Design Theory of Edge-Curves for Rotary Blades of Tractors

Abstract

The edge-curve of a rotary blade is one of the most important design factors. This is influential to the entwining phenomenon of grass and straw on it and also to tillage resistance characteristics, and othrs.
The authors introduce a new approach to the design theories of effective edge-curves as follows;
1. Considering the slip-out motion of the grass and straw on the edge-curve, the possibilities of ideal edge-curves to remove and push them into the soil were discussed.
2. Grass removing characteristics of the rotary blade are mainly determined by the value of “edge-curve angle” α between radius direction and the tangential line of the edge-curve.
3. The design theories are composed so as to meet the authors' experimental results for Asian paddy rice field condition as follows:
The edge-curve angle smaller than about 55 to 57.5° is apt to have entwining phenomenon of grass and straw on the tip portion of the blade.
The edge-curve angle smaller than about 65 to 67.5° is apt to have also entwining phenomenon on the neck (holding) portion of the blade.
4. The grass removing performance of the rotary blade will be achieved by the edge-curve with its angle α, linearly increasing from the tip toward the neck portion. The equation of edgecurve design is proposed in polar coordinate as follows;
r=r₀sin^1/kα₀sin^-1/k(α₀+kθ)
where, r₀ is the radius of the tip point of the edge-curve when θ is zero degree, α₀ is the edge-curve angle at the same starting point and k is the constant being selected by a designer who decides the increasing ratio of the edgecurve angle α from the tip toward the neck portion of the blade. The recommended value of α₀ is 55 to 57.5° when r₀ is 300 to 200mm and about 1/18 for k (that is, increases the edge-curve angle about by 10° in accordance with the increase of θ by 180°).
5. Some examples of the rotary blade having the theoretical edge-curve are shown.