Audemars Piguet "RD#5" - An "ergonomic" chronograph with an unprecedented reset mechanism

This year marks the 150th anniversary of Audemars Piguet's founding. Following the release of their revolutionary perpetual calendar, they released a flying tourbillon chronograph called the "RD#5." At first glance, it appears to be a conventional complicated watch, but its case is only 8.1mm thick. The company's secret to achieving this slimness was a completely new reset system that replaced the conventional reset hammer and heart cam.

The rack-based reset system provided an extremely light touch

　The RD collection is packed with Audemars Piguet's innovative spirit. The fifth piece chosen for the collection is the flying tourbillon chronograph, Audemars Piguet's specialty. It is called the "Royal Oak Jumbo Extra-Thin Flying Tourbillon Chronograph (RD#5)."

　Six months prior to the official announcement, I saw a prototype of the movement that would be used at Audemars Piguet's headquarters. If it hadn't been explained to me by Julio Papi, Director of Watch Conception, no one would have believed it was a flying tourbillon chronograph. Incidentally, Audemars Piguet had mastered the art of crafting a thin flying tourbillon with the 2022 Royal Oak Flying Tourbillon Extra-Thin (RD#3). However, even with the chronograph, this movement was still extremely thin. Papi explained, "The reason for its thinness is that we've omitted the reset hammer and heart cam, which are essential components of mechanical chronographs." It's true that eliminating these mechanisms would make a mechanical chronograph dramatically thinner. But how did they achieve such a feat?

　Mechanical chronographs typically have a horizontal or vertical clutch connecting the gear train to the chronograph gear train. To stop the chronograph, the clutch is released, and to reset, a reset hammer strikes a heart cam, returning the chronograph and counter hands to their zero positions. Regardless of the type of clutch or control, this mechanism has been common to all modern mechanical chronographs since Adolphe Nicole patented it in 1862. While this brought the revolutionary zero-reset mechanism to the chronograph, the large reset hammer across the movement and the heart cams attached to each chronograph axis increased the thickness of the movement. Since 1969, the widespread adoption of compact vertical clutches has allowed mechanical chronographs to have more space for additional mechanisms, but the reset hammer and heart cams have remained an obstacle to making them thinner.

　This does not mean that there have been attempts to make the reset hammer thinner. In the 1980s, Frédéric Piguet created the 1185 series of automatic chronographs, measuring just 3.7mm thick, by molding the reset hammers for the chronograph seconds hand, 30-minute counter, and 12-hour counter into one piece. Most modern chronographs influenced by this design also feature a similar integrated reset hammer, but no attempt has been made to eliminate these components altogether.

　As Giulio Papi explained, Audemars Piguet was able to make the flying tourbillon extremely thin by replacing the reset hammer and heart cam with a retrograde reset mechanism: a toothed rack that returns to zero like a retrograde, resetting the associated pinion and the associated chronograph seconds, 60-minute, and 12-hour counter hands to their original positions.

　Audemars Piguet patented this reset mechanism, and in the patent documents, the company explained the reason for its development as follows:

"The mechanism (with a reset hammer and heart cam) requires delicate adjustments of the spring force, cam shape, and axial position of the tactile element (push button), making it suitable for implementing a coaxial display, but it occupies a relatively large amount of space in the thickness direction. Therefore, an easy alternative approach is desirable that is comfortable to use, robust, reliable, and allows the counter arrangement on the dial to remain the same as before." (Translation by the author)

　The rack that meshes with the gear moves horizontally, and when it reaches a certain point, it is reset to its original position by spring force, and the gear also returns to its zero position. If a hand were attached to that gear, it would be possible to return the hand to its zero position. However, because the gear that drives the chronograph is constantly rotating, it is impossible to reset the meshing rack no matter how you look at it.

　In response, Audemars Piguet has developed a mechanism in which "when the return wheel (rack) reaches its final position, this notch (tooth) allows the indicator wheel (the pinion that turns the chronograph hands, etc.) and the return wheel to be temporarily decoupled (decoupled), and the action of an elastic return member (spring) causes the return wheel to move backward (retrograde) to its initial position. This occurs each time the indicator wheel rotates once" (quote from patent documents).

　In other words, even if the chronograph continues to run, only the rack returns to its original position every 60 seconds, 30 minutes, or 12 hours. When the reset button is pressed, the rack and the indicator wheel (i.e., the pinion that turns the chronograph hands) are connected, and all hands related to the chronograph are returned to their zero positions.

　The benefits of this mechanism extend beyond its slimness. Because the rack itself supports the indicator wheel, a regulating spring is unnecessary, reducing resistance when the chronograph is activated. Furthermore, by eliminating the heavy reset hammer, the thick spring that regulates it, and the heart cam that engages with the hammer, the pushers are significantly lighter to press. "The pusher often has a width of more than 1 mm, requiring approximately 1.5 kg of force. Our goal was to reduce these values ​​and improve the customer experience," explains Giulio Papi. Incidentally, Audemars Piguet chose an indirect reset (used in the former Lemania and ETA 7750) that resets the chronograph using accumulated spring force, rather than a direct reset (used in the AP 4400 series and the former Valjoux 72), in which pressing the pusher directly resets the chronograph. When the reset button is pressed, the reset hammer is released, and the spring force resets it to its original position, returning the heart cam that comes into contact with it to its zero position. The advantage of this mechanism is that you don't need to directly operate the mechanism with your fingers, so you only need to lightly press the button. In addition, the inertia of the rack is much smaller than that of the reset hammer, so the force required to press the button on this watch is even lighter. Although the exact figures have not been revealed, the force required to press the push button is said to be only 1/25 of that of the 4401. In addition, since the chronograph operates with a weaker force, the strain on the mechanism is also reduced.

　Another benefit is the design. Chronographs with large reset hammers have limited design freedom to begin with. However, this model, with its separate reset mechanism, has succeeded in achieving a design that is as close to symmetry as possible.

　By revising the reset mechanism, Audemars Piguet has achieved an unprecedented thinness in its flying tourbillon chronograph. However, what the company aimed for was not simply thinness. Like the previous RD series, this is a truly "usable complicated watch."