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DISTRIBUTION MECHANISM – SWISS LEVER ESCAPEMENT

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Video: Discussion on the Swiss lever escapement with Dominique Büser

Real operating speed (21,600 vph)

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1. General information

In general, the Escapement is the Distribution mechanism of a watch, a Pendulum clock, or a Clock. The Swiss lever escapement is valued for its robustness, precision, and relative ease of production.

From its very first appearance, this Escapement established itself as the standard, both for its technical qualities and its economic qualities.

2. History

The Swiss lever escapement appeared in the second half of the 19th century, improving on the principles of the English lever escapement, which was older and less efficient.

It was around 1842 that Georges-Auguste Leschot, a Swiss horologist, contributed to the industrialization of this escapement, which enabled its large-scale distribution. Over the following decades, Swiss manufacturers refined it, making it a cornerstone of modern horology.

3. Role of the escapement

the escapement has two main roles. First, it must keep the finishing train stationary in order to release it punctually and regularly. Indeed, if the escapement did not exist, all the energy from the barrel would dissipate within a few seconds through the finishing Train, which would then run away uncontrollably. The Escapement therefore blocks the energy of the Train and lets it escape only periodically… which is where its name comes from. (Note that in the animation above on the left, the Escape wheel advances in small jumps, but is stopped most of the time).

The Escapement must also sustain the oscillations of the regulating organ by transmitting a regular Impulse to it. Since the Regulating organ is an oscillator, the Escapement also serves to transform a linear motion (the rotation of the finishing Train) into an oscillatory motion (the Oscillations of the balance-hairspring) (animations above).

4. Description and components

The Swiss lever escapement belongs to the family of free escapements.

The Swiss lever escapement is composed of the following elements:

The escape wheel. It is generally made of steel, nickel-phosphorus (UV-LIGA), or silicon (DRIE (Deep Reactive Ion Etching))

The pallets (lever). It is generally made of steel, or even of brass or Glucydur, with pallet stones made of ruby / Jewel, or entirely made of nickel-phosphorus or silicon.

The double roller. It consists of the large and small roller, connected together by a pipe. It is made of relatively ductile materials, suitable for driving-in (Steel, Brass or Glucydur, etc.). Too brittle, Silicon is not suitable for manufacturing double rollers. The impulse pin (roller jewel), generally made of Ruby / Jewel (synthetic corundum), is shellacked or glued to the large roller.

5. Advantages and disadvantages of the Swiss lever escapement

Advantages

• Precision: The Swiss lever escapement is a well-developed escapement that provides excellent chronometric performance.
• Reliability: It withstands shocks and positional variations particularly well. It is thus perfectly suited to use in wristwatches.
• Industrial production: Whatever materials are used in manufacturing its components, the production of Swiss lever escapements is well-controlled and economical.
• After-sales service: Except for silicon escapements, the Swiss lever escapement can be regulated (escapement-making (finishing)) easily, regardless of the age of the watch.

Drawbacks

• Friction : Although the materials of the Escapement components (Steel-Ruby/Jewel, Silicon) are chosen to minimize friction, it remains significant. Thus, the Steel teeth of the Escape wheel interact with the Ruby/Jewel Pallet stones of the Pallets (lever) exclusively through sliding friction. The Swiss lever escapement alone absorbs about 30% of the nominal energy of the Barrel. Silicon escapements now achieve better efficiency.
• Maintenance : In traditional manufacturing (Steel and Ruby/Jewel), it requires delicate Lubrication, to be renewed regularly in order to ensure good performance and limit wear. Silicon escapements also offer a solution to this problem, most of them being able to operate without Lubrication.

6. Operation

the operation of the Swiss lever escapement can be divided into four phases that unfold unfailingly according to the same cycle at each vibration of the regulating organ. These four phases are:

1. Rest
2. Unlocking (drop angle)
3. Impulse
4. Safety features

Let us detail below the action of each component during each of the four phases:

1. Rest

During this phase, the entire finishing train (wheel train) is stopped, blocked by the escapement.

Double roller:

This is the only element of the escapement then in motion.

