Last Updated on May 22, 2024 by Jason
I look down at the vintage Tissot Visodate Seastar Seven watch that is ticking loudly on my wrist. The watch dates from the 1960s and I am always amazed that a watch nearing 60 years of age remains wearable and reliable. It has a fascinating history, but its legacy goes back far beyond its 60 years. The underlying technology within the watch dates back to the mid-18th century. In this post, we take a look at the lever escapement, the heart of virtually all mechanical watches over the last couple of centuries.
The escapement is a key component of a mechanical watch movement. It controls the transmission of power from the mainspring, through the wheel train into a series of impulses that are passed to the balance wheel. The ticking sound you hear when holding a watch to your ear is from the escapement. The sound is generated when the pallet fork locks and unlocks with the escape wheel at each vibration of the balance wheel. This happens multiple times a second and creates the illusion of a sweeping seconds hand. The reality is, that the second around half a dozen times a second, but each incremental tick is so small and quick they merge into what seems to be a continuous flow.
If the escapement didn’t exist, a fully wound watch would unwind uncontrollably in a matter of seconds. The hands would spring freely and there would be no timekeeping element to the watch. The release of power is controlled in such a way, that a way that a fully wound watch can run for several days.
Origin
In around 1750 the English horologist, Thomas Mudge (1715-1794), first came up with the concept of the lever escapement in his workshop in London. After a series of refinements, it is generally accepted that he finished his work on the lever escapement in 1755. There are a number of different types of escapement. However, for the purpose of this post, we are going to discuss the Swiss lever movement that evolved from Mudge’s original invention. The Swiss lever movement appeared in the mid-19th century and is still used in the majority of mechanical watches today. The English lever escapement, is slightly different, as it changes the positioning of the wheels. Whereas the balance wheel, lever and escape wheel are in a straight line in a Swiss lever design, with the English lever they form a 90-degree angle.
Components of the escapement
The lever escapement consists of three major components. Please refer to the images below.
- Escape wheel: A specially shaped wheel with “teeth” that interact with the pallet fork.
- Pallet fork: A Y-shaped lever with two pallet stones that interact with the teeth of the escape wheel.
- Balance wheel: An oscillating wheel which serves as the timekeeping element, connected to a hairspring, which controls the oscillation.
Other key components
Although, not part of the escapement itself, these are key components that interact with it.
- Mainspring: The energy source, providing power to the wheel train.
- Wheel train: Transfers energy from the mainspring to the escapement.
How it works
The escape wheel pinion meshes with the fourth wheel of the wheel train, which supplies power to it from the mainspring. The escape wheel has 15 specially shaped teeth designed to interact with the two pallet jewels teeth. The wheel is constructed of steel. The rotation of the escape wheel is controlled by the pallet jewels. These are fixed to the left and right sides of the pallet fork.
At the end of the pallet is a lever, which has a fork designed to receive the ruby impulse pin of the balance roller which is fixed to the balance wheel shaft. The balance wheel is returned towards its static centre position by an attached hairspring. The lever is mounted on a shaft and is free to move between two fixed banking pins.
At rest one of the escape wheel teeth will be locked against a pallet. As the escape wheel rotates clockwise, the escape wheel tooth is locked in place against the pallet and the lever is held in place by the left banking pin. The impulse pin is located within the lever fork and rotates according to the balance wheel. To get started, the lever fork must receive a small nudge from the anti-clockwise rotation of the balance wheel via the impulse pin, which rotates the lever slightly clockwise off the left banking pin. This unlocks the entrance pallet allowing the wheel to rotate clockwise a small distance until it is locked by the escape tooth interacting with the opposite pallet jewel.
Cycle
The impulse received by the entrance pallet as the tooth moves over the impulse face is transferred by the lever to the balance wheel 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 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. At this point, the lever will be resting against the left banking pin. The balance wheel rotates back from the opposite direction nudging the lever fork to move back towards the left banking pin. As a result, the cycle starts again.
Each back and forth movement of the balance wheel from and back to its centre position corresponds to a drop of one tooth. This is called a beat. A typical 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.
Summary
Before the lever escapement, time regulation relied on the verge escapement, foliot balance-bar, balance wheels, and anchor escapements. Each had its advantages and disadvantages, with varying levels of accuracy and reliability. However, none of these provided the reliability and accuracy that was needed for a personal timepiece. The lever escapement, introduced by Thomas Mudge in the mid-18th century, ultimately provided a superior solution that significantly enhanced the precision of portable timepieces and laid the foundation for modern watchmaking. The lever escapement’s interaction with the mainspring, wheel train, and balance wheel ensures the accuracy and reliability of mechanical watches. It was so successful that nearly 300 years later the technology is still used in modern mechanical watches. In fact, the vintage watch you are probably wearing on your wrist is still working today because of the ingenuity of the lever escapement.
Related content
Mechanical watches at Wikipedia.