Urwerk is universally known for its unique approach to time-telling, usually marrying the classic concept of wandering hours with avant-garde styling and reinvented mechanics. The brand’s signature brand of watchmaking is exemplified by the UR-150 “Scorpion”, which demonstrates the brand’s ability to reinvent the wheel in displaying the time.
The UR-150 is a time-only watch, showing just the hours and minutes, but with a technical execution that is sophisticated and elaborate. Nicknamed “Scorpion” due to its crisp retrograde hour hand that brings to mind the desert creature’s swift sting, the UR-150 features a central carousel that creeps slowly over the course of one hour, only for the hour hand to snap back and reset with a lively action at the top of each hour.
The concept of satellite and retrograde hours is not new to Urwerk – in fact the idea dates back to the Opus V – but in the UR-150 the mechanics underlying the time display have been refined, upgraded, and elaborated upon.
Dissecting the time display module of the UR-150 is a fascinating process that speaks to the talent of the watchmakers and constructors at Urwerk who conceived these radical time displays.
The mechanics are inventive, especially in the interconnected nature of the various sub-assemblies ranging from the retrograde to the satellite discs. The construction also incorporates finer engineering details like main shaft that supports two concentric but independent axes of the carrier platform and pointer arm.
The very first Urwerk with a retrograde display, the Opus V of 2005
Telling the time
The UR-150 is intuitive to read: a bright orange pointer frames the current hour while simultaneously pointing to the minutes. All of the numerals are large and legible, with hours on three satellite discs and minutes on a 240° segment, essentially two-thirds of a full circle.
As the hour passes, the carousel sweeps the 240° sector in the same fashion a classic wondering hour complication would (think the early F. P. Journe Vagabondage).
But at the hour’s end, the carousel doesn’t just switch to the next hour and continue with its circular trajectory.
Instead, the pointer snaps back a full 240° to the “zero” minute as the hours disks reshuffle. The entire process repeats every hour, with the swift retrograde action and reshuffling are made possible by a clever mechanism paired with an epicyclic, or planetary, gear set.
The illusion
The entire mechanism relies on a clever optical illusion. For their travel over the 240° minute sector over the course of an hour, the hour discs and pointer appear to be a single unit. This is far from the truth, but the system tricks the wearer into thinking they are affixed to each other.
In reality, the two assemblies, hours and minutes, are powered by the same cannon pinion in the base movement, but in very different ways.
The key sub-assemblies of the UR-150. Image – Urwerk
Retrograde motion
The retrograde hour pointer is driven by a modern take on the classic retrograde works.
A snail cam (yellow) directly engages with the base movement’s cannon pinion in a 1:1 ratio. This means that the snail cam makes one full revolution each hour.
A follower arm (red) is always in contact with the snail cam’s surface and a toothed segment of an intermediate pinion. This pinion links the arm with a toothed rack (also in red), which itself is attached to a spiral spring (blue). The spiral spring doesn’t need to be necessarily pretensioned, as there are tensioner blade springs ensuring the follower arm is always in firm contact with the cam.
Above (left) and below perspectives of the bare retrograde system. Image – Urwerk
The intermediate pinion is directly responsible for the hour pointer hand. Both are co-axial and the pinion is rigidly connected to the hand.
As the snail cam makes one revolution an hour, the radius at the point of contact with the arm’s feeler increases linearly, pivoting the arm against the snail cam. The intermediate pinion transmits the motion to the sprung rack on the opposite side, slowly tensioning the spiral spring.
When the cam reaches its peak radius at the top of the hour, the indirectly-tensioned feeler falls off the “cliff” of the snail cam, and simultaneously the spiral spring uncoils, rapidly driving the rack in reverse. This full cycle of the retrograde mechanism translates into a slow advancement of the intermediate pinion in a clockwise direction over 60 minutes, ending with a split-second reversal of direction that brings it back to its original position.
Correspondingly, the intermediate pinion makes two-thirds of a rotation over one hour, then performing the same rotation in reverse swiftly. And so we have the retrograde motion that sweeps over 240°.
Parts of the time-telling module. Image – Urwerk
This retrograde system differs from classic retrograde works because its components are spread out and the functions separated. Classic retrograde mechanisms usually rely on a directly sprung follower arm, while the UR-150’s system employs a dedicated rack tensioned by a spiral spring.
The reason for this more elaborate approach is likely the need for a stronger tensioning spring that acts more directly on the intermediate pinion. This can be attributed to the large and heavy retrograde hand, in contrast to classic systems where the pinion only carries a slim, conventional hand.
The correct ratio
With the retrograde motion out of the way, we will now focus on the platform carrying the hour discs. The three discs each carry four engraved hours (totalling 12 hour numerals) and arranged in an equilateral triangle around the movement’s centre.
