Maintaining power
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In horology, a maintaining power is a mechanism for keeping a clock or watch going while it is being wound.
The weight drive used by Christiaan Huygens in his early clocks acts as a maintaining power. In this layout, the weight which drives the clock is carried on a pulley and the cord (or chain) supporting the weight is wrapped around the main driving wheel on one side and the rewinding wheel on the other. The chain then loops down from the rewinding wheel and up again to the main driving wheel via a second pulley carrying a small tensioning weight which ensures the loop stays taut and the chain engages well with the main driving wheel and rewinding wheel. In the first illustration the clock is fully wound, the driving weight is up and the tensioning weight down, a ratchet on the winding wheel prevents it from turning back. The driving weight pulls the main wheel in the direction of the arrow. In the second illustration the driving weight has reached its lowest point and the tensioning weight is now up, the clock needs to be wound by turning the winding wheel (or by pulling the chain), but during that time the main wheel continues to feel the driving force and the clock will not stop.
The principle was later applied by the French clockmaker Robert Robin who automated the re-winding in his remontoire. The drive- and tensioning-weights were made much smaller and drove the escape wheel directly. It was re-wound by the main train of the clock which turned the fourth pulley and was controlled by a lever attached to the tensioning weight. When this had risen to its upper limit, it started the re-winding process. As the drive weight rose, the tensioning weight fell and at the bottom of its travel it stopped the re-winding.