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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 another pulley which has spikes which allow the cord to turn the pulley without slipping. It is this pulley which turns the train of gears in the clock movement. The cord is kept in engagement with this pulley by a small tensioning weight which is also carried on its own pulley. The cord is endless and is looped over a fourth pulley before going back to the drive weight. The fourth pulley also has spikes to stop the cord slipping and has a ratchet that only allows it to turn in one direction. As the drive weight descends and turns the clock, the tensioning weight rises. When the drive weight needs to be re-wound, either the fourth pulley is turned directly to lift the weight or the cord supporting the tensioning weight is pulled down and draws the fourth pulley round with it.
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.
Bolt and Shutter
This is a type of maintaining power which needs to be engaged before re-winding is started. It consists of a weighted arm (bolt) with a ratchet pawl on the end of it which engages with the edge of the first wheel to keep it turning while the weight or spring is wound. To make sure that it was always operated, the hole in the dial through which the clock is wound is covered with a shutter which can be moved out of the way by pushing down on a lever at the side of the dial. This lever also engages the bolt. A similar type of mechanism is sometimes used on turret clocks. Because these take much longer to wind, and are usually wound by trained staff, the bolt carries a segment of a gear wheel rather than a single pawl and is engaged manually.
John Harrison invented a form of maintaining power around the mid-1720s. His clocks of the period used a grasshopper escapement which malfunctioned if not driven continuously—even while the clock was being wound. In essence, the maintaining power consists of a disc between the driving drum of the clock and the great wheel. Attached to the disc is a spring which tends to push the great wheel forwards. While the clock is being wound, this spring continues to drive the clock. The reaction from the spring tries to drive the disc backwards, but this is prevented by a pawl which engages with ratchet teeth cut in the edge of the disc. At the end of winding the drive weight re-winds the maintaining spring ready for its next use. In normal operation the maintaining disc turns in the same direction as the drum and the great wheel and the ratchet teeth simply slip under the pawl. Although described here as using a driving weight it is equally applicable to spring-driven clocks and watches. The whole mechanism is completely automatic in its operation and has remained one of Harrison's lasting contributions to horology.