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History of Mechanical Clocks with Animations
Figure 2.1. Tower clock in Austria. After the introduction of tower clocks in Italy in the 13th century, the practice of building towers with clocks spread all over Europe. Morguefile.com.
1.3 The Early Mechanical Clocks
Who developed the first mechanical clocks? Historians say that during the renaissance, large mechanical clocks were developed and installed in towers in several large Italian cities, originally to coordinate the prayers of religious orders, but later to give the wider public access to the time for arranging meetings and business transactions. It was also a way for the cities to show off their wealth. There are reports of one of the first of these, a crude tower clock on Sens Cathedral installed in 1176, but perhaps the first tower clock resembling modern clocks with hands was installed about 1276, with more built in the next century.
Essential parts of a clock
The inventors of these first mechanical clocks are unknown, however we must credit them with putting together the essential parts of modern mechanical clocks. Of all the people we will talk about in this chapter, this person (or persons) deserves the lion's share of the credit for getting all the parts of a mechanical clock working together to tell time. The parts of a typical early clock are shown in Figures 2.2 and 2.3. These parts are:
- A power source was provided by weights lifted with ropes and pulleys. Later clocks sometimes used wound up springs instead.
- A resonator was provided to pace the clock. These
early clocks used a "verge-and-foliot" mechanism as shown in Figure
2.2, that functioned as a resonator, although not a very good resonator
by today's standards. Later mechanical clocks used a pendulum or a balance wheel. Most had periods in the range of ½ to 4 cycles per second.
Figure 2.2. Animation showing the parts of a verge and foliot clock. Mouse over over the figure to see the action and click on it to restart it. The animation has been speeded up by x3 to make its action more apparent. The frame which supports the axles of the gears and the verge is not shown in this figure. A different view of the right hand part of this figure is shown in Figure 2.3. The parts on the early tower clocks were often very large. See youtube.com for an example of such a clock circa 1386 at the Salisbury Cathedral in England.
- A gear train is a series of
gears arranged so that their teeth interact (mesh).
In clocks, it is common to use pairs of gears, i.e. sets of two gears (a large one
and a small one) mounted together on
the same shaft such as shown in Figure 2.2.
Turning one gear will cause the rest of the gears in the train to also turn.
The ratio of the number of teeth of two meshing gears will cause a change
in both the speed of turning of one gear compared to the other and in the
torque imparted to it (greater speed means less torque and vice versa).
In Figure 2.2 the gears are arranged to convert the large torque at the pulley to a
much weaker torque with larger motion at the site of the resonator. They are
also designed so that the 3 second cycle time of the verge and foliot will result in
the minute hand making one complete revolution in one hour and the hour hand
making one revolution in twelve hours.
Figure 2.3. Verge and foliot escapement (also shown on the right hand side of Figure 2.2). The drive weight (shown in Figure 2.2) applies a constant torque to the toothed escapement wheel. The foliot bar (with adjustable weights) swings to and fro on a shaft. The shaft, known as the verge, has projections called pallets which get in the way of the toothed escapement wheel's teeth. These pallets are knocked by the toothed wheel. With each knock, the toothed escapement wheel is allowed to turned one half tooth's worth of rotation, until it runs into the other pallet that is now positioned to block the wheel's motion. The toothed escapement wheel is connected to the clock hands through a gear train as shown in Figure 2.2. p ceperley 2010
- An escapement mechanism between the gear train and resonator serves to use the power from the weight and gear train to keep the resonator going. It also regulates the speed at which the gears and clock hands turn by allowing the escape wheel to turn only one tooth each time the resonator goes through a cycle.
Concerning the terminology, the "escapement" and "escapement mechanism", these strictly refer to the device between the gear train and the resonator, that uses the constant force of the gears to power the oscillating resonator and also limits the gears to turn synchronously with the resonator's cycle. However in many clock mechanisms and especially in the case of the verge and foliot, the resonator and escapement are intertwined enough that the whole mechanism including the resonator is often loosely referred to as the "escapement mechanism".
All mechanical escapement mechanisms are similar. In all, a spring or weight driven gear train applies a torque to a toothed escapement wheel. We see the toothed escapement wheel in Figures 2.2 and 2.3 for the case of the verge and foliot escapement. It is driven by the gear train as shown in Figure 2.2. The rest of the parts shown in Figure 2.3 are not directly attached to the driven gear train. They are instead driven by the pallets, the projections sticking out of the verge axle, interacting with the escapement wheel. The system is set up such that the escapement wheel can push one pallet out of the way (by rotating the verge axle and weights), only to have the other pallet be turned back into the wheel's path on the other side of the wheel. The wheel then pushes the second pallet out of the way but then the first pallet is turned into its path, and so on. With each pushing, the escapement wheel is allowed to turn one half tooth's worth. How fast the escapement wheel turns is controlled by this escapement process, i.e. how fast the pallets and the attached foliot and weights can be pushed out of the way given the torque that the gear chain applies. It is this torque, that is created by the drive weight through a set of gears that unfortunately varies considerably with the state of the gears (i.e., their lubrication, their wear, and the temperature, etc.) which causes variation in the speed of the clock.
The basic ideas developed in the verge and foliot escapement were used in practically all the improved mechanical clocks that were to follow, up through the present day. While improved resonators replaced the foliot and weights, future mechanical clocks kept the principle of using two pallets that alternately block the turning of a driven toothed escapement wheel. And while the early clocks provided a starting point, they left much room for improvement. Often they displayed 15 minutes to hours of error each day, were temperamental, and subject to excessive wear and high maintenance.
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