, used in most pendulum clocks.]]
In mechanical watches and clocks, an escapement is a device which converts continuous rotational motion into an oscillating or back and forth motion. It is the source of the "ticking" sound produced by watches and clocks.
An escapement drives the timekeeping element, usually a pendulum or balance wheel, in a clock or watch. It is powered by a coiled spring or a suspended weight that rotates a gear train. Without the escapement, the system would unwind uncontrollably. The escapement, controlled by the periodic swing of the pendulum or balance wheel, regulates this motion. It allows the gears to advance or 'escape' a fixed amount with each swing, to move the timepiece's hands forward at a steady rate. A second function of the escapement is to keep the pendulum or balance wheel moving by giving it a small push with each swing.
With each swing of the pendulum, one of its arms releases one tooth of a gear, making it change from a "locked" state to a "drive" state for a short period that ends when another tooth on the gear strikes the opposite arm of the pendulum, which locks the gear again. It is this periodic release of energy and rapid stopping that makes a clock "tick;" it is the sound of the gear train suddenly stopping when the escapement locks again.
History
The importance of the escapement in the
history of technology is that it was the key invention that made the all-mechanical
clock possible. Oscillating timekeepers are used in every modern clock.
Liquid-driven escapement
of the 3rd century BC working by the earliest known instance of an escapement mechanism]]
The earliest liquid-driven escapement was described by the
Greek engineer
Philo of Byzantium (3rd century BC) in his technical treatise
Pneumatics (chapter 31) as part of a
washstand. A counterweighted spoon, supplied by a water tank, tips over in a basin when full, releasing a spherical piece of
pumice in the process. Once the spoon has emptied, it is pulled up again by the counterweight, closing the door on the pumice by the tightening string. Remarkably, Philo's comment that "its construction is similar to that of clocks" indicates that such escapement mechanisms were already integrated in ancient water clocks. For example, in Su Song's clock, water flowed into a container on a pivot. The escapement's role was to tip the container over each time it filled up, thus advancing the clock's wheels each time an equal quantity of water was measured out. The development of mechanical clocks, though, depended on the invention of an escapement which would allow a clock's movement to be controlled by an oscillating weight. Unlike the continuous flow of water in the Chinese device, the medieval escapement was characterized by a regular, repeating sequence of discrete actions and the capability of self-reversing action:
Both techniques used escapements, but these have only the name in common. The Chinese one worked intermittently; the European, in discrete but continuous beats. Both systems used gravity as the prime mover, but the action was very different. In the mechanical clock, the falling weight exerted a continuous and even force on the train, which the escapement alternately held back and released at a rhythm constrained by the controller. Ingeniously, the very force that turned the scape wheel then slowed it and pushed it part of the way back . . . In other words, a unidirectional force produced a self-reversing action— about one step back for three steps forward. In the Chinese timekeeper, however, the force exerted varied, the weight in each successive bucket building until sufficient to tip the release and lift the stop that held the wheel in place. This allowed the wheel to turn some ten degrees and bring the next bucket under the stream of water while the stop fell back . . . In the Chinese clock, then unidirectional force produced unidirectional motion.
However the verge was the standard escapement used in every other early clock and watch, and remained the only escapement for 400 years. Its friction and recoil limited its performance, but the accuracy of these 'verge and foliot' clocks was more limited by their early foliot type balance wheels, which because they lacked a balance spring had no natural 'beat', so there was not much incentive to improve the escapement.
The great leap in accuracy resulting from the invention of the pendulum and balance spring around 1657, which made the timekeeping elements in both watches and clocks harmonic oscillators, focused attention on the errors of the escapement, and more accurate escapements soon superseded the verge. The next two centuries, the 'golden age' of mechanical horology, saw the invention of perhaps 300 escapement designs, although only about 10 stood the test of time and were widely used.
In the deadbeat, the pallets have a second curved "locking" face on them, concentric about the pivot the anchor turns on. During the extremities of the pendulum's swing, the escape wheel tooth rests against this locking face, providing no impulse to the pendulum, which prevents recoil. Near the bottom of the pendulum's swing the tooth slides off the locking face onto the angled "impulse" face, giving the pendulum a push, before the pallet releases the tooth. This was the first escapement to separate the locking and impulse actions of the escapement. The deadbeat was first used in precision regulator clocks, but due to greater accuracy superseded the anchor in the 19th century. It is used in almost all modern pendulum clocks.
Detent escapement
detent escapement mechanism.]]
's chronometer detent escapement.]]
