Anchor escapement

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Anchor escapement.
Anchor escapement.

In horology, 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[1][2] around 1657,[3] although some references credit clockmaker William Clement. Joseph Knibb probably built the first working anchor clock at Wadham College, Oxford, around 1670.[4].

Contents

[edit] How it works

The anchor escapement consists of two parts; the 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 pallets alternately catching and releasing an escape wheel tooth on each side.

Each time one pallet moves away from the escape wheel, releasing a tooth, the wheel turns and a tooth on the other side lands on the other pallet, which is moving toward the wheel. The swing of the pendulum pushes the escape wheel backwards for a distance until the pendulum reverses direction and the second pallet begins to move away from the wheel, with the tooth sliding along its surface, pushing it. Then the pallet releases the tooth, beginning the cycle again.

The backward motion of the escape wheel during part of the cycle, called recoil, is one of the disadvantages of the anchor escapement. It results in a reversal of the entire wheel train back to the driving weight, causing excessive wear to the gear teeth, backlash, and inaccuracy. It can also cause the points of the escape wheel teeth to dig into the pallet surface. The teeth are slanted backward, opposite the direction of rotation, and the surface of the pallets is slightly convex, to prevent this.[5]

Another reason the escape wheel teeth are slanted backward is as a safety measure. If the clock is moved without immobilising the pendulum, the anchor pallets can collide violently with the escape wheel. The slanted teeth ensure that the flat faces of the anchor pallets hit the sides of the teeth first, protecting the delicate points from being broken.[5]

The shaft of the anchor, called the crutch ends in a fork which embraces the shaft of the pendulum, giving it transverse impulses. The pendulum rod is hung from a short straight suspension spring attached to a sturdy support directly behind the anchor. The pivot of the anchor is aligned with the bending point of the spring. This arrangement is more stable than suspending the pendulum directly from the anchor.

The anchor is very tolerant of variations in its geometry, so the geometry varied widely. In late 1800s Britain the usual design[5] was a 90° angle between the pallets, which meant locating the anchor pivot a distance of √2 ≈ 1.4 times the escape wheel diameter from the escape wheel pivot. In a minute beating longcase clock, the escape wheel often had 30 teeth, so the pallets encompassed about 7 - 8 teeth. The impulse angle of the pallets, which equaled the swing of the pendulum, was 3°-4°.

[edit] History

The anchor was the second widely used escapement in Europe, replacing the venerable 400 year old verge escapement in pendulum clocks. In 1673, 17 years after he invented the pendulum clock, Christiaan Huygens published his mathematical analysis of pendulums, Horologium Oscillatorium, including his discovery that the wide pendulum swings of verge clocks caused the period of oscillation of the pendulum to vary with changes in drive force. The widespread realization that only small pendulum swings were isochronous motivated a search for an escapement that could deliver small swings.

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°.[3] This allowed clocks to use longer pendulums, which had a slower 'beat'. In addition to the improved accuracy due to isochronism, lower air drag meant they needed less power to keep swinging, and caused less 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[6]

The anchor escapement replaced the verge in pendulum clocks within about 50 years, although French clockmakers continued to use verges until about 1800. Many verge clocks were rebuilt with anchors. In the 18th century the deadbeat escapement replaced the anchor in precision regulators, but the anchor remained the workhorse in home pendulum clocks. During the 19th century the deadbeat form gradually took over in most quality clocks, but the anchor form was still used in some clocks into the 20th century.

[edit] Disadvantages

The anchor escapement is reliable and tolerant of large geometrical errors in its construction, but in operation it is simply a rearrangement of the old verge escapement, and retains two of the major disadvantages of 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.
  • It is a recoil escapement as mentioned above; 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 causes extra wear to the movement and disturbs the motion of the pendulum, causing inaccuracy.

One way to determine whether a pendulum clock has an anchor escapement is to observe the second hand. If it moves backward slightly after every tick, showing recoil, the clock has an anchor escapement.

[edit] Deadbeat escapement

Deadbeat escapement, showing: escape wheel (a), pallets showing concentric locking faces (b), pendulum rod (c).
Deadbeat escapement, showing: escape wheel (a), pallets showing concentric locking faces (b), pendulum rod (c).

The above two problems were remedied by a modification to the pallets, resulting in a much better variation of the anchor escapement: the Graham or deadbeat escapement. This is usually credited to English clockmaker George Graham who introduced it around 1715 in his precision regulator clocks.[7][8][9][10] However it was actually invented around 1675 by Richard Towneley, and first used by Graham's mentor Thomas Tompion in a clock built for Sir Jonas Moore, and in the two precision regulators he made for the Greenwich Observatory in 1676,[11] mentioned in correspondence between Astronomer Royal John Flamsteed and Towneley[12][13]

The deadbeat has 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 one of 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 swing, 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 position is isochronous for different drive forces, ignoring friction, and that the deadbeat escapement approximately satisfies this condition.[14] It would be exactly satisfied if the escape wheel teeth were made to fall exactly on the corner between the two pallet faces, but for the escapement to operate reliably the teeth must be made to fall above the corner, on the locking face.[15]

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 an increased drive force in the deadbeat causes a wider swing but no change in the period.

The deadbeat escapement was initially used in precision regulator clocks, but gradually became more popular. Virtually all pendulum clocks made today use it.

[edit] Footnotes

  1. ^ Milham, Willis I. (1923). Time and Timekeepers. MacMillan.  p.146
  2. ^ Glasgow, David (1885). Watch and Clock Making. London: Cassel & Co., p.293. 
  3. ^ a b Headrick, Michael (2002). "Origin and Evolution of the Anchor Clock Escapement". Control Systems magazine, 22 (2). Inst. of Electrical and Electronic Engineers. 
  4. ^ Chapman, Allen (2005). England's Leonardo: Robert Hooke and the Seventeenth Century Scientific Revolution. CRC Press, 84. ISBN 0750309873. 
  5. ^ a b c Britten, Frederick J. (1896). The Watch and Clock Maker's Handbook, 9th Ed.. London: E.F. & N. Spon, p.8-11. 
  6. ^ Milham 1945, p.146
  7. ^ Milham 1945, p.185
  8. ^ Glasgow 1885, p.297
  9. ^ "Clocks". Encyclopaedia Britannica, 11th Ed. 6. (1910). The Encyclopaedia Britannica Co.. p.541. 
  10. ^ Deadbeat escapement. Encyclopedia of Clocks and Watches. Old and Sold Antiques Marketplace. Retrieved on 2008-06-08.
  11. ^ Betts, Jonathan Regulators in Bud, Robert; Warner, Debra Jean (1998). Instruments of Science: An Historical Encyclopedia. Taylor & Francis. ISBN 0815315619.  p.121
  12. ^ Flamsteed, John; Forbes, Eric; Murdin, Lesley (1995). The Correspondence of John Flamsteed, First Astronomer Royal, Vol.1. CRC Press.  Letter 229 Flamsteed to Towneley (September 22, 1675), p.374, and Annotation 11 p.375
  13. ^ Andrewes, W.J.H. Clocks and Watches: The leap to precision in Macey, Samuel (1994). Encyclopedia of Time. Taylor & Francis. ISBN 0815306156.  p.126, this cites a letter of December 11, but he may have meant the September 22 letter mentioned above.
  14. ^ 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: 105. University Press. 
  15. ^ Beckett, Edmund (Lord Grimsthorpe) (1867). A Rudimentary Treatise on Clocks and Watches and Bells, 6th Ed.. London: Lockwood & Co.. 


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