  The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage $V$ larger than a threshold voltage $V_{\rm u}$. This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage $V_{\rm u}$. Associated with the process is also
hysteresis in the I-V characteristics.
