MISCELLANEOUS STRESS CALCULATIONS
Although the machine was largely designed from experiemce with the design and usage of similar equipment, some calculations are in order:
If 4 pedallers, each weighing 66 kg are seated in their positions then maximum bending moment in each crankshaft support frame member will be around 1050 Newton-metres. Taking the yield stress of mild steel as 2.10 exp 8 Newton/metre squared, then 75 x 50 x 2mm mild steel rectangular hollow section should just yield under a bending moment of 2060 Newton-metre. Thus we have a safety factor of approximately 2.0 here.
Similarly, maximum bending moment in the front transverse frame member will be 230 N-m, whereas the section 50 x 50 x 1.8mm MS SHS should just yield under 1130 N-m.
It can be shown that the 15.88mm diamater stub axles for the pedal cranks should fail under torsional moment of 79 N-m, equivalent to a force of 43 kg force at the end of the crank. Normal operational torque loading should not exceed 30 N-m, equal to 150 Watts work rate at 48 rev/minute (5 radian/second).
Previous unpublished tests at Morogoro show that we need an axial force of about 1 tonne to produce 2.0 fineness modulus maize flour using the Hunts MiniMill Plates. The thread we are using is 25mm diameter metric fine, with major diameter 24.65 mm and minor diameter 22.5mm. 10000 Newtons axial thrust on one thread only has a safety factor of 1.7. Thus the layshaft thread, nut and locknut system should not be prone to failure.
Using a yield stress of 8 x 10 exp 6 N/metre squared (in fact the value for GREEN hardwood), analysis of the wooden beaters gives a safety factor of 5 for shearing of the wood at the spoke nuts, and a safety factor of 1.3 for tensile stress failure due to bending at the middle of the beaters.
A similar analysis of the alternative mild steel beaters gives us a safety factor of 1.3.