Escapement

The escapement drives the pendulum in a pendulum clock, usually from a gear train. The gear train is powered to provide energy into the pendulum, typically using springs or weights. Without the escapement the system would simply "unwind" immediately, but the escapement stops this motion periodically, controlled by the pendulum. The pendulum moves the escapement back and forth, and makes it change from a "locked" state to "drive" state for a short period that ends when the gear train hits the next lock on the escapement. It is this periodic release of energy and rapid stopping that makes a clock "tick", it is the sound of the gear train suddenly stopping when the escapement locks again.

Reliability

Escapements have two general reliability problems. Either they "set", i.e. jam and stop moving, or they skip. Skipping one cycle in a thousand is enough to make a clock seriously inaccurate.

The escapement is the part of a clock most prone to wear, because it moves the fastest. The efficiency of an escapement's design, that is, how much energy is converted into pendular motion, directly affects how long a pendulum clock can operate between windings.

Types

Many escapements were designed. The following were the most successful:

Verge escapement

The earliest escapement (from about 1275) is the verge escapement, also known as the crown-wheel-verge escapement or the verge-and-foliot escapement with a small modification. The projections of a wheel resembling a crown push on two small cams, known as the palletts, mounted in a shaft, the verge. The verge is geared to a large spinning wheel, or in later designs, a horizontally spinning pendulum (the foliot). When the crown gear turns, the pallettes are pushed, spinning the wheel. The motion continues until the other pallett hits the teeth on the far side of the crown, suddenly stopping the motion, and forcing the wheel to reverse direction. A large amount of force is required to make the system stop and reverse, forcing these clocks to be fairly large and mechanically strong in order to withstand the forces when the palletts hit the crown teeth -- verge clocks did not go "tick, tick", they went "bang, bang".

Anchor escapement

A major advance in timekeeping was introduced by Robert Hooke, who designed the anchor escapement, also known as the "Anchor-Hooke" or recoil escapement. Similar in basic concept to the palletts on the verge, Hooke's anchor was modified so the palletts "just" entered the teeth of the gearing. Unlike the verge, however, the main gear was no longer a crown, but flat, and the anchor rotated in the same direction, typically sitting at the top of the gear. The anchor only had to move a small amount to release the gearing, thereby dramatically reducing the overall amount of drag in the system, as well as allowing for more compact clocks as the pendulum didn't have to swing as far. Not only did this improve timekeeping considerably, it was also easier to build because the escapement gearing could be cut from sheet metal, instead of the tubing needed for the crown gear.

Deadbeat escapement

The deadbeat escapement, attributed to George Graham, was an improvement over the anchor escapement. In the anchor, and verge, the pendulum continued swinging even after the teeth had locked, forcing the escapement to move backward against the force of the springs or weights. This required a large pendulum to store enough momentum to overcome this force and keep moving during this period. In Graham's mechanism the escapement the teeth on the anchor were curved to form a small arc of a larger circle. When the pendulum swung past the locking point, the teeth rotated freely inside the gearing so the escapement did not move (it was "dead"), and there was no force on the pendulum (other than drag). The pendulum was only driven for a very short period of time, which was controlled by the cutting of the teeth on the anchor. This was the first escapement to separate the two functions of "locking" or counting the swings, and "impulse" or the pushing of the pendulum.

Grasshopper escapement

One of the oddest mechanical escapements known is John Harrison's grasshopper escapement. In this movement, the pendulum pushes a nodding pawl, shaped something like a cricket's head. When the pawl unlocks the driving wheel, the gear train briefly pushes the pendulum, and then the pawl engages and locks at the next place on the driving wheel. Regrettably, the special shape of the driving wheel makes the cricket-head movement more difficult to manufacture than cheaper escapements. Cricket-head escapements carved by Harrison from wood in the middle of the 18th century are still operating. Most escapements wear far more quickly, and waste far more energy.

Elecromechanical escapements

In the late 19th century, electromechanical escapements were developed. In these, a switch or phototube turned an electromagnet on for a brief section of the pendulum's swing. These are the most precise escapements known. They were usually employed with vacuum pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.

Twin pendulum clock

In the 20th century W.H. Shortt invented a twin pendulum clock with an accuracy of one hundredth of a second per day. In this system the time keeping "master" pendulum, which is made from a special alloy whose length does not change with temperature, swings as free of external influence as possible sealed in a vacuum chamber and does no work. It is in mechanical contact with its escapement for only a fraction of a second every 30 seconds. A secondary "slave" pendulum turns a ratchet, which every 30 seconds triggers an electromagnet. This electromagnet releases a gravity lever onto the escapement above the master pendulum. A fraction of a second later, the motion of the master pendulum releases the gravity lever to fall farther. In the process, the gravity lever gives a tiny impulse to the master pendulum, which keeps that pendulum swinging. Then the gravity lever falls onto a pair of contacts, completing a circuit that does several things: (1) energizes a second electromagnet to raise the gravity lever above the master pendulum to its top position, (2) sends a pulse to activate one or more clock dials, and (3) sends a pulse to a synchronizing mechanism that keeps the slave pendulum in step with the master pendulum. Since it is the slave pendulum that releases the gravity lever, this synchronization is vital to the functioning of the clock.

This form of clock became a standard for use in observatories. It was the first type of clock accurate enough to detect seasonal variations in the Earth's rotation.

Gear trains

To convert the motion of the escapement into an accurate analogue representation using 'hands' a gear train divides the motion of the escapement. Usually there are at least two gears: an hour gear, and a minute gear. These two gears are directly connected to the indicators (hands).

It is customary to make smaller gears more precisely, from more expensive materials in order to reduce wear.

Modern gear trains use involute gears, with tooth shapes that are an engineered compromise between efficiency and wear. Older clocks use cycloidal gears. The oldest clocks had hand-cut gears, some use gears made from interpenetrating cages of rods known as lantern pinons.

The indicators and face show the current time. Premium pendulum clocks often drive bells, whistles (cuckoo clocks) and dolls in order to help announce the time.

The slowest part of the gear train is attached to an energy storage device. This is either a spring, or a set of weights that pull on a cogwheel.

Escapements powered by either clockwork or twisted rubber-bands were also used as the output-devices in early model radio control systems, prior to the introduction of digital 'proportional' systems using the modern electric motor-driven servo actuator in the 1970's.

See also

• Tourbillon
• Balance wheel

External links

• American Watchmakers-Clockmakers Institute (http://www.awi-net.org/)
• Federation of the Swiss Watch Industry (http://fhs.ch/en/)
• Daniels coaxial escapement (http://www.bhi.co.uk/hj/August04AoM.PDF) (sketch and accuracy review)