Anchor escapement
From Wikipedia, the free encyclopedia
The anchor escapement is a type of escapement, the mechanism in a clock that maintains the swinging of a pendulum for accurate timekeeping. It was first claimed to be invented by Robert Hooke, although some references credit William Clement, but Joseph Knibb of Oxford has the best evidence in his favour. The anchor escapement appeared first in about 1671. This followed the invention of the pendulum in 1657.
All mechanical clocks use an escapement, to control the motion of the hands around the dial. The rate at which the hands travel around the dial is determined by the periodic time of the swing of the pendulum. 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.
Prior to the introduction of the anchor escapement, most mechanical clocks used the verge-and-foliot system. The main difference is that the teeth of the verge escape wheel project to the side of the wheel whereas the teeth of the anchor project from the rim of the wheel like most gear teeth
The application of the pendulum to clocks, increased accuracy by about ten times over older systems, and from that point on almost every clock mechanism for the next 50 years was based on the anchor with additional modifications to further reduce drag or recoil. One direct modification was the Graham escapement, which eliminated recoil and was called a dead beat escapement. One escapement which is completely different to the anchor was John Harrison's complex grasshopper escapement . However the escapement used in his famous longitude-measuring chronographs was actually the verge. The original chronometer made by Harrison is on display at the Guildhall museum in London and the previous clocks made by him are in the Greenwich observatory and are open to public view. Harrison used his grasshopper escapement in longcase clocks. The verge escapement is more commonly thought of as a turret clock or tower clock escapement. It was used in watches in a rather different form. Instead of a verge and foliot, a watch had a balance, much like a modern balance.
