Introduction
The fourth wheel is the gear that directly drives the escape wheel, making it one of the most critical components in the upper train of a mechanical clock. If the fourth wheel is incorrect—wrong pinion size, wrong arbor length, or wrong tooth count—the escapement will skip, flutter, lose power, or stop entirely. This guide explains how to choose the correct fourth wheel for your clock movement.
Understanding What the Fourth Wheel Does
Drives the escape wheel
The fourth wheel’s pinion meshes with the escape wheel, powering the escapement.
Maintains train timing
Its tooth count and pinion size determine the upper-train gear ratio.
Supports the second hand (in many clocks)
In many movements, the second hand mounts directly to the fourth wheel arbor.
Key Features of Fourth Wheels
Tooth count
Must match the movement’s designed train ratio.
Pinion size
The pinion must mesh correctly with the third wheel for smooth power transfer.
Arbor length
Determines how the wheel sits between the plates and aligns with the escape wheel.
How to Identify the Correct Fourth Wheel
Match the tooth count
Even a one-tooth difference disrupts timing and escapement performance.
Check the pinion diameter
Incorrect pinion size causes binding, skipping, or power loss.
Verify arbor length and pivot size
The arbor must fit the movement plates and bushings without friction or excess play.
Matching Fourth Wheels to the Movement
Identify the movement manufacturer
Hermle, Urgos, and Kieninger specify exact tooth counts and pinion sizes for each model.
Match the escape wheel
The fourth wheel must drive the escape wheel with the correct spacing and geometry.
Check the third wheel compatibility
The pinion must mesh smoothly with the third wheel to maintain power flow.
Common Fourth Wheel Materials
Brass fourth wheels
Most common and ideal for smooth, low-friction operation.
Steel pinions
Provide durability and long-term wear resistance.
Alloy arbors
Used in modern clocks for improved strength and precision.
Common Mistakes When Choosing Fourth Wheels
Incorrect tooth count
Causes timing errors and escapement instability.
Wrong pinion size
Leads to binding, skipping, or power loss.
Incompatible arbor length
Results in poor alignment with the escape wheel or third wheel.
Recommended Internal Links
- How to Choose the Right Clock Escape Wheels
- How to Choose the Right Clock Escape Wheel Arbors
- How to Choose the Right Clock Verge Assemblies
- How to Choose the Correct Clock Crutch Assemblies
FAQs
How do I know which fourth wheel my clock needs?
Match the tooth count, pinion size, arbor length, and pivot diameter to the original movement.
Why is my escape wheel skipping?
The fourth wheel may have the wrong tooth count or may not align correctly with the escape wheel.
Can I reuse an old fourth wheel?
Only if the pinion teeth and pivots are in excellent condition.
Do fourth wheels from different brands interchange?
Only if the tooth count, pinion size, and arbor dimensions match exactly.
Why does my clock lose power near the escapement?
An incorrect fourth wheel can cause friction, poor mesh, or power loss.
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