During the rest phase, the double roller freely performs its ascending supplementary arc α, then its descending supplementary arc β (see below, Figure 1, point 1). This is the angle traveled by the double roller (and therefore by the balance) during the period when the escape wheel and the pallets (lever) are stopped.

Pallets (lever):

The pallets (lever) are at rest. The rod of the pallets (lever) is held against the banking pin (below, Figure 1, point 2) by the tooth of the escape wheel under the effect of the draw (see the tab The Draw).

Escape wheel:

Like the pallets (lever), the wheel is at rest. One of its teeth rests on the resting surface of the pallet stone of the pallets (lever) (see below, Figure 1, point 3).

Figure 1: The rest

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2. The unlocking (drop angle)

During this phase, the escape wheel and the finishing train move very slightly backward.

Double roller:

The unlocking (drop angle) phase begins when the impulse pin (roller jewel) comes into contact with the fork (Figure 2, point 4). The unlocking (drop angle) phase of the double roller corresponds to the angle α it travels through, while the impulse pin (roller jewel) pushes the fork of the pallets (lever) (Figure 3).

Pallets (lever):

The unlocking phase of the pallets (lever) corresponds to the angle β that it travels during the action of the escape wheel tooth on the resting plane of the pallet stone (Figure 3). The unlocking phase ends when the wheel tooth is, theoretically, on the resting tip of the pallet stone (Figure 4, point 5).

Escape wheel:

Through the lever formed by the pallets (lever) and its geometry, the escape wheel moves back slightly during the unlocking phase (geometric recoil, approx. 0.15-0.25°) then continues its motion by briefly detaching from the resting plane of the pallet stone (dynamic recoil, approx. 0.04°). The sum of the geometric recoil and the dynamic recoil defines the angle γ (Figure 3). Finally, the escape wheel resumes its usual direction of rotation under the effect of the driving energy. As with the pallets (lever), the unlocking (drop angle) phase is considered to end when the wheel tooth is, theoretically, on the resting tip of the pallet stone (Figure 4, point 5).

Figure 4

End of unlocking

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3. Impulse

It is during this phase that the escapement transmits the energy received from the barrel via the finishing Train (wheel train) to the regulating organ.

Double roller:

The Impulse begins when the Fork catches up with the Impulse pin (roller jewel) (Figure 5, point 6). The impulse phase of the double roller corresponds to the angle δ traveled while the Fork pushes the Impulse pin (roller jewel) (Figure 6). The impulse phase begins before the Dead point and ends after it (Figure 7).

Pallets (lever):

The impulse phase of the Pallets (lever) corresponds to the angle ε (Figure 6) that it describes while the impulse tip of the wheel tooth travels across the impulse face of the Pallet stone. The impulse phase ends when the impulse tip of the wheel tooth is on the impulse tip of the Pallet stone (Figure 8).

Escape wheel:

The Impulse phase of the Escape wheel corresponds to the angle it travels during the action of the Impulse plane of its tooth on the Impulse plane of the Pallet stone. The Impulse begins when the resting tip of the Pallet stone comes into contact with the Impulse plane of the Pallet stone (Figure 5, point 7) and ends when the Impulse tip of the tooth reaches the Impulse tip of the Pallet stone (Figure 8, point 8).

Figure 8

End of impulse

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4. Safety features

The safety features are angles traveled by the escape wheel and the pallets (lever), for nothing, just after the impulse phase.

Double roller:

At the beginning of the safety phase, the impulse pin (roller jewel) loses contact with the fork of the pallets (lever) (Figure 9, point 9). The double roller then freely begins its ascending supplementary arc α’ then its descending supplementary arc β’ (Figure 9).

Pallets (lever):

The safety angle of the pallets (lever) is called the run to the banking. It is the angle traveled by the rod (baguette shape) of the pallets (lever) once the tooth of the escape wheel comes into contact with the resting plane of the pallet stone (Figure 10, point 11) and the moment when the pallets (lever) rod comes into contact with the banking pin under the effect of draw (Figure 11, point 12).

