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. The anchor escapement was probably invented by British scientist Robert Hooke around 1657, although some references credit clockmaker William Clement. who disputed credit for the invention with him. Joseph Knibb probably built the first working anchor clock at Wadham College, Oxford, around 1670..
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 momentum of the pendulum continues to move the second pallet toward the wheel, pushing the escape wheel backwards for a distance, until the pendulum reverses direction and the 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.
Neither the anchor escapement nor the deadbeat form, below, are self starting. The pendulum must be given a swing to get them going.
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.
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°. 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
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 more accurate deadbeat form of the 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 is still used in a few pendulum clocks today.
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:
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, showing recoil, the clock has an anchor escapement.
The deadbeat form of the anchor escapement was initially used only in precision clocks, but due to its superior accuracy its use spread during the 1800s to most quality pendulum clocks. Almost all pendulum clocks made today use it.
In contrast to the backward slant of the anchor escape wheel teeth, the deadbeat escape wheel teeth usually slant forward to ensure that the tooth makes contact with the locking face of the pallet, preventing recoil.
When the deadbeat was invented, clockmakers initially believed it had inferior isochronism to the anchor, because of the greater effect of changes in force on the pendulum's amplitude.
Patent No. 7,604,395 Issued on Oct. 20, Assigned to ETA SA Manufacture Horlogere Suisse for Anchor Escapement (Swiss Inventors)
Oct 24, 2009; ALEXANDRIA, Va., Oct. 29 -- Andres Cabezas Jurin of Yverdon-les-Bains, Switzerland, and Thierry Conus of Lengnau, Switzerland,...
Ingenium: Five Machines That Changed the World.(Books: A selection of new and notable books of scientific interest)(Brief article)(Book review)
Jul 21, 2007; INGENIUM: Five Machines That Changed the World MARK DENNY The automobile, the computer, and countless other technological...