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Written by: Jessica Chappell
Written on: October 1st, 2001
Tags: physics, history & society
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At the time of publication, Jessica Chappell was a junior majoring in Civil Engineering with a Structural emphasis at the University of Southern California. She enjoys swimming and water skiing.
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Volume I Issue IV > The Long Case Clock: Engineering Behind a Grandfather Clock
The first record of man keeping time was approximated to have occurred in 700 B.C. with the use of the sundial. The next true advancement in accurate time keeping was the engineering feat of pendulum clock technology. Long case pendulum clocks have been used since 1657, and remain popular today. The Grandfather Clock was created on the basis of three main scientific principles: potential energy, period of oscillation, and kinematics. These principles are engineered into a long lasting mechanical device, one of beautiful form and practical function.

Introduction

The Long Case clock, more commonly known as the Grandfather Clock, is a common piece of furniture in many American homes. Its aesthetically pleasing facade and pleasant chimes combine with the functionality of the clock itself to form an integral part of a living room or hallway. But the long case clock that has been passed down from generation to generation was built on a few simple principles: potential energy, natural periodicity, and kinematics.

The History of Time

Man's first device to keep time was the sundial, a tool that determined equal divisions of the day by recording the progression of the shadow of a tall object. According to author Ernest Edwardes, the first reference to a sundial is in Isaiah 38:8 and has been dated to approximately 700 B.C.:
Behold, I will bring again the shadow of the degrees, which is gone down in the sundial of Ahaz, ten degrees backward (13). The King James Version of the Bible in The Grandfather Clock
The sundial is an effective guideline for calculating elapsed time but because of the Earth's elliptical orbit around the sun, the time kept by a sundial over the course of a year was less than accurate and left much room for improvement [1].
Although sundials were by far the most common time keeping devices, they were not the only ones used. Both the clepsydr and clepsammia, also known as "water clocks" and "hour glass," respectively, were less common devices [2]. Water clocks relied on the seepage of water through small holes in a vessel. The mechanical devices were extremely expensive to make and therefore only the extremely wealthy acquired them. Still the time was not kept with precision because the period (or interval) between movements of the clock's mechanism was adversely affected by temperature, barometric pressure and the change of pressure within the vessel caused by the decline in the water level [2]. The hourglass kept a precise interval of time if undisturbed and on a perfectly level base, but required much attention to observe the exact moment the last grain of sand fell and to be promptly turned over. This disadvantage kept the hourglass a peripheral method of time keeping.

A Driving Force

The machine in every mechanical clock requires a force to set its moving parts in motion. The primeval water clock utilized potential energy or stored energy in the form of elevated water. The constant gravitational force on the Earth's surface acted upon the stored water which, in turn, drove the gears of the clock. This same principle of stored energy is utilized in clocks today but, instead of water, weights or springs are used.
Potential energy is stored by winding a chain or rope with a weight attached around a drum. The equation for potential energy due to gravity is mass times gravity times the height of the object, or PE = mgh. The drum is attached to a mechanism to provide resistance and keep the drum from instantly unraveling. In order to convert this energy to work for the clock mechanisms the weight must be lowered at a slow, constant rate. A mere friction force would not have the precision necessary to keep time because, much like the water clock, friction is affected by temperature and humidity. This is where the observations of Galileo Galilei come into play.
Author Jo Ellen Barnett recounts a fable about Galileo's observation of a pendulum:
While attending prayers at the Cathedral of Pisa in 1583, the nineteen-year-old Galileo Galilei (1564-1642) got sidetracked by the swinging alter lamp. The longer he looked at it the more it seemed that the swing took the same amount of time whether it make a wide arc or a tiny one. ...By the time prayers were over he had discovered the length of time (period) of a pendulum's swing depends on how long the pendulum is and not on the width of its swing [1]. Barnett, Time's Pendulum
While Barnett does not make mention of Galileo applying his discovery of the pendulum's "natural periodicity" to a time keeping device, Edwardes does state that his son, Vincenzio, produced a timekeeping design of his father's [2]. All sources, however, credit the Dutch Astronomer Christiaan Huygens with creating the first pendulum clock in 1657. This semi-accurate pendulum clock had an error rate of less than one minute per day, but it only provided for constant movement of the clock's hour hand, and could not measure smaller divisions of time. Huygens' design was improved by William Clement in 1670. Still, even before the pendulum had been implemented in clock mechanisms, it was Marin Mersenne who discovered that a 39.1 inch long pendulum would complete its swing in precisely one second [3]. William Clement exlpoited this "Royal Pendulum" length in his design, allowing a minute and second hand to be added to the clock face alongside the hour hand. These pendulum clocks varied by no more than ten seconds per day, far surpassing the technology of sundials and making them obsolete. While the "Royal pendulum" was widely used, longer lengths were also commonly used because the longer period offered even less variation. Some pendulums were up to 65 inches long, giving a beat of 1.25 seconds [3].
The period of a pendulum is altered by fluctuations in temperature, which decrease the precision of the instrument. The equation of the period, T, of oscillating motion is two times pi times the square root of length divided by gravity, or T=2sqrt(L/g) - so the period is proportional to the square root of length. Unfortunately, the length of the pendulum can modulate because the metal bar of the pendulum expands and contracts due to temperature changes. This problem has been combated in many ways, the most effective of which is the "gridiron pendulum." Several alternating rods of brass and steel connect these pendulums, because these metals are relatively unaffected by temperature changes. Furthermore, the small expansions and contractions of the individual metals counteract one another, resulting in minimal change in the length of the pendulum.

