Lagrange’s method, equation (1), is used to derive the equations of motion resolved in the generalized coordinates (p1y...p4y, p1z ...p4z) as illustrated in figure 3. Together with the stiffnesses indicated in figure 4, these form the expressions for the potential energy U in equation (2).In the sequel, it is assumed that the spring is massless giving T = 0. Viscous damping is applied over the generalized coordinates, givingwhere and denotes each generalized coordinate and the corresponding damping coefficient.