Component record · Train
Rack and Snail
Partially verifiedA self-correcting hour-striking mechanism in which a toothed rack falls against a stepped cam (the snail) mounted on the hour wheel to set the correct number of strikes.
What it does
Determines and limits the number of blows struck at each hour, automatically staying correct even if the striking and going trains have previously fallen out of step, since the snail's position is taken directly from the current hour.
How it works
The snail is a cam with twelve unevenly stepped levels, one for each hour, mounted so its rotation tracks the current hour directly from the hour wheel rather than from a separately advancing count wheel.
Just before striking, the rack is released to fall until a pin on it rests against the snail's current step; how far the rack has fallen sets how many teeth of the rack the striking train's gathering pallet must 'gather' (pick up one at a time), and hence how many blows are struck.
Because the snail's position is always derived from the actual current hour position rather than from a separately counted sequence, rack striking automatically corrects itself even after the hands have been reset without separately adjusting the striking train — a key advantage over count-wheel striking.
Evolution
Rack striking progressively displaced count-wheel striking in higher-grade clocks from the late 17th century onward because of its self-correcting property, though the simpler count-wheel system remained common in less expensive clocks.
Known variations
- Simple hour rack-and-snail striking
- Combined with quarter-repeating or quarter-striking racks in more complex clocks and repeating watches
Identification for collectors
- A stepped cam (snail) geared to the hour wheel, with a falling rack visible in the striking train, distinguishes rack striking from count-wheel striking
Common failure modes
- Worn gathering pallet teeth causing miscounts
- Rack not falling fully due to binding, causing an incorrect strike count despite correct hour position
Related photographs
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No confidently matched photograph of this subject has been found yet. These photographs show a closely related record, and are not examples of the type described on this page.
Sources & references
- Britten, F. J., Old Clocks and Watches and Their Makers
- Attribution of rack striking's invention to Edward Barlow versus other contemporaries is debated in the secondary literature.
Where sources disagree, the disagreement is stated rather than resolved silently.