  We present the kinematics of a sample of bipolar planetary nebulae (PNe)
which cover a wide range of observed morphologies and collimation degrees, from
bipolar PNe with a marked equatorial ring and wide lobes to highly collimated
objects. We use an empirical model in order to derive the expansion velocity,
collimation degree, and inclination angle of the PN with respect to the plane
of the sky. The equatorial expansion velocities measured in the objects in our
sample are always in the low to medium range (3-16km/s),while their polar
expansion velocities range from low to very high (18-100 km/s). None of the
objects in our sample, even those that show an extreme collimation degree, seem
to be (kinematically) younger than ~1000 yr. We compare our results with the
state-of-the-art theoretical models for the formation of bipolar PNe. We find
good agreement between the observed expansion velocities and numerical models
that use magnetic fields with stellar rotation as collimation mechanism.
