It is well-known that the measured value of the angle of repose varies dependig upon the method of measurement. The present author indicates that, although the angle of repose is placed in one category by its definition, it should be divided into two different groups. They are named the maximum angle of repose α_RM and the stable angle of repose α_RS. On the analogy of the tilting box method, the maximum angle of repose α_RM corresponds to the angle of slope at the moment that a box begins to slip, whereas the stable angle of repose α_RS corresponds to the angle of the residual slope after slipping (Fig. 1).
The relation between these two angles of repose and the measuring method has been investigated based on the previous results in the literatures. The sliding method (the tilting box test or the revolving cylinder test) gives the maximum angle of repose, while the draining method gives the stable angle of repose. The pouring method (the free pile method or the restricted pile method) yields the values scattered between the maximum angle of repose and the stable angle of repose, which varies also with the technique of supplying powder. These facts are clearly seen in the reports of Takahashi²⁾ (Fig. 3) and of Train¹²⁾ (Fig. 4).
In order to evaluate the flowability of powder, the maximum angle of repose should be chosen as the angle of repose. The tilting box method is the best for measuring the maximum angle of repose, because of its reliability of measured values, its clearness of forming (or tilting) speed, its possibility of measurement at any bulk density, and its accurate inspectation of two dimensional scale effect.