  We compute the critical coupling constant for the dynamical chiral-symmetry
breaking in a model of quantum chromodynamics, solving numerically the quark
self-energy using infrared finite gluon propagators found as solutions of the
Schwinger-Dyson equation for the gluon, and one gluon propagator determined in
numerical lattice simulations. The gluon mass scale screens the force
responsible for the chiral breaking, and the transition occurs only for a
larger critical coupling constant than the one obtained with the perturbative
propagator. The critical coupling shows a great sensibility to the gluon mass
scale variation, as well as to the functional form of the gluon propagator.
