Pendulum clock
From Free net encyclopedia
A pendulum clock uses a pendulum as its time base. From their invention until about 1930, the most accurate clocks were pendulum clocks. Pendulum clocks cannot operate on vehicles, because the accelerations of the vehicle drive the pendulum, causing inaccuracies. See chronometer for a discussion of the problems of navigational clocks.
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History
The pendulum clock was invented by Christian Huygens in 1656, based on the pendulum introduced by Galileo Galilei.
Pendulum clocks remained the mechanism of choice for accurate timekeeping for centuries, with the Fedchenko observatory clocks produced from after WWII up to around 1960 marking the end of the pendulum era as time standards considered.
Pendulum clocks remain popular for domestic use.
Mechanism
Pendulum clocks have several parts:
The pendulum itself, a mass on the end of a rod. The escapement which passes energy to the pendulum to keep it swinging and also releases the gear train in a step-by-step manner. The gear train which slows the rapid rotation of the escapement down to a suitable speed to match the characteristics of the drive motor. An indicating system which shows how often the escapement has rotated and therefore how much time has passed.
Gravity swing pendulum
The pendulum swings with a designed period which varies with the square of its effective length. To keep time accurately, pendulums are usually made to not vary in length as the temperature changes. John Harrison invented the gridiron pendulum, which used the differential expansion of brass and steel to achieve a zero-expansion pendulum. Later clocks used mercury or a combination of zinc and steel to do the same job. By the end of the nineteenth century, materials were available which had a very low inherent change of length with temperature and these were used to make a simple pendulum rod. These included the nickel/iron alloy Invar and fused silica (quartz). The latter is still used for pendulums in gravity meters. Pendulums are sometimes polished and streamlined to reduce the effects of air drag (which is where most of the driving power goes) on the clock's accuracy. In the late 19th century and early 20th century, pendulums for clocks in astronomical observatories were often operated in a chamber which had been pumped to a low pressure to make the pendulum's operation even more accurate.
Thermal compensation
Owing to the expansion of metal, the length of a simple pendulum will vary with temperature, slowing the clock as the temperature rises. This can be compensated for by using a mechnanism which effectively shortens the moment arm to compensate. Early high precision clocks used the liquid metal mercury to lift a portion of the pendulum mass in compensation for the increased length of the suspension. Other techniques that do not used this toxic metal include using a sliding "banjo" of metals with differing thermal expansion rates (such as steel and brass), to maintain a constant effective length.
Torsion spring pendulum
This pedulum is a wheel-like mass (most often four spheres on cross spokes) suspended from a vertical strip (ribbon) of spring steel. Rotation of the mass winds and undwinds the suspension spring, with the energy impulse aplied to the top of the springs. As the period of a cycle is quite slow compared to the gravity swing pendulum it is possible to make clocks that need to be wound only every thirty days, or even only once a year, the latter sometimes called a "perpetual clock" or "anniversary clock", the latter sometimes given as a wedding memorialisation gift.
Escapement
The escapement drives the pendulum, usually from a gear train. This is the part that ticks. Most escapements have a locking state, and a drive state. In the locking state, nothing moves. The motion of the pendulum switches the escapement to drive, and the escapement then pushes on the pendulum for some part of the pendulum's cycle. A notable but rare exception is Harrison's grasshopper escapement. In precision clocks, the escapement is often driven directly by a small weight or spring which is re-set at frequent intervals by an independent mechanism called a remontoire. This frees the escapement from the effects of variations in the gear train.In the late 19th century, electromechanical escapements were developed. In these, a mechanical switch or a phototube turned an electromagnet on for a brief section of the pendulum's swing. These were used on some of the most precise clocks known. They were usually employed with vacuum pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.
In the 20th century W.H. Shortt invented a free pendulum clock with an accuracy of one hundredth of a second per day. In this system the time-keeping pendulum does no work and is kept swinging by a push from a weighted arm (gravity arm) which is lowered onto the pendulum by another (slave) clock just before it is needed. The gravity arm then pushes on the free pendulum which relesases it to drop out of engagement at a time which is set entirely by the free pendulum. Once the gravity arm is released it trips a mechanism to re-set itself ready for release by the slave clock. The whole cycle is kept synchronised by a small blade spring on the pendulum of the slave clock. The slave clock is set to run slightly slow and the re-set circuit for the gravity arm activates a pivoted arm which just engages with the tip of the blade spring. If the slave clock has lost too much time its blade spring pushes against the arm and this accelerates the pendulum. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became the standard for use in observatories from the mid-1920s until superseded by quartz technology.
Clock Train
The gear train for clocks is unusual in an engineering sense in that it is used to increase rather than decrease speed. A (relatively) large force moving slowly is transmitted through a set of gears to drive the escapement which moves quickly with a small force. In early clocks, the gears were made from iron but it soon became usual to use steel for the small gears (pinions) and brass for the large gears (wheels). This combination of materials runs together smoothly without needing lubrication. The teeth of the gears were originally shaped by eye and cut by file but by the late 18th century they were being cut by machine to a uniform shape and spacing. Some clocks use lantern pinions which do not have teeth as such but the teeth of the mating gear fit in the spaces between rods which run between flanges which straddle the gear. Some modern clocks use gearing which does not have the traditional (cycloidal) tooth shape but use the current involute form. The bearings in most clocks consist of a steel shaft (arbor) fitting in holes in the brass plates of the clock. They are lubricated with oil which needs to be replaced from time to time. A few clocks have used jewels for the faster-turning shafts and some use modern ball-bearings. The drive motor for the train is usually either a weight hanging on a cord wrapped round a barrel or some kind of spring. The usual intervals for winding the clock are daily, every week or every month. Month- and year-going clocks are relatively scarce and usually a torsion spring and wheel as a pendulum, a different mechanism than the gravity swing pedulum.
Time Indication
The indicating system is usually by two hands moving round a circular dial which carries twelve large markers for the hours and sixty markers for the minutes. Many clocks have a small third hand indicating seconds on another dial. The gear train is usually arranged so that one of the arbors turns once in an hour. This is used to drive the minute hand through a slipping clutch which allows the position of the hand to be adjusted by being pushed round the shaft. The hour hand is usually driven not from the main train but from the minute hand through a small set of gears.
See also
External links
it:Orologio a pendolo nl:Slingeruurwerk pl:Zegar wahadłowy ru:Механические часы sl:Ura na nihalo sv:Pendyl