotor would be a disaster even if it was long enough to give the same threshing and seperation areas as the large rotor. But too large or BIG rotors also have problems with separation. This is where "Roosters" reference to centrifugal force comes in. Centrifugal force is a big factor in separation in a rotary. The simple explanation; the amount of centrifugal force is closely related to the number of degrees per second that a rotor turns through. A BIG diameter rotor with a certain peripheral or tip speed will have low RPM compared to the RPM of a small rotor with the same tip speed. The small rotor will be turning through a large number of degrees per second compared to the large rotor. The small diameter rotor will have a proporionately higher centrifugal force and theoretically better separation. In combine terms it is not going to matter much. The diameters of rotors of most combines are very similar and design of the rotor and cage are far more influential in the threshing and separation efficiency even up to somewhat larger diameter rotors than we are currently using. The downside of bigger diameter rotors is packing them into a user friendly and bureaucrat friendly transport package. The engineers also have to fit bigger grain tanks, heavier drives and other assorted larger bits around a large cage and rotor. Another downside is the lower RPMs that the larger rotors will operate at to keep the same tip speeds. These means increased drive torque on all mechanical components, particularly where heavy drive, very low RPM crop conditions arise. Much heavier gear boxes and heavier drives are required. The costs go up exponentially as these components increase in size and specifications. Never under estimate the engineer's problems as they compromise and trade-off to try and get a half decent machine which is why when they have a good design, they are loath to change it. Most combine engineers usually over come their problems by making things somewhat complicated and ultimately difficult to maintain and service. I can think of a couple of makes that follow this pattern. The odd genius engineer makes his combine a reasonably simple machine while it still does every thing required of it. The old AC engineers were the best of the lot at this. Having designed and built a large harvesting machine that other for one conveyor, used only air flow for it's entire operation, I got an inkling on what combine enginers have to put up with to build a machine that suits every body. They, the for the most part, have my respect and even, at times, my sympathy. Apologies "Sidekick"! I probably have not answered your question to your satisfaction. Here ends the Epistle! Cheers.
