The lever escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

is a key component of the typical movement

Movement (clockwork)

In horology, a movement is the internal mechanism of a clock or watch, as opposed to the case, which encloses and protects the movement, and the face which displays the time. The term originated with mechanical timepieces, whose movements are made of many moving parts...

 found in most mechanical wristwatches, pocket watch

Pocket watch

A pocket watch is a watch that is made to be carried in a pocket, as opposed to a wristwatch, which is strapped to the wrist. They were the most common type of watch from their development in the 16th century until wristwatches became popular after World War I during which a transitional design,...

es and many small mechanical non-pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

s.
The invention of the lever escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

 is attributed to Thomas Mudge

Thomas Mudge (horologist)

Thomas Mudge was an English horologist who invented the lever escapement, the greatest single improvement ever applied to pocket watches.-Early life:...

, and its modern form was developed by subsequent workers including Breguet

Breguet

Breguet may refer to: * Breguet , watch manufacturer* Breguet Aviation, a defunct French aircraft manufacturer; was formally known as "Société Anonyme des Ateliers d'Aviation Louis Breguet"...

 and Massey

Massey

Massey may refer to:In places:* Massey, New Zealand, a suburb Auckland* Massey, Ontario, Canada* Massey Island, Nunavut, Canada* Massey, Maryland, United StatesPeople with the surname:* Massey In education:...

. It is a detached escapement

Escapement

In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

, which means that the time-keeping element runs entirely free of interference from the escapement during most of the operating cycle.

The rotation of the escape wheel is controlled by the pallets

Pallet fork

The pallet fork or pallet lever is an integral component of the lever escapement of a typical mechanical watch. Its purpose is to release the escape wheel one tooth at a time, at each swing of the balance wheel, and also give the balance wheel small pushes to keep it going.The lever is shaped like...

. The escape wheel has specially shaped teeth of either ratchet or club form, which interact with the two jewels called the entrance and exit pallets. The escape wheel, except in unusual cases, has 15 teeth and is made of steel. These pallets are attached solidly to the lever, which has at its end a fork to receive the ruby impulse pin of the balance roller which is fixed to the balance wheel

Balance wheel

The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...

 shaft. The balance wheel is returned towards to its static center position by an attached balance spring (not shown in the diagram). In modern design it is common for the pallet mountings and the fork to be made as a single component. The lever is mounted on a shaft and is free to rotate between two fixed banking pins.

At rest one of the escape wheel teeth will be locked against a pallet. As shown in the diagram, the escape wheel rotates clockwise and the entrance tooth is locked in place against the entrance pallet, the lever held in place by the left banking pin. The impulse pin is located within the lever fork and the balance wheel is near its center position. To get started, the lever fork must receive a small impulse from the anti-clockwise rotation of the balance wheel via the impulse pin (say by being shaken) which rotates the lever slightly clockwise off the left banking pin. This unlocks the entrance pallet allowing the wheel to rotate clockwise.

As the powered escape wheel rotates clockwise, the entrance tooth tooth slides across the sloping impulse plane of the entrance pallet. This turns the pallets about their axis, which places the exit pallet into the path of the rotating escape wheel. Once the entrance tooth leaves the impulse plane of the entrance pallet, the wheel is able to turn a small amount (called the drop) until the exit tooth of the escape wheel lands on the locking face of the exit pallet. The wheel is said to be locked on the exit pallet. From the release from the entrance pallet to this point, the escape wheel will have turned through exactly one half of the 24 degree angle between two teeth.

The impulse received by the entrance pallet as the tooth moves over the impulse face is transferred by the lever to the balance wheel

Balance wheel

The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...

 via the ruby impulse pin on the roller of the balance wheel. The lever moves until it rests against the right banking pin; it is held in this position by the force of the exit tooth against the exit pallet jewel (called the draw). This means that in order to unlock the wheel it must be turned backwards by a small amount, which is done by the return momentum of the balance wheel via the impulse pin..