The carrier platform being assembled into the module. Image – Urwerk
As mentioned, the motion of the discs is independent of the retrograde action, as the carrier platform moves in a far slower circular trajectory. The platform is indirectly driven by the cannon pinion, via a gear affixed to and co-axial with the snail cam.
The cleverness of the disc display is getting the transmission ratios correct. The snail cam makes a full rotation every hour, but that translates only two-thirds of a rotation on the intermediate pinion (and by extension the pointer arm). As the hour discs appear to be locked to the pointer, it is reasonable to deduce that any given hour disc also revolves around the centre of the dial at two-thirds of a rotation each hour.
Above (left) and below perspectives of the retrograde system along with the supporting shaft. Image – Urwerk
To complete the illusion, the carrier platform needs to be slowed down. Hence the gear (gold) on the snail cam has fewer teeth than the carrier platform’s gear (dark grey), indirectly putting the cannon pinion at a mechanical advantage relative to the platform.
The platform thus revolves at a rate of two-thirds of a rotation per hour, meaning it takes the carrier one and a half hours to complete a full revolution.
Planetary gears
Moving to the upper side of the platform, a key element here is the synchronisation of the hour discs with the pointer arm. Traditional wandering hour systems usually rely on a Maltese cross or star-shaped wheel and spring assembly in order to accurately index the hour numeral as the time passes.
Above (left), side and below perspectives of the almost completed module. Note the angled satellites and the sun gear. Image – Urwerk
The UR-150 module instead relies on the differential effect of a planetary gear set. On the same axis that joins the intermediate pinion of the retrograde mechanism and pointer arm sits a central “sun” wheel, right above the carrier platform.
This sun wheel is in turn geared to each of the three hour discs and also to a fourth gear that carries an air brake. It is worth mentioning that the hour disc gears have slightly slanted teeth, each tooth inclined at 10° relative to the central sun gear.
Each hour wheel becomes a “satellite” gear and the platform is the “carrier”. A movement of the sun gear turns the hour wheels when the carrier is stationary still, and vice versa with the sun wheel stationary and the carrier mobile.
Connecting the dots
With the constituent subassemblies all in place, everything comes together. A key challenge is ensuring the components are synchronised from the assembly.
When the retrograde mechanism begins turning (when the snail cam is at its smallest radius and the pointer arm indicates “zero” minutes), the orange frame in the pointer arm must frame a given hour on the hour discs in a perfectly indexed position.
Above (left) and below perspectives of the complete module, with only the hour discs missing. Note how the pointer arm fits over the components. Image – Urwerk
As the watch runs, the snail cam advances the pointer, but also causes the carrier platform to turn. As explained, the carrier and pointer revolve at the same rate of two-thirds of a rotation per hour, in the same direction. Their net relative velocity is zero, so both are in fact stationary one relative to each other.
This creates the illusion that the carrier and pointer are rigidly connected, while also keeping the hour discs stationary and correctly indexed, since both the carrier and sun gear are stationary in their reference frame.
The snap and shuffle
At the top of each hour, the the retrograde mechanism snaps the pointer arm back to zero, while the carrier continues rotating at its constant rate of two-thirds of a turn per hour. This is where the differential effect comes into play.
The sun wheel snaps back with the pointer arm and causes the three hour discs and airbrake to rotate along with it. Since the carrier is slow-turning, this happens as if the carrier was stationary and only the sun wheel was mobile.
Completed movement, with the time-telling module and custom winding rotor installed. Image – Urwerk
The large 240° angle travelled by the pointer arm also causes all the satellite discs to spin at a lively and synchronised pace, creating the reshuffling motion. As the pointer reaches the “zero” marker, the next hour numeral is already in position, arranged by the shuffle. The cycle repeats every hour.
The hour numerals engraved on the satellite discs are arranged in a careful sequence, so the correct indexing of the next hour is always assured.
Also, the two-thirds rotation of the sun gear needs to be converted in a multiple of 0.25 for the hour wheels (since they need to index in steps of one quarter of a turn). The ratio between the sun and each of the satellites appears to be 9:8.
Exploded view of UR-150’s module. Image – Urwerk
The addition of the air brake is a fun detail which is not only reminiscent of Urwerk’s “turbine” winding, but also rooted in traditional watch- and clockmaking, echoing Urwerk cofounder Felix Baumgartner’s personal background.
In times past, some large standing clocks or tower clocks employed air brakes in remontoirs or in the escapement. The air resistance generated by the air brake would break the momentum of a component that tended to accelerate too abruptly.
In the UR-150, the air brake is supposed to buffer the retrograde action and shuffle, although the actual utility is debatable.
The inertia generated by the momentum of the four satellite discs, and the pointer arm’s own inertial moment, almost surely result in the pointer rebounding slightly as it travels back to the “zero” marker — something that the tiny air brake is not likely to prevent.
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