Basic Mechanical Parts

There are four basic internal parts that cause each hand on the pendulum clock to work: the escapement, the weight, the gears, and the setting mechanism.

Escapement

In order to convert the regularity of the pendulum to the movement of the clock in the most effective fashion, clockmakers engineered another device called the escapement. The escapement consists of three parts: the escapement gear, the anchor, and the pendulum. The escapement gear has specially designed teeth and determines the rate at which the other gears in the clock turn. The anchor is connected to the pendulum and latches to the escapement gear teeth. The anchor is roughly the shape of an upside-down U. It has two points of contact with the escapement gear; with each swing of the pendulum, one of the gear's teeth is allowed to pass. This provides the constant movement of the second hand of the clock. The minute and hour hands of the clock turn due to the same basic process, but the gears powering the minute and hour hands turn after many more oscillations of the pendulum. Thus, the pendulum becomes the means of a consistent release of the potential energy stored in the weights.

Weight

A weight is attached to a chain that wraps around the gear farthest from the escapement gear; this weight provides potential energy to the pendulum, maintaining its swinging at a constant rate. The unwinding of the chain around the gear and the subsequent sinking of the weight represents potential energy loss of the weight. Therefore, mechanical clocks must periodically be wound to replace the weight's potential energy.

Gears

If the gear connected to the escapement anchor was attached to the drum, every 60 seconds (depending on the length of the pendulum) the drum would complete one full revolution. This would translate to a clock that needs to be wound every 20 minutes and only has a second hand [4]. Gear ratios overcome this problem. For each of the time keeping pieces - hour hand, minute hand, and second hand - a corresponding gear must be turning at an appropriate rate. The ratio for seconds to minutes as well as minutes to hours will be 60:1, while hours to the traditional half-day clocks will be 12:1. Further ratios are necessary to create a desirable winding interval or an interval that is once daily or weekly for ease in maintaining (show series of gears here). Usually, several gears are placed between the weighted gear and the escapement gear. With more gears separating the two ends, the weighted gear moves more slowly, causing the weight to drop and lose energy at a slower rate. Though the ratios of the gears seem easy enough to understand, it becomes difficult to align each gear to access a hand on the face of the clock. This problem is usually solved by utilizing tubular shafts to run the movement of the hands of the clock one within another [4]. However, even with the tubular shafts, arranging the gear works remains a challenging task (Fig. 1).
Skender/SXC
Figure 1: Gears are arranged with appropriate ratios to ensure proper revolution.

Setting Mechanism

The setting mechanism is generally a small lever connected to a gear to pull the gear out of alignment with the others. This stops the clock's movement, allowing it to be wound, and the time to be set.

The Long Case

Contrary to common belief, the Long Case clock was not created to house a long pendulum. Instead, it was a means of allowing the weights used in potential energy storage to fall for a longer distance, which decreases the frequency of winding [1]. Once the Royal Pendulum was invented, it became the perfect housing for both the weight and pendulum.
The Grandfather Clocks have two main parts: trunk and hood. The trunk houses the pendulum and weight. The hood displays the dial, or face, of the clock and encloses the clock mechanisms. Both the trunk and the hood are traditionally highly ornamental. Much of the ornamental nature of the dial depends on the information displayed.

Ancillary Features

The primary function of a Grandfather Clock is to tell time. Therefore, an hour and minute hand are all that is required to accomplish the task. However, Long Case clocks are of an extremely ornamental nature and have a tendency to become long lasting family heirlooms. Accordingly, they are often embellished with other features, and are always created with the utmost craftsmanship. Many case clocks include a second hand, the day of the week, and a moon dial. For each additional dial, additional gears must be fit into the same amount of available space within the hood. A rare inclusion in a clock is the Perpetual Calendar [2]. As the name implies, the clock automatically compensates for the varying lengths of the months and the extra day in February for leap year. The expense, skill, and engineering required to create such an arrangement makes Perpetual Calendar clocks rare and difficult to set.

Putting It All Together

The design and manufacture of a Grandfather Clock require great craftsmanship and the accumulated knowledge of physics and engineering. The principles of energy and kinematics along with the properties of a pendulum create an accurate timepiece which - despite advances in technology - still stands as a symbol of excellence in the world of science and engineering.

References

    • [1] Jo Ellen Barnett. Time's Pendulum. New York: Plenum Trade, 1998.
    • [2] Ernest L. Edwardes. The Grandfather Clock. Altrincham: John Sherrat & Son, 1949.
    • [3] N. Hudson Moore. The Old Clock Book. New York: Frederick A. Stokes Company, 1911.
    • [4] Marshall Brain. "How Pendulum Clocks Work." HowStuffWorks. Internet: http://electronics.h​owstuffworks.com/gad​gets/clocks-watches/​clock.htm [1 Aug. 2001].