After the exit tooth drop, the balance wheel rotates anti-clockwise, free of interference from the escapement until the balance spring pulls it back clockwise, and the impulse pin re-enters the fork. This will unlock the escapement, releasing the escape wheel so that the exit tooth can slide over the impulse plane of the exit pallet, which transfers a clockwise impulse to the balance wheel's impulse pin via the lever fork, while pushing the lever up against the left banking pin. The escape wheel drops again until the entrance tooth locks on the entrance pallet now being held in place by the left banking pin via the lever. The balance wheel continues clockwise, again free from interference until it is pulled back by the balance spring to the center position. The cycle then starts again.

Each back and forth movement of the balance wheel from and back to its center position corresponds to a drop of one tooth (called a beat). A typical watch lever escapement beats at 18,000 or more beats per hour. Each beat gives the balance wheel an impulse, so there are two impulses per cycle. Despite being locked at rest most of the time, the escape wheel rotates typically at an average of 10 rpm or more.

Draw

The reliability of the modern lever escapement depends upon draw; the pallets are angled so that the escape wheel must recoil a small amount during the unlocking. The draw holds the lever against the banking during the detached portion of the operating cycle. Draw angle is typically about 15 degrees to the radial.

Early lever escapements lacked draw (indeed some makers considered it injurious as a cause of extra friction in unlocking) as a result a jolt could result in the escapement unlocking.

Lever watch movement

Most modern mechanical watches are jeweled lever watches, using manmade ruby

Ruby

A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

 or sapphire

Sapphire

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

 jewels for the high-wear areas of the watch.

Pin pallet escapement

A cheaper, less accurate version of the lever escapement is used in alarm clock

Alarm clock

An alarm clock is a clock that is designed to make a loud sound at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but...

s, kitchen timer

Timer

A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

s, mantel clock

Mantel clock

Mantel clocks — or shelf clocks — are relatively small house clocks traditionally placed on the shelf, or mantel, above the fireplace. The form, first developed in France in the 1750s, can be distinguished from earlier chamber clocks of similar size due to a lack of carrying handles.These clocks...

s and, until the late 1970s, cheap watches, called the Roskopf, pin-lever, or pin-pallet escapement after Georges Frederic Roskopf

Georges Frederic Roskopf

Georges F. Roskopf , the inventor of the pin-pallet escapement, was born in Germany and became a naturalized Swiss citizen.- Early life and introduction :...

, who invented it in 1867. It functions similarly to the lever, except that the lever pallet jewels are replaced by vertical metal pins. In a lever escapement, the pallets have two angled faces, the locking face and the impulse face, which must be carefully adjusted to the correct angles. In the pin pallet escapement, these two faces are designed into the shape of the escape wheel teeth instead, eliminating complicated adjustments. The pins are located symmetrically on the lever, making beat adjustment simpler. Watches that used these escapements were called pin lever watches, and have been superseded by cheap quartz watches.

Future directions

One recent trend in escapement design is the use of new materials, many borrowed from the semiconductor fabrication

Semiconductor fabrication

Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...

 industry. A problem with the lever escapement is friction. The escape wheel tooth slides along the face of the pallet, causing friction, so the pallets and teeth must be lubricated. The oil eventually thickens, causing inaccuracy, and requiring cleaning and reoiling of the movement about every 4 years. A solution is to make the escape wheel and other parts out of harder materials than steel, eliminating the need for lubrication. Materials being tried include silicon

Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 (called silicium in the watch industry), nickel phosphorus, diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

, and diamond-on-silicon. Ulysse Nardin

Ulysse Nardin

Ulysse Nardin is a watch manufacturer founded in 1846 in Le Locle, Switzerland. Historically Ulysse Nardin was best known for being a manufacturer of marine chronometers, but today Ulysse Nardin produces complicated mechanical watches.-History:...

 in 2001 and Patek Philippe in 2005 introduced watches with silicon escape wheels.

External links

• An Analysis of the Lever Escapement, by H. R. Playtner, 1910, from Project Gutenberg
  Project Gutenberg
  Project Gutenberg is a volunteer effort to digitize and archive cultural works, to "encourage the creation and distribution of eBooks". Founded in 1971 by Michael S. Hart, it is the oldest digital library. Most of the items in its collection are the full texts of public domain books...