Escape wheel:

The safety angle of the escape wheel is called the drop (impact). The drop (impact) corresponds to the angle traveled by the wheel from the moment one of its teeth leaves the impulse plane of the pallet stone (Figure 9, point 10) until another tooth comes into contact with the resting plane of the other pallet stone (Figure 10, point 11).

Figure 11

End of the Run to the banking

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Note:

For a complete understanding of how the Swiss lever escapement works, please refer to the two animations at the top of this page.

7. Definition of various angles

1. The double roller (de facto the Balance)

Description of the different angles of the double roller (and therefore of the balance) during two vibrations (one oscillation).

(*) Important:

When the escape wheel performs its dynamic recoil (see operation tab), pallets (lever) continues its movement, driven by the Impulse pin (roller jewel). When the escape wheel resumes its forward movement, the tooth of the wheel regains contact with the impulse plane of the pallet stone of the pallets (lever) and the impulse phase transmitted to the balance begins. During this period, contact between the impulse pin (roller jewel) and the fork of the pallets (lever) has been briefly interrupted and the double roller (and therefore the balance) has freely traveled a small angle between the end of unlocking (drop angle) and the beginning of impulse (angles described above as the free travel of the balance).

2. The Pallets (lever)

2.1 Total lift angle (Figure 12)

The total lift angle of the pallets (lever) corresponds to the displacement of the rod (baguette shape) of the pallets (lever) between the two banking pins (or limiting pins). It corresponds to the sum of the unlocking (drop angle), impulse, and run to the banking angles, and its value is generally between 12° and 20°.

Figure 12

Lift angle of the pallets (lever)

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2.2 Virtual depth angle (Figure 13)

This is the angle between the rest tip of the pallet stone (see the pallets (lever)) and the point where the tooth of the escape wheel falls onto the rest plane of the pallet stone of the pallets (lever).

2.3 Run to the banking angle (Figure 13)

This is the angle covered by the pallets (lever) between the end of the impulse and the resting of the fork against the banking pin (or the banking stud) (see operation)

Figure 13

Virtual depth and run to the banking angles of the Pallets (lever)

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2.4 Total depth angle (Figure 14)

The total depth angle corresponds to the sum of the virtual depth angle (Figure 13 – ) and the run to the banking angle (Figure 13 – )

Figure 14

Total depth angle of the pallets (lever)

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8. Components preventing overbanking

While the balance travels through its supplementary arc (ascending or descending), the pallets (lever) rests against one or the other of the banking pins (or the banking studs) due to the effect of the draw (see draw tab).

If a shock manages to overcome the torque of the draw, the pallets (lever) swings against the other banking pin. At the end of the additional descending arc of the balance, the impulse pin (roller jewel) strikes against the back of the horn (lug), which causes the watch to stop immediately. (Figure 15).

Figure 15

Overbanking situation

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In order to prevent overbanking, the Swiss lever escapement has four components:

• The dart (safety pin)
• The small roller table
• The horns
• The impulse pin (roller jewel)

When the balance travels through its supplementary arc, the dart (safety pin) rests against the edge of the small roller table, preventing the pallets (lever) from swinging against the other banking pin and thus preventing overbanking (Figure 16).

Figure 16

Components preventing overbanking (dart and roller table)

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The roller table has a notch aligned with the impulse pin (roller jewel). During the unlocking and impulse phases (see operation), this notch allows the dart (safety pin) to pass through and the pallets (lever) to swing normally from one pin to the other. During these phases, it is the inside of the horns (lugs) that comes to rest against the impulse pin (roller jewel), thus preventing overbanking (Figure 17).

Figure 17

Organs preventing overbanking (horn and impulse pin)

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9. The impacts

The different impacts of the Swiss lever escapement are perceptible to the ear. This is the “tick-tock” of mechanical watches. But when the human ear perceives a tick or a tock, the escapement has actually emitted five sounds resulting from five impacts. These sounds are so close together that the ear only hears one (a tick or a tock).

