Complication · Striking & chiming
Rack Striking
Partially verifiedA striking-control system using a pivoted, toothed rack that falls onto a stepped cam (the snail) mounted on the hour wheel, so the number of teeth the rack is allowed to fall past — and hence the number of blows struck — is read directly from the current hour position, making the system self-correcting.
Definition
A striking-control system using a pivoted, toothed rack that falls onto a stepped cam (the snail) mounted on the hour wheel, so the number of teeth the rack is allowed to fall past — and hence the number of blows struck — is read directly from the current hour position, making the system self-correcting.
Historical origin
Generally attributed to the English clockmaker Edward Barlow (Edward Booth) in the late seventeenth century (commonly dated to around 1676), with a related mechanism also claimed by the Reverend Edward Barlow's contemporaries in a disputed priority episode typical of the period; rack striking progressively displaced the older count-wheel system in higher-grade clocks and watches from the early eighteenth century onward because of its self-correcting advantage.
How it works
A snail — a cam with twelve (or twenty-four) steps of increasing radius, one for each hour — is fixed to rotate with the hour wheel; at each hour, a toothed rack is released and falls until a pin or arm on the rack meets the snail's edge, so the distance the rack falls (and hence the number of teeth engaged for the following count) is determined directly by the current hour, not by a separately maintained sequence of notches.
The striking train then runs, driven by the going or a separate striking barrel, advancing a gathering pallet that picks up the rack's teeth one at a time until the rack is fully returned, producing one strike per tooth gathered; because the fall distance was set directly from the current hour position, the correct count is produced even if the hands have previously been moved without the clock striking through every intervening hour.
This self-correction is rack striking's core advantage over count-wheel striking (see Count-Wheel Striking): whatever hour the hands currently show, the snail step for that hour determines the next strike count directly, so the striking and going functions cannot become permanently desynchronised in the way a count wheel can.
Pocket watch context
The standard mechanism underlying quarter and minute repeating pocket watches from the eighteenth century onward, where its self-correcting property is particularly valuable since a repeater must reliably strike the correct time on demand at any moment, not only at the top of each hour.
Wristwatch context
Remains the standard striking-control principle in modern minute repeater and grande/petite sonnerie wristwatches, miniaturised to fit within a wristwatch movement.
Clock context
Became the standard mechanism in higher-grade English, French, Swiss and German striking clocks (longcase, bracket, mantel and carriage clocks) from the early eighteenth century onward, though cheaper mass-produced clocks continued to use count-wheel striking well into the nineteenth century for cost reasons.
Identification
- A pivoted, toothed rack falling against a stepped snail cam, rather than a notched count wheel and detent
- Moving the hands manually (where the movement allows it) does not desynchronise the subsequent strike count, since the snail step is read fresh from the current hour position each time
- Standard in minute and quarter repeating watch movements from the eighteenth century onward
Servicing notes
Rack-striking geometry (rack hook, gathering pallet, snail profile and warning function) must be precisely restored on reassembly; incorrect rack-hook or gathering-pallet timing is a common cause of a rack-striking clock or repeater striking the wrong count or jamming, and this is specialist work for a trained clockmaker or watchmaker.
General guidance only. Complicated mechanisms should be assessed and serviced by a qualified watchmaker or clockmaker specialist, not adjusted at home.
Related photographs
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 to Edward Barlow is widely repeated in the secondary literature but the exact priority and date are not settled beyond dispute in sources consulted.
Where sources disagree, the disagreement is stated rather than resolved silently.