(21)

Here, Γ _e represents the θr or zrside surface of the element region shown in Figure 2.n _θ and n _z are the inward unit normal vectors concerning the side surface andS _e is an element surface (θz surface). Coefficients such as S _θ,S _z, and D _θ are given by the following equations using Eq. (12) evaluated by the screw diameter at the element position and the barrel diameter, respectively.