Echappement naturel

The échappement naturel was the invention of Abraham Louis Breguet, one of the most eminent watchmakers of all time. Following the introduction of the detent chronometer escapement with a temperature compensated balance, very close rates could be achieved in marine chronometers and to a lesser degree in pocket chronometers. This achievement was due, other things being equal, to the minimal interference with the balance during unlocking and impulse. A further key advantage of this escapement was that there was no need for oil on the escapement’s working surfaces and hence no deterioration in the friction between the working surfaces as the oil aged. A drawback was that the detent escapement as it was used in pocket chronometers was prone to stopping as a result of motion. Most escapements are capable of being stopped by a sudden movement but the detent escapement gives an impulse to the balance only when it is moving in one direction. The escapement is therefore not self-starting. The lever escapement, as used in most modern mechanical watches, avoided this problem. In common with most other escapements it gave an impulse to the balance in both directions of the balance swing. This creates another problem in doing so because the introduction of a lever between the balance and the final (escape) wheel of the escapement requires lubrication on the acting surfaces.

Breguet realised that this problem (more serious at the time because of the rather poorer oils then available) could be removed and many of the advantages of the detent escapement retained if an impulse were given in both directions of the balance. In order to do this he geared two escape wheels together, one escape wheel driven by the fourth wheel of the wheel train, which in turn was geared to and drives a second escape wheel. A pivoted detent swings freely between them and locks each escape wheel in turn on a coaxial set of locking teeth.

Photographs of the escapement from above and below in the prototype watch and a diagram of the important features are shown in the figures.

As the two escape wheels A and B rotate meshed together, one of them is locked when an impulse/locking tooth, standing upright from the escape wheel, rests on the wide straight locking stone of the detent. In the figure the balance is rotating counter-clockwise and just at the point that the unlocking pallet C on the balance is about to move the detent D to release peg E on escape wheel A which is resting on the jewelled locking surface underneath the detent arm. The impulse pallet F mounted on the balance staff just below the unlocking pallet C will have moved in front of peg G whilst the balance completes its unlocking arc. When the peg E is released by the detent, the escape wheels accelerate together and peg G will deliver an impulse on the jewelled rear face of impulse pallet F. After the impulse arc, tooth H from the escape wheel B locks on the other end of the detent locking jewel which has now been moved into its path. The depth of locking is limited by a banking pin J.

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Wikipedia