400-Day / Anniversary Clocks
Summary:
400-Day clocks—also known as anniversary clocks or torsion clocks—are long-running mechanical clocks designed to operate for up to a full year on a single wind. Their defining feature is the torsion pendulum, a rotating assembly powered by a delicate suspension spring. Typically displayed under a glass dome, these clocks became popular both for their engineering novelty and their elegant, decorative appearance.
Key Identifiers:
Glass Dome or Four-Glass Case
Most anniversary clocks feature:
A removable glass dome covering the movement
Or a four-glass brass case with beveled panels
Exposed movement and pendulum prominently visible
The transparent design highlights the torsion pendulum’s rotation.
Torsion Pendulum Assembly
The most recognizable feature:
A rotating pendulum with four brass balls
Suspended by a thin torsion spring (suspension spring)
Rotates slowly back and forth rather than swinging
The extreme sensitivity of this spring is central to the clock’s long runtime—and fragility.
400-Day Mechanical Movement
Most models include:
An 8-, 30-, 240-, or 400-day escapement, depending on design
Time-only operation (no strike or chime)
A platform-style anchor escapement interacting with the torsion spring
German makers such as Kundo, Schatz, Kern, and Gustav Becker were leading producers.
Adjustable Beat Scale & Locking Bar
These clocks often include:
A beat adjustment scale atop the movement
A pendulum locking bar at the base to secure the pendulum for transport
The locking bar is crucial anytime the clock is moved.
Operational Guide:
Placement & Leveling
Anniversary clocks are extremely sensitive to placement.
Position on a stable, vibration-free surface
Level the clock in all directions
Avoid drafts, sunlight, or heavy foot traffic
Even slight tilting or vibration can stop the torsion pendulum.
Step 1:
Unlocking and Installing the Pendulum
Before operation:
Slide or release the pendulum locking bar
Carefully attach the pendulum to the lower suspension fork if not already installed
Ensure the suspension spring is vertical and not twisted
Never twist the pendulum during installation—this can break the upper spring.
Step 2:
Starting the Pendulum
Gently rotate the pendulum ¼–½ turn to one side
Release smoothly
The pendulum should rotate back and forth with wide, slow turns (typically 270–360 degrees total movement)
If rotation is shallow or stopping prematurely, check for level and beat alignment.
Step 3:
Winding the Clock
400-day clocks have a single mainspring:
Insert the key into the rear arbor
Wind slowly until resistance increases
Do not force or over-wind
Most models will run 6–12 months on a full wind.
Step 4:
Setting the Time
Turn the minute hand forward only
Avoid moving hands backward
Never touch the hour hand directly
Setting the time too quickly can disturb the torsion pendulum.
Step 5:
Regulating the Speed
Regulation on a torsion clock is more sensitive than on pendulum clocks.
Pendulum Adjustment
On the pendulum disc or ring:
Turn adjustment nut clockwise → Speeds up the clock
Turn counterclockwise → Slows the clock
Changes must be very small, followed by:
24–48 hours before re-evaluating
These clocks respond slowly due to their long oscillation period.
Beat Adjustment
If the clock ticks unevenly:
Adjust the upper beat scale lever slightly
Ensure the fork and anchor are centered and balanced
Incorrect beat is a common cause of poor running.
Step 6:
Routine Maintenance
Case & Glass Care
Clean the dome or glass gently with ammonia-free cleaners
Remove dome carefully; do not bump the pendulum
Keep brass polished using non-abrasive products
Movement Care
Service every 8–15 years, depending on conditions
Torsion clocks require specialized knowledge—avoid DIY disassembly
Keep the suspension spring protected from drafts and accidental bumping
Handling & Transport
Always lock the pendulum before moving
Remove glass dome first
Transport upright and cushioned
Neve carry by the dome
A broken suspension spring is the most common damage during transport.