A striking mechanism is the part of a clock movement that sounds the hours, and sometimes the quarters or half hours, on a bell, gong or rods. It usually forms a separate gear train, with its own weight or spring, which is released by the going train at the appropriate moment and controls a hammer that strikes the correct number of blows.
There are two principal systems. The older count wheel, or locking plate, uses a wheel with notches spaced at increasing intervals to regulate the number of blows; it is simple and robust, but it can fall out of step with the hands and must then be corrected by hand. Rack striking, developed in England in the later seventeenth century and closely linked to the repeating work of Edward Barlow, uses a stepped snail on the hour wheel to determine how far a toothed rack falls, so that the strike always matches the time shown and can be repeated at will. Rack striking became standard on most English clocks by the eighteenth century. When assessing a clock, a restorer will check the condition of the pivots and hammer springs, the correct sequence of strikes, and whether parts of the striking work are original.
