The image shows a CAD model of a regulator for steam engines, developed and applied for a patent by the Siemens brothers, which is absolutely different from preliminary centrifugal regulators.
This regulator is based on the difference of the velocity of a circular pendulum and of the velocity of the steam engine, which has to be controlled.
The control mechanism was implemented in remarkably different ways.
Here is shown the solution through a combination of screw and nut, where the screw is realized by means of two friction wheels and the nut by means of a double helically wound track.