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A striking clock is a clock that sounds the audibly on a bell, gong, or other audible device. In 12-hour striking, used most commonly in striking clocks today, the clock strikes once at 1:00 am, twice at 2:00 am, continuing in this way up to twelve times at 12:00 mid-day, then starts again, striking once at 1:00 pm, twice at 2:00 pm, and the pattern continues up to twelve times at 12:00 midnight.
The striking feature of clocks was originally more important than their ; the earliest clocks struck the hours, but had no dials to enable the time to be read. The development of mechanical clocks in 12th century Europe was motivated by the need to ring bells upon the canonical hours to call the community to prayer. The earliest known mechanical clocks were large striking clocks installed in towers in monasteries or public squares, so that their bells could be heard far away. Though an early striking clock in Syria was a 12-hour clock, many early clocks struck up to 24 strokes, particularly in Italy, where the 24-hour clock, keeping Italian , was widely used in the 14th and 15th centuries. As the modern 12-hour clock became more widespread, particularly in Great Britain and Northern Europe, 12-hour striking became more widespread and eventually became the standard. In addition to striking on the hour, many striking clocks play clock chimes on the quarter-hours. The most common sequence is Westminster Quarters.
Today the time-disseminating function of clock striking is almost no longer needed, and striking clocks are kept for historical, traditional, and aesthetic reasons. Historic in towns, universities, and religious institutions worldwide still strike the hours, famous examples being Big Ben in London, the Peace Tower in Ottawa, and the Kremlin Clock in Moscow. Home striking clocks, such as , , grandfather clocks and are also very common.
A typical striking clock will have two , because a striking clock must add a striking train that operates the mechanism that rings the bell in addition to the timekeeping train that measures the passage of time.
Before European clocks, China developed a water-driven astronomical clockwork technology, starting with the first century AD scientist Zhang Heng (78–139). The Tang dynasty China Chinese Buddhism and inventor Yi Xing (683–727) created a rotating celestial globe that was turned by a water clock mechanism driven by a waterwheel. This featured two wooden gear jacks on its horizon surface with a drum and a bell, the bell being struck automatically every hour and the drum being struck automatically every quarter-hour. It is recorded that Confucianism students in the year 730 were required to write an essay on this device in order to pass the Imperial examinations. The use of clock jacks to sound the hours were used in later of Song dynasty China, such as those designed by Zhang Sixun and Su Song in the 10th and 11th centuries, respectively.
A striking clock outside of China was the clock tower near the Umayyad Mosque in Damascus, Syria, which struck once every hour. It is the subject of a book, On the Construction of Clocks and their Use (1203), by Riḍwān ibn al-Sāʿātī, the son of a clockmaker. The Florence writer Dante Alighieri made a reference to the gear works of striking clocks in 1319. One of the older clock towers still standing is St Mark's Clocktower in St Mark's Square, Venice. The St Mark's Clock was commissioned in 1493, from the famous clockmaker Gian Carlo Rainieri from Reggio Emilia, where his father Gian Paolo Rainieri had already constructed another famous device in 1481. In 1497, Simone Campanato moulded the great bell, which was put on the top of the tower where it is alternately beaten by the Due Mori ( Two Moors), two bronze statues handling a hammer.
The astronomical clock designed by Richard of Wallingford in 1327 and built around 1354, also struck 24 hours.
Some rare clocks use a form of striking known as "Roman Striking" invented by Joseph Knibb, in which a large bell or lower tone is sounded to represent "five", and a small bell or high tone is sounded to represent "one". For example, four o'clock would be sounded as a high tone followed by a low tone, whereas the hour of eleven o'clock would be sounded by two low tones followed by a high tone. The purpose is to conserve the power of the striking train. For example, "VII" would be a total of three strikes instead of seven, and "XII" would be four strikes instead of twelve. Clocks using this type of striking usually represent four o'clock on the dial with an "IV" rather than the more common "IIII",British Horological Institute, Workshop on Roman Numeral Clock Faces, 1999 FAQ: Roman IIII vs. IV on Clock Dials so that the Roman numerals correspond with the sequence of strikes on the high and low bells. One small table clock of this type sold from the George Daniels collection at Sotheby's on 6 November 2012 for £1,273,250.
The countwheel has the disadvantage of being entirely independent of the timekeeping train; if the striking train winds down, or for some other reason the clock fails to strike, the countwheel will become out of sync with the time shown by the hands, and must be resynchronized by manually releasing the striking train until it moves around to the correct position.
The snail-shaped cam is a part of the timekeeping train that revolves every twelve hours; often the snail is attached to the same pipe on which the hour hand is mounted. The diameter of the cam is largest at the one o'clock position, permitting the rack to move only a short distance, after which the striking train is stopped; it is smallest at the 12 o'clock position, which allows the rack to move the farthest. Striking stops when the last tooth of the rack has been taken up by the gathering pallet.
Because the number of strikes on the hour is determined by the position of the snail which rotates in tandem with the hour hand, rack striking seldom becomes desynchronized. Rack striking also made possible the repeating clock, which can be made to repeat the last hour struck by pressing a button. Rack striking became the standard mechanism used in striking clocks down to the present.
Clocks that have more elaborate functions than just striking the hours, such as chiming the quarter hours, or playing tunes, are called "chiming clocks" by clockmakers. The additional functions are usually run by a second complete striking mechanism separate from the (hour) striking train, called the "chiming train". These clocks have three weights or mainsprings, to power the timing train, striking train, and chiming train.
The release lever (L) holds the rack (M) up when the clock is not striking. On the shaft of the minute hand (not shown), which rotates once per hour, there is a projection. As the change of the hour approaches, this projection slowly lifts the release lever, allowing the rack to fall until its point rests on the snail (N). The amount the rack can fall, and thus the number of strikes, is determined by the position of the snail. Exactly on the hour the striking train (G, H, K) is released and begins to turn. As it turns, the pins (G) repeatedly lift the hammer (F) and allow it to drop, ringing the gong (E). The gear ratios are arranged so that the gear wheel (H) makes one revolution each strike. A small pin (S) on this wheel engages the rack teeth, lifting the rack up by one tooth each turn. When the rack reaches the end of its teeth it stops the striking train from turning (using a mechanism not shown in the diagram, in such a way that gear (H) is held stationary with the pin (S) not engaging the rack, so that the rack is able to fall freely again on the next hour). So the number of strikes is equal to the number of teeth of the rack which are used, which depends on the position of the snail.
Some also contain speakers and sound chips that electronically imitate the sounds of a chiming or striking clock. Other quartz striking clocks use electrical power to strike bells or gongs.
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