The theoretical flow ripple of an external gear pump is studied for pumps of
similar size using different numbers of teeth on the driving and driven gears. In this
work, the flow ripple equation is derived based upon the flow of incompressible fluid
across the changing boundaries of a control volume. From this method, it is shown that
the instantaneous length of action within the gear mesh determines the instantaneous flow
ripple. A numerical and a closed-form approximation are presented for the instantaneous
length of action and it is mentioned that the difference between these two solutions is
negligible.