Escapement
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Image:Escapement.gif 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 continuously, but the escapement makes this motion periodic, controlled by the pendulum. The pendulum moves the escapement back and forth, and makes it change from a "locked" state to a "drive" state for a short period that ends when the next tooth on the gear hits the locking surface 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. An escapement is also found in a mechanical watch, powering and regulated by a balance wheel and hairspring instead of a pendulum.
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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 dependent 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, the impulse should be evenly distributed on either side of the lowest point of the pendulum's swing. This is because pushing a pendulum when it's moving towards mid-swing makes it gain while pushing it while its moving away from mid-swing makes it lose. If the impulse is evenly distributed then it gives energy to the pendulum without changing the time of its swing. 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 of the pendulum as little as is possible.
The escapement has 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 have been designed and developed over the years. The following are notable:
Verge escapement
The earliest escapement (from about 1275) is the verge escapement, also known as the crown-wheel-and-verge escapement. It pre-dates the pendulum and was originally controlled by a foliot, a horizontal bar with a weight at each end. A vertical shaft (verge) is attached to the middle of the foliot and carries two small plates (pallets) sticking out like flags from a flag pole. One pallet is near the top of the verge and one near the bottom and looking end-on down the verge the pallets are a little over ninety degrees apart. The escape wheel is shaped somewhat like a crown and turns about a horizontal axis. As the wheel tries to turn, one tooth of the wheel pushes against the upper pallet and starts the foliot moving. As the tooth pushes past the upper pallet, the lower pallet swings into the path of the escape wheel. The momentum of the moving foliot pushes the escape wheel backwards but eventually the system comes to rest. It is now the turn of the lower pallet to push the foliot and so on. The system has no natural frequency of oscillation - it is simply force pushing inertia around.
The next stage of development was to use the same idea but attach it to a pendulum. The axis of the verge became horizontal, one half of the foliot disappeared and the crown wheel rotated about a vertical axis. On a much smaller scale the same escapement was used for watches with a balance wheel and spring replacing the pendulum. John Harrison's first chronometer used a heavily-modified verge escapement and demonstrated that the verge could be capable of good timekeeping.
Anchor escapement
In England, the anchor escapement largely superseded 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 escapement continued to be used with its geometry modified to accommodate a smaller arc of operation. The teeth of an anchor escape wheel project radially from the edge of the wheel as with any ordinary gear wheel. Above the wheel are the anchor shaped pallets. Rather like the animation at the top of this page but upside down.
Deadbeat escapement
A clock with a deadbeat escapement was made by Thomas Tompion in 1675 although it was left to Tompion's successor George Graham, to make it widely known. It was an improved version of 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 traditional form of gears in clocks only works well going forwards so the recoil introduces high loads into the system, leading to friction and wear.
In Graham's escapement the pallets are curved about the same axis that they turn on: there is no recoil, so the locking face of the pallets provide no impulse. The impulse is provided by putting an angled plane surface on the end of the pallet so that as the escape wheel is released its tooth pushes along this wedge, impulsing the pendulum. This was the first escapement to separate the locking and impulse actions of the escapement. The escapement was adopted widely for precision and high-quality clocks and led to a number of later escapements which share its lack of recoil See Rawlings. The Science of Clocks
Grasshopper escapement
A rare but interesting mechanical escapement is John Harrison's grasshopper escapement. In this escapement, the pendulum is driven by two hinged arms (pallets). As the pendulum swings, the end of one arm catches on the escape wheel and drives it slightly backwards; this releases the other arm which moves out of the way to allow the escape wheel to pass. When the pendulum swings back again, the other arm catches the wheel, pushes it back and releases the first arm and so on. The grasshopper escapement is more difficult to manufacture than other escapements and is something of a rarity. Grasshopper escapements made by Harrison in the 18th century are still operating. Most escapements wear far more quickly, and waste far more energy. In the bearings of his clocks he used Lignum vitae , a wood which is very hard, and is self lubricating.
Gravity escapement
A gravity escapement uses a small weight or a weak spring to give an impulse directly to the pendulum. The earliest form consisted of two arms which were pivoted very close to the suspension spring of the pendulum with one arm on each side of the pendulum. Each arm carried a small dead beat pallet with an angled plane leading to it. When the pendulum lifted one arm far enough its pallet would release the escape wheel. Almost immediately another tooth on the escape wheel would start to slide up the angle face on the other arm thereby lifting the arm. It would reach the pallet and stop. The other arm meanwhile was still in contact with pendulum and coming down again to a point lower than it had started from. This lowering of the arm provides the impulse to the pendulum. The design was developed steadily from the middle of the 18th century to the middle of the 19th century. It eventually became the escapement of choice for turret clocks and has recently been perfected in the inertially-detached gravity escapement invented by James Arnfield. This frees the pendulum from any part in unlocking the clock train; all it does is lift a gravity arm and then later on part company from it at a lower point.
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. On some clocks the pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.
Hipp clock
In the middle of the 19th century Matthias Hipp invented an ingenious 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 also carried a small vane, 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 then 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.
Free pendulum clock
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 "master" pendulum, whose rod 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 triggers an electromagnet every thirty seconds. 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. 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. 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.
See also
External links
- American Watchmakers-Clockmakers Institute
- Federation of the Swiss Watch Industry
- Daniels coaxial escapement (sketch and accuracy review)
- Watch and Clock Escapements. 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. Published in 1904, from Project Gutenbergde:Uhrenhemmung