In 1986, Audemars Piguet released a new caliber that, at the time was a remarkable technical achievement. This is the Caliber 2870. Not only was this the first production tourbillon with automatic winding, the tourbillon was also the smallest ever produced:  a diameter of 7.2 millimeters and height of 2.5. For the first time, a tourbillon cage was made of titanium (by means of computer-controlled electroerosion), and an automatic winding hammer was produced of an extremely dense platinum-iridium alloy. The wheel train was also of a very unusual design, playing a large role in the compactness and flatness of the design. 

Almost as remarkable as the movement itself, is the way in which Audemars constructed and utilized the caliber. The caliber itself is the case. The entire ebauche and case (save the thin front bezel and sapphire) are constructed of a single piece of 18 karat gold. As illustrated left, many of the lower automatic winding and train pivots are visible on the back of the case (these are cap jewels). The result is an automatic tourbillon barely 5.0 millimeters thick. The case measures 29 X 33 millimeters.

The rear crown provides hand setting, but not winding. Initial winding is accomplished by shaking the watch to swing the winding hammer. The very efficient winding provides a power reserve of about 50 hours at full wind.

The 2870 case/ebauche uses an additional main bridge (1, right), barrel bridge (2), and sub-bridge for the train wheels (not visible). All are made of gold.  The tourbillon bridge (3) is made of steel. The main bridge is shown below, inverted.

The extremely novel movement is shown left with the main bridge removed.   (The mainspring barrel is removed in this view, 1). The wheel train sub-bridge (2) and full platinum-iridium winding hammer (3) are now visible.


As illustrated right, the transfer of power from the mainspring barrel (1) to tourbillon carriage (7) is accomplished in a most unusual way. Wheel 2 (right, and below, inverted) is a three-part wheel. The top level is able to rotate independently of the other two. The wheels ride on a ruby column which, itself, rotates on a post on the bottom plate.

The mainspring barrel drives the bottom level of wheel 2. The middle section of wheel 2 drives the bottom level of wheel 3. (The two parts of wheel 3 are also independent of each other.) The bottom level of wheel 3, in turn, drives the pinion of wheel 4, and thence on to the tourbillon carriage (7) via wheels 5 and 6. Meanwhile, the bottom level of wheel 3 also drives the cannon pinion (8), which rides on a post on the plate. (The cannon pinion carries the minute hand.) In a conventional manner, the cannon pinion drives the upper level of wheel 3 (which is a conventional minute wheel), which in turn drives the hour wheel (removed in the illustration) and attached hour hand. Hand setting is accomplished by pulling wheel 9 down into engagement with the top level of wheel 2, which drives the top level of wheel 3 and thus the motion works train in a conventional manner. This unusual train produces 18,000 beats per hour and a 50 second rotation of the tourbillon. Note that the tourbillon carriage, as seen from the dial side, rotates counterclockwise. This is of little consequence in a 50 (as opposed to 60) second tourbillon, which cannot be used as a seconds indicator in any case.


While not quite as unusual as the gear train, the automatic winding train is still quite original.

A steel arm (1) carries the platinum-iridium hammer (2). By means of a pair of pawls, the arm winds wheel (3) in a counterclockwise direction only. The upper section of wheel 3 winds the barrel (6) by means of intermediate wheels (4) and (5). The winding is surprisingly efficient, and allowed Audemars to dispense with a conventional stem and crown (and the thickness it would have added to the movement).

The double pawl winding is similar to a system used in Patek’s early automatics (calibers 12-600 and 27-460), and, to a lesser extent, IWC’s caliber 85 and the recent Chopard L.U.C. 1.96. The pawls are illustrated below.


The hammer stops against a spring (left, arrow) at either end of its travel. The unidirectional winding works with the hammer swinging left (as seen from the dial side). The hammer is visible through a cut out in the dial.






The hammer itself, riveted to a steel arm, is a particularly elegant piece of engineering and design. An extraordinary amount of mass has been provided in very small volume. The triple AP logo makes hammer movement visible. The wheel train and winding systems together utilize 30 jewels.




© 2012 Bourne In Time Inc.