Stability and Hopf Bifurcation in an Hexagonal Governor System With a   Spring
  In this paper we study the Lyapunov stability and the Hopf bifurcation in a system coupling an hexagonal centrifugal governor with a steam engine. Here are given sufficient conditions for the stability of the equilibrium state and of the bifurcating periodic orbit. These conditions are expressed in terms of the physical parameters of the system, and hold for parameters outside a variety of codimension two. 
Jorge Sotomayor, Luis Fernando Mello and Denis de Carvalho Braga