The analysis of these sounds, based on the intensity of each one and the spacing between them, makes it possible to detect malfunctions or incorrect adjustments of the escapement. Similarly, the graphs from a timing machine make it possible to identify problems related to the escapement. For each vibration, these five impacts are as follows:

1. The unlocking (drop angle)
2. The start of the impulse of the escape wheel
3. The start of the impulse of the balance
4. The end of the drop (impact)
5. The end of the run to the banking

1. Unlocking (drop angle)

The first impact of a vibration occurs when the impulse pin (roller jewel) comes into contact with the inside of the fork (Figure 18).

Figure 18

Impact during unlocking

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2. Start of the escape wheel impulse

Shortly after the unlocking, the resting tip of the escape wheel falls onto the impulse plane of the pallet stone. This is the second impact (Figure 19).

Figure 19

Impact at the start of the escape wheel impulse

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3. Start of the balance impulse

The fork catches up with the impulse pin and transmits its impulse to the balance via the double roller. This is the third impact (Figure 20).

Figure 20

Impact at the start of the balance impulse

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4. End of drop

A tooth of the escape wheel comes into contact with the resting surface of a pallet stone of the pallets (lever) (Figure 21).

Figure 21

Shock at the end of the Drop (impact) of the Escape wheel

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5. End of the Run to the banking

Finally, at the end of its Run to the banking, the Rod (baguette shape) of the pallets (lever) comes into contact with one of the Banking pins (or a stop) (Figure 22).

Figure 22

Shock at the end of the Run to the banking of the Pallets (lever)

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10. Analysis of rate diagrams

Watch rate testing devices (timing machines), provide a great deal of information such as the instantaneous daily rate, the amplitude and the value of the guide mark. However, a good reading of the diagrams provided by timing machines makes it possible to identify many problems related to the finishing train, the escapement and the regulating organ.

Thus, when a perfectly regulated watch is running with no defects, the diagram shows a single horizontal line (Figure 23).

Figure 23

When the Guide mark is too large, two parallel lines appear on the diagram (Figure 24). The correction consists in adjusting the Guide mark (using the movable stud holder or the collet), then readjusting the Daily rate.

Figure 24

When the diagram line is ascending, the watch is running fast (Figure 25 left). Conversely, when it goes down, the watch runs slow (Figure 25 right). It is then a matter of adjusting the rate (by modifying the active length of the hairspring or the moment of inertia of the balance).

Figure 25

In some cases, a variation in the rate will be observed during movement measurements, both in the horizontal position and in the vertical position (Figure 26). The correction, in this case, consists of tightening the banking pins.

Figure 26

When the diagram shows a regular sinusoidal line, it is useful to measure the time interval between each peak of the sine wave. If the frequency corresponds to the rotation speed of the fourth wheel (seconds wheel) (60 sec if the movement has a seconds hand), an out-of-flat (hairspring defect) or out-of-round (hairspring defect) defect exists on the fourth wheel (seconds wheel) (Figure 27). The correction then consists of checking and correcting the riveting of the seconds wheel on its pinion, or replacing the fourth wheel (seconds wheel).

Figure 27

The diagram sometimes shows an irregular and random line. In such cases, the amplitude is often too low or subject to numerous variations (Figure 28). In such a case, it is recommended to carry out a complete overhaul of the movement.

Figure 28

If the diagram occasionally shows nearly vertical lines, if the sounds of the escapement speed up and become irregular, and a sharp increase in the amplitude is observed (or the total loss of amplitude information), this means that the amplitude is occasionally too high and that the balance knock (rebate) (Figure 29). It is then possible to replace the mainspring with a spring developing a lower torque, or to modify the value of the total depth of the lever’s pallet stones.

Figure 29

Multiple and irregular lines indicate that the balance knocks continuously (amplitude too high) (Figure 30). Here again, the defect will be corrected by replacing the mainspring with a spring of lower torque or by modifying the total depth of the pallet stones.

Figure 30

A diagram showing a sinusoidal curve with a tight frequency (a few seconds) indicates an Out-of-round (hairspring defect) problem of the the escape wheel (Figure 31). It is then recommended to replace the escape wheel.

Figure 31

If the diagram shows a random line superimposed on a straight line, this means that the entry pallet stone of the pallets (lever) is sticking or is dirty (Figure 32). It is then recommended to carefully clean the pallets (lever) and the escape wheel and, if necessary, to replace the pallets (lever) or the entry pallet stone.

