Verge escapement

The verge escapement is the earliest known type of escapement, the mechanism in a clock that regulates the swinging of a pendulum for accurate timekeeping. Its origin is unknown, but appears to date to about 1275. Verge escapements were used for almost all clocks until Robert Hooke introduced the anchor escapement around 1657, and continued to be built until the 1800s.

The verge escapment, also known as the crown-wheel-verge relies on the angular momentum of a large wheel to slow the motion of a powered crown gear . A vertically-mounted shaft, the verge, is situated just in front of the crown, with two small cams, the palletts, positioned to enter the teeth of the crown on either side. When the clock is started the crown will rotate until it hits one of the palletts, which slows the crown as it is forced to rotate the wheel. After some amount of rotation of the verge the pallett will come out of the tooth and the crown will start to spin again. However the wheel will continue rotating, and soon the second pallett will enter a tooth on the "far side" of the crown. This instantly stops the crown from rotating, and forces it to reverse the direction of the wheel.

The crown requires a fair amount of power in order to reverse direction of the wheel, and verge clocks tend to be very large and mechanically strong. The action itself, when the palletts engauge, is very sudden and powerful. As a result, verge clocks do not "tick, tick", they "bang, bang".

There are a number of problems with the verge mechanism:

• Accuracy is a great problem: the oscillator has no natural or resonant frequency. The period is strongly sensitive to the torque on and defects in the escape wheel.
• The clock had to be kept very flat, or the wheel would not turn properly.
• Another is that the wheels would expand and contract with temperature, so the clocks tended to keep different time in the summer and winter. This was offset by replacing the wheel with the foloit, a long bar with weights that could be moved in and out to counter expansion and contraction. These mechanisms are often referred to as the verge-and-foliot escapement.
• Another problem is that the banging motion when the palletts hit the teeth causes the entire system to "shudder", leading to inaccuracies in timekeeping. The system also generally had very high friction and drag, forces with a random nature that led to additional inaccuracy. These issues are not all that simple to solve, and it was not until Hooke's anchor escapement 400 years later that a real improvement in timekeeping was possible.

See also

• Salisbury Cathedral—oldest known operating clock, with verge and foliot escapement

External links

• Joe Monincx—builds novelty clocks with a variant V&F of his own