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Clock Tower

July 25th, 2010 admin No comments

Clock Tower

Striking clock

Passing strike

The most basic sort of striking clock simply sounds a bell once every hour. This sort of striking clock is called a passing strike clock. It is far simpler to create such a clock; all that must be done is to attach a cam to a shaft that rotates once an hour; the cam raises and then lets fall a hammer that strikes the bell. Originating before the mechanical clock itself, in water clocks, such clocks were the earliest striking clocks; they rang once for each canonical hour, and were used as reminders to summon monks or nuns to their prayers. This sort of striking is still found in some skeleton clocks. It does not require a separate gear train to arm and release the single stroke sounded.

The Tang Dynasty Chinese Buddhist monk and inventor Yi Xing (683727) created a rotating celestial globe that was given motive power by hydraulics of a turning waterwheel (acting as a large escapement), in the tradition of Zhang Heng (78139). 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 Confucian 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 clock towers 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 was constructed by the Arab engineer al-Kaysarani in 1154. The Florentine writer Dante Alighieri made a reference to the gear works of striking clocks in 1319. The most famous original striking clock tower still standing is possibly the one in St Mark's Clocktower in St Mark's Square, Venice. The St Mark's Clock was assembled in 1493, by 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 (h. 1,56 m., diameter m. 1,27), which was put on the top of the tower where it's alternatively beaten by the Due Mori (Two Moors), two bronze statues (h. 2,60) handling a hammer.

Counting the hours

During the great wave of tower clock building in 14th century Europe, around the time of the invention of the mechanical clock itself, striking clocks were built that struck the bell multiple times, to count out the hours. The clock of the Beata Vergine (later San Gottardo) in Milan, built around 1330, was one of the earliest recorded that struck the hours. In 1335, Galvano Fiamma wrote:

There is there a wonderful clock, because there is a very large clapper which strikes a bell 24 times according to the 24 hours of the day and night, and thus at the first hour of the night gives one sound, at the second two strokes, and so distinguishes one hour from another, which is of greatest use to men of every degree.

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", where 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, the hour of 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. Clocks using this type of striking usually represent four o'clock on the dial with an "IV" rather than the more common "IIII".

Countwheel

Countwheel striking: the unequally spaced notches in the external countwheel on the right side of this tower clock movement regulate the number of times the bell is struck.

Two mechanisms have been devised by clockmakers to enable striking clocks to correctly count out the hours. The earlier, what appeared in the first striking clocks in the 14th century, is called countwheel striking. This uses a wheel that contains notches on its side, spaced by unequal, increasing arc segments. This countwheel governs the rotation of the striking train. When the striking train is released by the timekeeping train, a lever is lifted from a notch on the countwheel; the uneven notches allow the striking train to move only far enough to sound the correct number of times, after which the lever falls back into the next notch and stops the striking train from turning further.

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 synch 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.

Rack striking

Rack striking: the snail-shaped cam (N) makes the clock sound the correct number of times by checking the fall of the rack (M).

In the late seventeenth century, rack striking was invented. Rack striking is so called because it is regulated by a rack and snail mechanism. The distance a rack is allowed to fall is determined by a snail-shaped cam, thereby regulating the number of times the bell is allowed to sound. The rack and snail was invented by British clergyman Edward Barlow in 1676.

The snail-shaped cam is a part of the timekeeping train that revolves every twelve hours; often, the snail is attached to the center wheel that drives the hour hand of the clock. Like the countwheel, the cam stops the striking train when the appropriate count has been reached. 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 twelve o'clock position, which allows the rack to move the farthest.

Because it is linked to the timekeeping train, rack striking seldom becomes desynchronized from the hands. 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.

Parts of mechanism

All hour striking mechanisms have these parts (the letters below refer to the diagram at left):

Power source - This is usually identical to the device that powers the clock's timekeeping mechanism: in weight driven clocks it is a second weight on a cord (P), in spring driven clocks it is another mainspring. Although older one-day (30-hour) clocks often used a single weight or mainspring to drive both the timekeeping and striking trains, better clocks used a separate power source, because the striking mechanism consumes a lot of power and often has to be wound more frequently, and also to isolate the delicate timekeeping train from the large movements that occur in the striking train. Winding a striking clock requires winding both the timing and striking parts separately.

