DESIGN CONSIDERATIONS - THRESHING MECHANISM
The wire hoop mechanism already mentioned is only really effective for rice. The peg-type mechanism is suitable for beans only. The rasp-bar-and wire-concave mechanism can be used for almost all crops, provided that clearance stators-to-beaters is correct and that beater speed is correct for the crop being threshed. These mechanism types are usually used on drum-type threshers, but an engine powered disc-type portable rice thresher was developed in recent years at IRRI, where the threshing was done in an axial and not a radial axis.
The first prototype pedal thresher constructed at Morogoro used plain wooden beaters with a wire concave, but separation of grain from the heads was found to be less than 95% with conventional semi-automatic feeding (letting the heads go into and through the beater/concave interface and into the output collection area). Therefore it was decided to adopt a feeding pattern similar to that used for treadle rice threshers - holding the heads at the beater-stator interface for about 1 second, and then withdrawing the heads. In this way, separation tended to 100%, and it was possible to dispense with the concave completely to save costs. With hindsight, the use of wooden beaters with a rasp pattern and a wire concave may have resulted in an acceptable degree of separation (and maybe not).
Thus a choice exists between a wire concave system with semi-automatic feeding and either cast-iron, steel or wooden rasp beaters; or a concave-less system with manual feeding and wooden or steel plain or rasped beaters. Concave-less design results in lower initial capital cost but higher real or effective labor costs. Concave-less also gives consistently high separation (if the feeders do their job properly). Steel beaters give better flywheel effect and lower wind / air resistance losses, but probably produce more grain cracking - we didnt experiment with steel beaters at all do we dont have experience. A flat-faced wooden (or steel) beater 25mm deep x 1000 mm wide in theory absorbs about 15 watts when travelling at 10 metres/second or 37 watts at 13.5 metres/second - probably not much compared with the power output of 4 operators.
Threshing velocity refers to the velocity of the beaters, hoops or pegs which strike and rub the heads of grain. Too high a velocity results in breaking or even shattering of the grain, thus rendering it unsuitable for use as seed and making separation from the chaff extremely difficult. Too low a threshing velocity causes insufficient separation of grain from the plant head. Typical threshing velocities, as used by Sutton (1969) are 13.5 metre/second for sorghum, millets, wheat and beans, and 23.5 metre/second for rice. However a threshing velocity of only 8.4 metre/second is specified for the Indian treadle rice thresher (1965), which despite the low velocity ensures effective threshing by the hand holding of the heads in the beater-stator interface for 1-2 seconds. The design in this report utilises 13.5 metre/second.