Anchor escapement

A more accurate variation without recoil called the deadbeat escapement was invented by Richard Towneley around 1675 and introduced by British clockmaker George Graham around 1715.

On the two arms of the anchor are curved faces which the teeth of the escape wheel push against, called pallets.

The teeth are slanted backward, opposite the direction of rotation, and the surface of the pallets is slightly convex, to prevent this.

One way to determine whether an antique pendulum clock has an anchor or deadbeat escapement is to observe the second hand.

The pendulum rod is hung from a short straight suspension spring attached to a sturdy support directly behind the anchor.

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°.

[8] In addition to the improved accuracy due to isochronism, this allowed clocks to use longer pendulums, which had a slower 'beat'.

The varying force applied to the wheel train by the large exterior hands, exposed to wind, snow, and ice loads, was better handled by gravity escapements.

This is often erroneously credited to English clockmaker George Graham who introduced it around 1715 in his precision regulator clocks.

[11][12][13][14] However it was actually invented around 1675 by astronomer 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 new Greenwich Observatory in 1676,[15] mentioned in correspondence between Astronomer Royal John Flamsteed and Towneley.

[8] When an escape wheel tooth is resting against one of the dead faces, its force is directed through the anchor's pivot axis, so it gives no impulse to the pendulum, allowing it to swing freely.

When the pallet on the other side releases the escape wheel, a tooth lands on this "dead" face first, and remains resting against it for most of the pendulum's outward swing and return.

In contrast to the backward slant of the anchor escape wheel teeth, the deadbeat escape wheel teeth are radial or slant forward to ensure that the tooth makes contact with the "dead" face of the pallet, preventing recoil.

In 1826 British astronomer George Airy proved this; specifically, he proved that a pendulum that is driven by a drive impulse that is symmetrical about its bottom equilibrium position is isochronous for different drive forces, ignoring friction, and that the deadbeat escapement approximately satisfies this condition.

[21] Recent analyses point out that the nonisochronism of the anchor escapement can cancel the circular error of the pendulum.

Due to this effect, a carefully adjusted anchor escapement with polished pallets might be more accurate than a deadbeat.