Striking train - This is a gear train (G,H) that scales down the force of the power source and transmits it to the hammer mechanism which rings the gong. In antique clocks, to lower the manufacturing cost, it was often exactly the same as the timing train that ran the clock's timekeeping part, and installed parallel to it, on the left side as one faces the clock.

Regulator - A device to prevent the striking train from running too fast, and control the speed of striking. If it wasn't present, the striking train when released would run out of control under the force of the spring or weight. In most clocks it is a simple fan fly (K), a flat piece of sheet metal mounted on the fastest turning gear shaft. When the striking train turns, this beats the air, and the air friction limits the speed of the train. Striking watches and some modern clocks use a centrifugal governor instead.

Count mechanism - This is the critical part mentioned above, that releases the striking train at the proper time and counts out the proper number of strikes. It is the only part of the striking mechanism that is attached to the clock's timekeeping works. Virtually all modern clocks use the rack and snail. The snail (N) is usually mounted on the clock's center wheel shaft, which turns once every 12 hours. There is also a release lever (L) which on the hour releases the rack and allows the timing train to turn.

Hammer and gong - The hammer lever (F) is actuated by pins or teeth (G) on one of the striking train wheels. As the wheel turns the pin lifts the hammer lever, until the lever slips off the pin, allowing the hammer to drop, hitting the gong (E). Early house clocks used traditional hemispherical shaped bells. Later house clocks used gongs made of long steel tubes or bars, which have a sound more like large church bells. Mantel and other small clocks use thick hardened steel wires, which are coiled into a spiral to save space.

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.

How it works

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 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. 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.

Types of striking clocks

Specialized types of striking clocks:

Chiming clock - Strikes on the hours and chimes on the quarter hours, often playing fragments of a tune such as Westminster Quarters.

Repeater - a striking clock which can repeat the strikes at the push of a lever, for telling the time in the dark.

Musical clock - plays tunes on a music box in addition to counting the time

Automaton clock - with mechanically animated figures that periodically perform various displays, usually as a part of the clock striking the hours.

Cuckoo clock - a specific type of automaton clock which originated in Germany, which displays an animated bird and plays imitation birdcalls in addition to striking on a bell or gong.

Ship's bell clock - strikes the ship's bells of a nautical watch instead of the hours.

Some quartz clocks 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.

Notes

^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. p. 197. ISBN 0780800087.

^ Needham, Volume 4, Part 2, 473475.

^ Needham, Volume 4, Part 2, 475.

^ Needham, Volume 4, Part 2, 165.

^ Abdel Aziz al-Jaraki (2007), When Ridhwan al-Sati Anteceded Big Ben by More than Six Centuries, Foundation for Science Technology and Civilisation

^ Needham, Volume 4, Part 2, 445.

^ Boardman, Pete. "Why do clocks show 12 hours?". History. 24 Hour Clocks and Watches. http://homepage.mac.com/pete.boardman/24hourclock/history/index.html. Retrieved April 23, 2008.

^ British Horological Institute, Workshop on Roman Numeral Clock Faces, 1999. See also FAQ: Roman IIII vs. IV on Clock Dials

^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. ISBN 0780800087. p.197

^ "Rack Striking". Encyclopedia of Antiques. Old and Sold Antique Marketplace. http://www.oldandsold.com/articles02/clocks-r.shtml. Retrieved 2008-04-20.

^ Milham, Willis I. (1945). Time and Timekeepers. New York: MacMillan. ISBN 0780800087. p.202-204

References

Eric Bruton, The History of Clocks and Watches (Time Warner, repr. 2002) ISBN 0-316-72426-2

David Landes, Revolution in time: Clocks and the making of the modern world Harvard University Press (1983) ISBN 0-674-76802-7

Needham, Joseph (1986). Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering. Tapei: Caves Books Ltd.