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Exhaust (watchmaking)

In clock making , exhaust is a mechanism generally placed between the energy source ( spring , weight, etc.) and the regulating member. The escapement is to maintain and count the oscillations of the pendulum of a clock or the balance of a watch 1 .

In a gravity clock, it is also intended to contain the acceleration effects due to the gravitas exerted by the weights. When one has the idea of ​​giving the clocks a new engine by using the gravity of solid bodies, one can easily see that these bodies have the same disadvantage as water, that of accelerating in the fall and falling more At the end only at the beginning of their descent. The escapement driven by the pendulum has the object of continuously retarding the descent of the weight, of destroying the acceleration, and of leaving it in all the moments of its fall only the uniform velocity which it has in the first moment. Instead of the acceleration one obtains the uniformity of motion which is the principle of the regularity of the clocks 2 .

It is used to transmit the time information from a regulating device ( balance ) to the device to be regulated (for example, clockwise) and to supply the energy regulating device.


Animation of an anchor escapement, used in pendulum clocks.
Animation of an exhaust with Swiss anchor, used in watches.

The exhaust which has been used for the longest time to measure time is that known as a wheel of encounter, or exhaust with rod, with regulator to foliot . It was used on the oldest known mechanical clock in the Western world, that of the Dunstable priory , dated 1283.

The principle of operation is simple: in front of a toothed gear with front teeth (whose teeth are oriented like those of a bell saw, and of the same shape), a shaft with perpendicular (vertical) axis carries an orthogonal rule where slide Of the inertia adjustment masses (foliot) and two offset pallets pressing in turn on a tooth of the wheel. When the wheel drives the upper pallet, the lower pallet is not meshed and the foliage is rotated in one direction, when the tooth escapes, the lower pallet is interposed between two teeth and allows a rotation which, by a shock transmits A pulse and causes the leaflet to rotate in the other direction. The shock n ‘

The various organs added to the device (adjustment of the moment of inertia of the leaflet by moving masses, adjustment of the play by moving the lower pivot of the leaflet) did not exceed a regularity better than one hour per day. Christian Huygens had to wait to replace the flywheel with a pendulum. The first mechanisms dissipate a great deal of energy, and the pendulum has a large and variable stroke, so that the problem of the resulting variations in movement is compensated: Huygens also solves (in 1694) the theoretical problem of the isochronous trajectory : The pendulum must describe a cycloid arc so that its period does not depend on the ” Amplitude . Huygens add the string to the suspension flanges of the regulator to approach this ideal trajectory (the cycloid is an involute of a cycloid, it would ideally necessary trochoidal cheeks infinite curvature in the vicinity of the cusp, but this part of the curve Does not need a precise correction, because being very short it represents only a small part of the oscillation time).

The main improvement will come from England : George Graham invented the anchor mechanism , still used on many nineteenth century clocks, illustrated above. The principle is to make the driving force act only by impulse when the regulator passes in the vicinity of its position of equilibrium: the rest of its course, the rocker oscillates almost freely and its course is thus much more regular. This new mechanism also makes it possible to reduce the amplitude of the stroke, making cycloidal correction unnecessary. After many improvements, the main progress still comes from England: Thomas Mudge separates the anchor from the pendulum to create the free anchor. The mechanism is very interesting for oscillators with circular balances: the anchor is carried by a forked lever, the balance bears a spur which at rest is in equilibrium in the middle of the fork. In its oscillating movement, the pin engages with the fork at the moment when the escapement acts: the rocker receives the escape pulse and then releases the rocker which freely continues its travel without suffering the passive friction of the anchor , Which was then the main cause of movement irregularity of the spring-loaded mechanisms. The devices using this principle will be numerous, and surprisingly ingenious. The problem had to be solved because the navigation required accurate chronometers ,

Different types of exhaust

Exhaust with Swiss anchor, exhaust, cylinder exhaust, exhaust with yoke or wheel of encounter, exhaust of Graham. In 1558 for the astrolabe of Notre – Dame Cathedral of Saint – Omer , the clock mechanism was driven by a weight – originally a couleuvrine of 47.2 kg – suspended from a cable wound on a drum. The regulation is ensured by an escapement consisting of an encounter wheel, a vane shaft surmounted by a foliot allowing adjustment through the regulators.


  1. Exhaust and stepper motors , Federation of Technical Schools of Switzerland
  2. ↑ Ferdinand Berthoud. History of Time Measurement by Clocks, Volume 1. Ferdinand Berthoud. The Republic, 1802 read online [ Archive ]

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