Figure 32

The vertical lines marking a normal rate line indicate that the hairspring (balance spring) occasionally touches the regulator pins or the stud (hairspring stud) (Figure 33). This defect will be corrected by checking the geometry and centering of the hairspring.

Figure 33

A stabilization of the rate and the amplitude being too slow between two measurement positions indicates a problem of lubrication of the pivots of the balance or the finishing train (Figure 34).

Figure 34

11. Draw (recoil)

Draw is a safety feature that keeps the rod (baguette shape) of the pallets (lever) in contact against the banking pins when the balance performs its additional arc. This prevents the dart (safety pin) from coming into contact with the edge of the small roller and disturbing the period of the regulating organ.

The mechanical action of the draw results from the pressure exerted by a tooth of the escape wheel on the resting surface of one or the other of the pallet stones of the pallets.

Thus, the force F1 exerted by the wheel tooth acts perpendicularly to the resting surface of the pallet stone and creates a moment of force through the lever arm of the distance d1 with the pivot point of the pallets. This moment of force has the effect of pressing the rod of the pallets against the banking pin with a force F2 and a lever arm d2, corresponding to the distance between the pivot point of the pallets and the center of the banking pin (Figure 35).

Figure 35

The pallets (lever) is thus held in contact against the banking pin. If a shock manages to overcome the force of the draw, the pallets (lever) will move and the dart (safety pin) may briefly come into contact with the periphery of the small roller, but the force of the draw will immediately bring it back to its rest position.

Value of the draw moment of force

The draw moment of force can be defined by the following calculations:

The distance d1 varies according to the value of the angle α measured at the rest point B1. A variation of the angle α thus causes an increase or decrease in the moment of force M of the draw (Figure 36).

Figure 36

The draw angle β is the value between a perpendicular raised at the rest point B on the line joining this point to the pivot point of the pallets (lever) and the rest plane of the pallet stone BP (Figure 37).

Figure 37

As we have just seen, the draw angle β is defined at the resting tip B of the Pallet stone. It can also be defined at the resting point B1 of the Pallet stone. In this case, the draw angle is called α (Figure 38). We thus understand that during Unlocking (drop angle) (see operation), the draw angle varies from β to α.

Figure 38

The draw angle is defined during the construction of the escapement (ideally, it is 9° for the combination of a steel wheel and a ruby / jewel pallet stone). But too small a draw angle reduces the draw force and therefore its safety. It also increases the coefficient of friction.

Conversely, too large an angle increases the moment of force required for unlocking and increases the recoil value of the escape wheel (loss of impulse value). Thus, a draw angle that is too small or too large disrupts the regulating qualities of the regulating organ. This is how an ideal draw angle can be defined (for a steel escape wheel and ruby / jewel pallet stones), generally between 13° and 16°.

Finally, note that, during unlocking (drop angle), the drop angle increases on the entry pallet stone by the angle ε (β = α + ε) (Figure 38).

whereas it decreases on the exit pallet stone by the same value ε (β = α – ε) (Figure 39).

Figure 39

12. Stops on the resting and impulse planes

1. Stop on the resting plane

the stop is observed on the resting plane when the movement is fully wound. Then manually position the balance so that the impulse pin (roller jewel) is at the start of the unlocking (drop angle) phase (see “operation”) (Figure 40).

Figure 40

If the balance fails to unlock the escape wheel from the resting plane of the pallet stone by the sole force of the hairspring (balance spring), the movement is considered to have stopping on the resting surface.

The stopping on the resting surface is most pronounced when the movement is at maximum winding, because the force exerted by the tooth of the escape wheel on the resting surface of the pallet stone (and therefore the draw) is then at its maximum. As we describe in the chapter “draw”, the draw increases during unlocking (drop angle) on the entry pallet stone and decreases during unlocking (drop angle) on the exit pallet stone.

Movements with a diameter greater than 24 mm must under no circumstances exhibit stopping on the resting surface. However, stopping on the resting surface cannot be prevented when it comes to a movement with a diameter of less than 24 mm. Indeed, when the size of the movement is reduced, the moment of inertia and the torque of the hairspring (balance spring) decrease more quickly than the torque of the mainspring.