The detent or chronometer escapement is a type of detached escapement, and was most commonly used on marine chronometers, although some precision watches during the 19th century also used it. While low-friction escapements existed already, they were too large for small "movements" (as watch-work is referred to).
Electromechanical escapements
In the late 19th century, electromechanical escapements were developed for pendulum clocks. In these, a switch or phototube energised an electromagnet for a brief section of the pendulum's swing. On some clocks the pulse of electricity that drove the pendulum also drove a plunger to move the gear train.
Hipp clock
In the middle of the 19th century Matthias Hipp invented a switch for a clock which was impulsed electro-magnetically. The pendulum drove a ratchet wheel via a pawl on the pendulum rod and the ratchet wheel drove the rest of the clock train to indicate the time. The pendulum was not impulsed on every swing or even at a set interval of time. It was only impulsed when its arc of swing had decayed below a certain level. As well as the counting pawl, the pendulum carried a small vane, known as a Hipp's toggle, pivoted at the top, which was completely free to swing. It was placed so that it dragged across a triangular polished block with a vee-groove in the top of it. When the arc of swing of the pendulum was large enough, the vane crossed the groove and swung free on the other side. If the arc was too small the vane never left the far side of the groove, and when the pendulum swung back it pushed the block strongly downwards. The block carried a contact which completed the circuit to the electromagnet which impulsed the pendulum. The pendulum was only impulsed as it required it.
This type of clock was widely used as a Master clock in large buildings to control numerous slave clocks. Most telephone exchanges used such a clock to control timed events such as were needed to control the set up and charging of telephone calls by issuing pulses of varying durations such as every second, six seconds and so on.
Free pendulum clock
In the 20th century William Hamilton Shortt invented a free pendulum clock, patented in September 1921 and manufactured by the Synchronome Company, with an accuracy of one hundredth of a second a day. In this system the timekeeping "master" pendulum, whose rod is made from a special steel alloy with 36% nickel called Invar whose length changes very little with temperature, swings as free of external influence as possible sealed in a vacuum chamber and does no work. It is in mechanical contact with its escapement for only a fraction of a second every 30 seconds. A secondary "slave" pendulum turns a ratchet, which triggers an electromagnet slightly less than every thirty seconds. This electromagnet releases a gravity lever onto the escapement above the master pendulum. A fraction of a second later (but exactly every 30 seconds), the motion of the master pendulum releases the gravity lever to fall farther. In the process, the gravity lever gives a tiny impulse to the master pendulum, which keeps that pendulum swinging. The gravity lever falls onto a pair of contacts, completing a circuit that does several things:
# energizes a second electromagnet to raise the gravity lever above the master pendulum to its top position,
# sends a pulse to activate one or more clock dials, and
# sends a pulse to a synchronizing mechanism that keeps the slave pendulum in step with the master pendulum.
Since it is the slave pendulum that releases the gravity lever, this synchronization is vital to the functioning of the 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 a blade spring on the pendulum of the slave clock. If the slave clock has lost too much time its blade spring pushes against the arm and this accelerates the clock. 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 a standard for use in observatories (roughly 100 such clocks were manufactured ), and was the first clock capable of detecting small variations in the speed of Earth's rotation.
The synchronizing mechanism used a small spring attached to the shaft of the slave pendulum and an electromagnetic armature that would catch the spring if the slave pendulum was running slightly late, thus shortening the period of the slave pendulum for one swing. The slave pendulum was adjusted to run slightly slow, such that on approximately every other synchronization pulse the spring would be caught by the armature .
See also
Lever escapement
Co-axial escapement
Horology
Su Song
Galileo's escapement
Riefler escapement
Shortt-Synchronome_clock
Master clock
Tourbillon
References
, p. 56-58
Notes
External links
Mark Headrick's horology page, with animated pictures of many escapements
Performance Of The Daniels Coaxial Escapement, Horological Journal, August 2004
Watch and Clock Escapements, The Keystone (magazine), 1904, via Project Gutenberg: "A Complete Study in Theory and Practice of the Lever, Cylinder and Chronometer Escapements, Together with a Brief Account of the Origin and Evolution of the Escapement in Horology."
US Patent number 5140565, issued 23 March 1992, for a cycloidal pendulum similar to that of Huygens
findarticles.com: Obituary of Professor Edward Hall, The Independent (London), 16 August 2001
American Watchmakers-Clockmakers Institute, non-profit trade association
Federation of the Swiss Watch Industry FH, watch industry trade association
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Category:Timekeeping components
Category:Horology
Category:Mechanical power control
Category:Hellenistic engineering
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