Complication · Striking & chiming
Count-Wheel (Locking-Plate) Striking
VerifiedAn early and mechanically simple striking-control system in which a notched wheel (the count wheel or locking plate) determines how many blows are struck at each hour by the spacing of its notches, rather than by counting against the position of the hands.
Definition
An early and mechanically simple striking-control system in which a notched wheel (the count wheel or locking plate) determines how many blows are struck at each hour by the spacing of its notches, rather than by counting against the position of the hands.
Historical origin
The count-wheel (or 'locking plate') system is among the oldest mechanical striking-control methods, used in European clockwork from at least the fourteenth–fifteenth centuries and remaining standard in many clocks (and some watches) until progressively displaced from the late seventeenth century onward by rack striking, which offered the advantage of self-correction if the striking and going trains fell out of step.
How it works
The count wheel carries a series of notches spaced at increasing intervals around its edge, one group of notches for each hour to be struck; when the striking train is released (commonly by a pin on the hour wheel of the going train), a detent rides against the count wheel's edge and the striking train runs until the detent drops into the next notch, stopping the strike.
Because the count wheel is turned by, but not directly synchronised to, the position of the hour hand, if the striking and going trains are ever disturbed independently (for example, by manually moving the hands without allowing the clock to strike through each hour in sequence), the count-wheel strike can fall out of correspondence with the actual time shown and will continue striking the wrong count until manually corrected.
This vulnerability to desynchronisation is the count wheel's principal disadvantage relative to rack striking (see Rack Striking), which reads the actual hour position directly at each strike and therefore self-corrects; count-wheel systems are nonetheless valued by collectors and restorers for their comparative mechanical simplicity and the lighter, quieter action typical of many examples.
Pocket watch context
Found in early striking and quarter-repeating pocket watches predating the general adoption of rack striking in repeating watch work; a count-wheel repeating mechanism is generally regarded as an earlier and technically more basic form than an equivalent rack-striking repeater.
Wristwatch context
Not used in modern wristwatch striking complications, which are built almost exclusively on rack-striking principles; count-wheel striking is essentially a historical form encountered chiefly in antique pocket watches and clocks.
Clock context
Widely used in longcase, bracket, lantern and church tower clocks from the medieval period into the eighteenth and, in less expensive production, the nineteenth century, and still commonly encountered today in antique clock movements undergoing restoration.
Identification
- A visibly notched wheel or plate engaging a detent as part of the striking train, distinct from a rack-and-snail arrangement of a stepped snail cam and a pivoted, toothed rack
- If the hands have been moved without allowing the clock to strike through the full sequence of intervening hours, the strike count will typically no longer match the time shown — a practical diagnostic sign of count-wheel (as opposed to self-correcting rack) striking
- Common in movements predating roughly the early eighteenth century, though continued in cheaper production well after rack striking became available for higher-grade work
Servicing notes
Resetting a count-wheel striking sequence correctly after disassembly requires careful attention to notch alignment with the hour position; a misaligned count wheel is a frequent source of a clock striking the wrong number of blows after inexpert reassembly, and this realignment should be carried out by a trained clockmaker.
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
- de Carle, D., Watch and Clock Encyclopedia
Where sources disagree, the disagreement is stated rather than resolved silently.