Escapement
The escapement drives the pendulum in a pendulum clock, usually from a gear train. The gear train is powered to provide energy into the pendulum, typically using springs or weights. Without the escapement the system would simply "unwind" immediately, but the escapement stops this motion periodically, controlled by the pendulum. The pendulum moves the escapement back and forth, and makes it change from a "locked" state to "drive" state for a short period that ends when the gear train hits the next lock on the escapement. 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.
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Reliability
Escapements are as reliable as the quality of workmanship allows However a badly worn escapement will cause problems.
Accuracy
Ultimately the accuracy of a clock is dependant on the period of swing of the pendulum. Pendulums are made of metal and expand and contract with heat. Temperature compensation is essential for any clock to keep time accurately. Escapements play a big part in accuracy as well. The precise point in the pendulum's travel at which impulse is supplied, will determine how closely to time the pendulum will swing. Ideally it should be as close to the lowest point in its swing as possible. See Rawlings The Science of Clocks
The crucial element in escapement design is to give maximum energy to the pendulum in order to keep it swinging, and to interfere with the free swinging od the pendulum as much as is possible.
The escapement hass no control over how long the clock runs for, that is determined by the gearing between the spring or weight barrel, and the hour wheel.
Types
Many escapements were designed. The following were the most successful:
Verge escapement
The earliest escapement (from about 1275) is the verge escapement, also known as the crown-wheel-verge escapement or the verge-and-foliot escapement with a small modification. The teeth of the escape wheel, resembling a crown, are interrupted by two small cams, known as the pallets, mounted on a shaft, the verge. The verge is connected to the pendulum (the foliot). When the pendulum swings, the pallets are moved, allowing the escape wheel to advance by one tooth. The motion continues until the pallet moves out of the way of the tooth altogether and the wheel escapes or turns until arrested by the next pallet. John Harrison's first chronometer used the verge escapement and demonstartates that the verge is capable of good timekeeping.
Anchor escapement
In England, the Anchor escapement largely superceded the verge, because the angle through which the pendulum needed to swing was very much reduced. This allowed the use of longer pendulums and saw the introduction of the longcase or grandfather clock. In France however the Verge continued to be used. The teeth of an anchor escape wheel project radially from the edge of the wheel as with any ordinary gear wheel. Above the wheel is the anchor shaped pallets. Rather like the animation at the top of this page but upside down.
Deadbeat escapement
The deadbeat escapement, attributed to George Graham, was an improvement over the anchor escapement. A pendulum continues to swing even after the teeth have locked, and with the verge and the anchor, this reverses the direction of the gear train. The real problem with the recoil is that the pendulum doesn't swing freely. Unless a pendulum swings as freely as possible, it cannot swing isochronously or with exactly equal periods of time.
In Graham's escapement the pallets were curved to coincide with the same radius as the movement of the pallets. So there was no recoil. So the locking face of the pallets provides no impulse. This was the first escapement to separate the locking and impulse actions of the escapement. Se Rawlings. The Science of Clocks
Grasshopper escapement
One of the oddest mechanical escapements known is John Harrison's grasshopper escapement. In this movement, the pendulum pushes a nodding pawl, shaped something like a cricket's head. When the pawl unlocks the driving wheel, the gear train briefly pushes the pendulum, and then the pawl engages and locks at the next place on the driving wheel. Regrettably, the special shape of the driving wheel makes the Grass hopper escapement more difficult to manufacture than cheaper escapements. Grass hopper escapements made by Harrison in the middle of the 18th century are still operating. Most escapements wear far more quickly, and waste far more energy. He used Lignum Vitae for the movement, a wood which is very hard, and is self lubricating. Movement is a noun used almost exclusively in horology to mean the whole works.
Electromechanical escapements
In the late 19th century, electromechanical escapements were developed. In these, a switch or phototube turned an electromagnet on for a brief section of the pendulum's swing. These are amongst some of the best escapements known. They were sometimes 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.
Twin pendulum clock
In the 20th century W.H. Shortt invented a twin pendulum clock with an accuracy of one hundredth of a second per day. In this system the time keeping "master" pendulum rod, which is made from a special alloy whose length does not change 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 every 30 seconds triggers an electromagnet. This electromagnet releases a gravity lever onto the escapement above the master pendulum. A fraction of a second later, 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. Then the gravity lever falls onto a pair of contacts, completing a circuit that does several things: (1) energizes a second electromagnet to raise the gravity lever above the master pendulum to its top position, (2) sends a pulse to activate one or more clock dials, and (3) 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.
This form of clock became a standard for use in observatories. However seasonal variations in the Earth's rotation were discovered by observatory clocks made in the late 17th Century. Thomas Tompion made 2 such clocks. One of which is in the British Museum, and the other in the Greenwich Observatory.
Gear trains
To count the number of ticks of the escapement and move the hands over the dial at the correct rate, gearing is used. In a longcase or granfather clock, a driving weight is wrapped around a barrel, to which is attached the great wheel. This is the first gear in the train of the clock movement. Next is the centre wheel which carries the minute hand and goes round once an hour. There is then the third wheel which drives the escape wheel. The hour wheel is driven directly from the m,otion of the centre arbour. On the centre arbour or shaft is a pinion which drives the hour wheel. Wheels drive pinions ion clocks. Pinions have less than 16 teeth and their teeth are properly called leaves. If a mobile has 16 teeth or more it is called a wheel and its teeth are called teeth!
In the strike train there is a weight driven great wheel with pins projecting from the side to opeate the bell hammer. Then comes a hoop wheel which has a hoop with a slot in it so that a detent on the end of a lever can stop the runnintg of thew train when it has struck the correct number of hours. Next is warning wheel dtiving a fly, or fan. The fly limits the speed of the train to control the speed at which the clock strikes.
Turret clocks and car gear boxes use involute gears, but clocks and watches use cycloidal gears. These may be epicycloidal or hypocloidal. But it is impoortant that there is as smooth a drive from gear to gear, called the velocity ratio, as possible. The oldest clocks had hand-cut gears, but from approx 1600 very basic wheel cutting machines were used
Some pinions were known as lantern pinons. This is because the teeth were made of pieces of hardebned steel wires called trundles, mounted between two collets or flanges. They looked like old lanterns. For reference see W.O.Davies. Gears for Small Mechanisms. This is the definitive reference book on this subject.
See also
External links
- American Watchmakers-Clockmakers Institute
- Federation of the Swiss Watch Industry
- Daniels coaxial escapement (sketch and accuracy review)