Anchor escapement
In clockmaking, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and advances the clock's wheels at each swing. It was probably invented by British scientist Robert Hooke, although some references credit clockmaker William Clement. Joseph Knibb probably built the first working anchor clock at Wadham College, Oxford, around 1670.[1].
How it works
All mechanical clocks use an escapement, to advance the clock's wheel train with each 'tick'. The rate at which the hands travel around
The anchor escapement consists of two parts; an escape wheel, which is a vertical wheel with teeth on it rather like saw teeth, and the anchor, shaped vaguely like a ship's anchor, which swings back and forth on a pivot just above the escape wheel. On the two arms of the anchor are angled flat faces which the teeth of the escape wheel push against, called pallets. The central shaft of the anchor is attached to the pendulum, so the anchor swings back and forth, with
The reason the escape wheel teeth are slanted forward, in the direction the wheel turns, is as a safety measure. If the clock The slanted teeth ensure that the flat faces of the anchor pallets hit the
To keep the pendulum swinging it needs to be given an impulse. This is done by the escapement. There are two essential components to the escapement. One is the escape wheel, which has teeth rather like those of a saw. The other is the pallets. In the anchor escapement they are shaped like a ships anchor and at the extremities have a triangular hook. Each hook, or pallet as they are called, is moved in between the escape wheel teeth alternately, by the swinging of the pendulum. Each time one of the pallets moves away from the tooth, the escape wheel moves a little until another tooth lands on the other pallet. One of the disadvantages of the anchor escapement is that like the verge, it causes what is known as recoil. This is the pushing backwards of the escape wheel during the supplementary arc of the pendulum. The description, anchor recoil, is found in all text books. Recoil is the reversal of the entire train of wheel or gears in the clock all the way back to the driving weight. This causes excessive wear on the teeth and leaves of the pinions that make up the gear train. See Rawlings, The Science of Clocks and Watches.
History
The anchor was the second escapement in Europe, replacing the venerable 400 year old verge escapement in pendulum clocks. In the ferment of innovation following his application of the pendulum to clocks in 1657, Christiaan Huygens realized that the wide pendulum swings in the verge caused the period of oscillation of the pendulum to vary with changes in drive force. The chief advantage of the anchor was that by locating the pallets farther from the pivot, the swing of the pendulum was reduced from around 100° in verge clocks to only 4°-6°. [2] This allowed clocks to use longer pendulums, which had a slower 'beat'. In addition to the improved accuracy due to isochronism, these needed less power to keep swinging, and caused less friction and wear on the clock's movement. The anchor also allowed the use of a heavier pendulum bob for a given drive force, making the pendulum more independent of the escapement (higher Q), and thus more accurate. These long pendulums required long narrow clock cases, giving birth to the longcase or 'grandfather' style clock. The anchor increased the accuracy of clocks so much that around 1680-1690 the use of the minute hand, formerly the exception in clocks, became the rule[3]
The anchor escapement is reliable and tolerant of large geometrical errors in its construction, but it has some of the same disadvantages as the verge:
- Like the verge it is a frictional escapement; the pendulum is always being pushed by an escape wheel tooth throughout its cycle, and never allowed to swing freely. This disturbs the motion of the pendulum harmonic oscillator, causing a lack of isochronism.
- Like the verge it is a recoil escapement; some of the force used to reverse the direction of the pendulum comes from pushing the escape wheel backward during part of the cycle. This caused extra wear to the movement and disturbed the motion of the pendulum, causing inaccuracy.
One way to determine whether a pendulum clock has an anchor or deadbeat escapement is to observe the second hand. If it moves backward slightly after every tick, the clock has an anchor escapement.
Deadbeat escapement
The above problems were remedied by a modification to the pallets by George Graham around 1715, resulting in a much better escapement: the Graham or deadbeat escapement. He cut a second face on the pallets, called the 'locking' face, with a curved surface concentric with the pivot that the anchor turns on. When an escape wheel tooth is resting against these faces, it's force is directed through the pivot axis, so it gives no impulse to the pendulum, allowing it to swing freely. During the extremities of the pendulum's excursion, the tooth is in this locked position. Near the bottom of the pendulum's swing the tooth slides off the locking face onto the slanted impulse face of the pallet, allowing the escape wheel to turn and give the pendulum a push, before dropping off the pallet. The drag of the escape tooth on the locking face does add a small amount of friction to the pendulum's swing (this is called a frictional rest type escapement), but it is usually negligible.
In 1826 George Airy proved that a pendulum in which the drive impulse is symmetrical about its equilibrium point is isochronous for different drive forces, ignoring friction, and that the deadbeat escapement approximately satisfies this condition.[4] It would be exactly satisfied if the escape wheel teeth were made to fall exactly on the corner between the two pallet faces, but to make the escapement operate reliably the teeth are usually made to fall above the corner, on the impulse face.[5]
Initially clockmakers believed that the anchor was more isochronous than the deadbeat, and so more accurate. They got this impression because an increase of force on the escape wheel will increase the pendulum amplitude of the deadbeat more than the anchor. What happens is that the force of the escape tooth during the recoil part of the anchor's cycle tends to decrease the pendulum's swing, while the force during impulse part tends to increase it. These often cancel out, so that the pendulum's swing is often unchanged with changes in force. However when measured by the period, the force in both the recoil and impulse section of the anchor's cycle decreases the time of swing. So an increase in drive force in the anchor knocks the pendulum back and forth in a fixed arc faster, whereas
References
- ^ Chapman, Allen (2005). England's Leonardo: Robert Hooke and the Seventeenth Century Scientific Revolution. CRC Press. p. 84. ISBN 0750309873.
- ^ Headrick, Michael (2002). "Origin and Evolution of the Anchor Clock Escapement". Control Systems magazine,. 22 (2). Inst. of Electrical and Electronic Engineers. Retrieved June 6, 2007.
{{cite journal}}: CS1 maint: extra punctuation (link) - ^ Milham 1945, p.146
- ^ Airy, George Biddle (November 26, 1826). "On the Disturbances of Pendulums and Balances and on the Theory of Escapements". Trans. of the Cambridge Philosophical Society. University Press: 105. Retrieved April 25, 2008.
{{cite journal}}: Unknown parameter|vol=ignored (|volume=suggested) (help) - ^ Beckett, Edmund (Lord Grimsthorpe) (1867). A Rudimentary Treatise on Clocks and Watches and Bells, 6th Ed. London: Lockwood & Co.