In movements with a diameter of less than 24 mm, it is not tolerated to have a stop on the resting surface after 24 hours of running, as the difficulty in performing the unlocking significantly disturbs the isochronism of the regulating organ.

1a. Causes of the stop on the resting surface

The causes of a stop on the resting surface can be the following:

• A lack of freedom or an insufficient lubrication of the escapement wheels. (noticeable by a amplitude that is too low)
• A defective surface condition of the resting surface of one or the other pallet stones, or insufficient lubrication of the escape wheel by the pallet stones.
• A defective profile or surface finish of the resting plane of the escape wheel teeth.
• Too strong a draw angle.
• A lack of shake of the impulse pin (roller jewel) in the fork of the pallets (lever).
• Sides of the fork entry that are too short, resulting in excessive dart (safety pin) shake at the beginning of the supplementary arc of the balance.
• An incorrect setting to the guide mark

To avoid or limit the stop on the resting plane (depending on the diameter of the movement), it is necessary to determine the position of the balance when the stop on the resting plane occurs. It is then possible to reduce the total depth of each pallet stone so as to angularly shift the end of the unlocking (drop angle) so that it occurs before the stop position on the resting plane. This solution, however, implies a reduction in the value of the run to the banking.

Another solution, more complicated to implement, consists of increasing the inclination of the pallet stones’ impulse planes. This will reduce the total depth value without modifying the angle of the back to the run. Care must then be taken to maintain a sufficient value for the virtual depths.

When developing small-diameter movements, one can minimize the stop on the resting plane by increasing the ratio between the lift angles of the pallets (lever) and the balance (see table below). This implies reducing the diameter of the rollers and lengthening the fork of the pallets (lever). Thus, the balance begins its unlocking (drop angle) with a larger angle than that of the dead point, which increases its torque and limits the stop on the resting plane. However, a high balance lift angle increases the disruptive influence of the escapement on the isochronism of the regulating organ.

2. Stop on the impulse plane

The stop on the impulse plane is observed by winding the mainspring by two teeth of the ratchet wheel. The balance is then manually positioned so that one of the teeth of the escape wheel is at the beginning of the impulse plane of the pallet stone (Figure 41). If the impulse is not given completely by this weak driving force, it is then considered that the movement has a stop on the impulse plane.

Figure 41

All movements have a stop on the impulse plane. To check whether this is acceptable, the mainspring is wound very gradually until the movement starts spontaneously, taking care to observe the rotation of the ratchet wheel. The accepted values for overcoming the stop on the impulse plane are as follows:

Small diameter Movements: starts running after 4 to 5 teeth of Ratchet wheel Winding.

Medium diameter Movements: starts running after 1/2 turn of Ratchet wheel Winding.

Large diameter Movements: starts running after 1 to 2 turns of Ratchet wheel Winding.

2.a Causes of stopping on the Impulse plane

The causes of stopping on the Impulse plane may be as follows:

• A lack of driving force (Mainspring too weak, lack of freedom in the finishing Train, gearing defects, poor Lubrication).
• A lack of freedom of the Pallets (lever) or the Balance.
• A defective surface finish or profile of the Impulse plane of the Pallet stone.
• A surface finish or profile defect on the impulse plane of the escape wheel teeth.
• Poor lubrication of the escape wheel – pallets (lever).
• A lack of shake (play) of the impulse pin (roller jewel) in the fork of the pallets (lever).
• A defective surface finish or profile of the entry of the fork of the pallets (lever).
• Poor setting to the guide mark.

Important

The stopping on the impulse plane increases as the diameter of the movement increases. This is not actually a defect, as it does not affect either the accuracy of the movement or the isochronism of the regulating organ. However, when it is too significant, it does not allow the movement to start spontaneously under the sole influence of the driving force, and requires manually giving a first impulse to the balance (by quickly rotating the movement, of the watch for example).

To achieve the best regulation and the best possible isochronism, it is preferable to have the lowest possible balance lift angle.

3. Accepted values & ratios of lift angles