DESIGN CONSIDERATIONS - GRAIN MILLING UNIT
Addendum 1999 - Note by the way that this machine can be produced as a thresher without the grain milling unit and also it can (with a lot of cutting) be produced as a mill without a thresher.
It is apparent that on energy grounds, hammer milling is preferable to plate-milling, which in turn is preferable to pounding.
TPI's (1977) method for obtaining high speed appears attractive but requires awkward fitting, ties up an important means of transport, and causes tyre wear.
Small laboratory mills designed to be powered by 1 kilowatt electric motors rotate a speeds of about 8000 rev/minute. The writer has obtained speeds of 3500 rev/minute from pedal powered systems (4 man centrifugal water pump, using 2 stage chain then belt drive) - thus 8000 rpm is probably feasible using 2 stage belt then belt drive, but power losses may eliminate some or all of the specific energy advantage of hammer over plate milling. Anyway we never got around to trying that out...
Using a roller bearing plate mill with plates like the Hunts Minimill would require 26 Newton Metres torque for fine flour. 4 pedallers doing 57 rev/minute with chain drive of ratio 44:18 would require 95 watts mechanical power per operator - that gives 151 minutes endurance for a 60 year old male European laborer - therefore quite a satisfactory solution.
Such a roller bearing plate mill can be made from scratch, or a standard journal-bearing-type plate mill can be adapted by installing a standard tapered roller bearing or ball type thrust bearing in place of the usual bronze thrust washer. Thus radial loads are still absorbed by the journal bearings but the much more energy-consuming thrust load is absorbed by the high-efficiency anti-friction bearing.