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Mainspring in Focus

The term ‘Creep’ refers to the slipping of the welded bridle around the outside of a lubricated barrel Creep.

To compensate, some brands have grooved the barrel wall to stop the bridle from slipping. The problem is, though, that the torque generated from the winding with a grooved wall sends too much power to the escapement.

The creep is something of a compromise.

Shortening the bridle, which is spot welded to the spring, and straightening it, reduces creep but leaving the watch with elevated amplitude.

The magnet test with determine whether a mainspring is good quality and made of nickel/cobalt/chromium as these materials are anti-magnetic.

The term Strength, when used in regards to the mainspring is the thickness and width measured in Newton Metres – torque.

The mainspring should be the height of the barrel wall less 0.5mm clearance.

It is important to have the correct spring for the barrel is to ensure the watch gains the maximum power reserve possible i.e. number of turns of barrel. To do this, you need to figure out how much space you have inside your barrel. First things first, you need to work out the area of the barrel. To find the area of the circle, you have to square the radius and multiply by pi. But don’t forget you have to compensate for the barrel arbour, which takes up roughly one third of the barrel’s diameter.

Here’s a working example for you:

If a barrel of 10mm has a radius of 5mm then the area would be 78.57 (25×3.1428), but that’s before you’ve subtracted the area occupied by the barrel arbour. Since the barrel arbour tends to occupy one third of the would-be functional space of the barrel and the mainspring in either a relaxed of wound state occupies another third (the final third remains for the tensioning of the spring), you need to perform the following calculation:

The equation for working out the length of the mainspring is thus:

Pi (R2 – r2) / 2e

R= barrel radius
r = arbour radius
e = spring thickness

You have to double the thickness of the spring to account for the necessary space.

In similar equations, N will stand for the number of turns between the wound and unwound extremes of the spring and L the length of the spring.

To find the number of turns to unwind a barrel (N’), do the following equation:

N’ = 1/e (R – the square root of R2 – (Le/Pi)

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The quartz crystal, cut and finished in the shape of a tuning fork, is responsible for delivering a intervallic voltage, drawn from its piezoelectric frequency, to the circuit, but how the circuit chooses to use that voltage is a different matter.

The load, or drain, on the battery increases with the introduction of mechanical friction. Powering the circuit doesn’t really require all that much energy. The battery is just doing what it would do naturally and allowing power to flow all around it. Imagine the battery like a human body. When the body is standing still it can be said to be doing nothing, but blood still flows and its heart still beats. Assuming the body has a finite energy resource it can maintain this static pose until power runs out. Now if you ask that body to physically push something (for the sake of this example, let’s say a boulder), the energy will deplete faster, the body’s lifespan will decrease and it will burn out. That’s the problem faced by the battery.

When allowing power to flow around the circuit and through the quartz crystal, the battery is barely flexing its muscles. As soon as the wheels come into play, powered of course by the bipolar motor as previously discussed, the friction of driving the train asks an awful lot of the power source.

In a Quartz Watch that has a seconds hand an impulse is needed every second. However, for watches with no seconds hand, such a load can be reduced. It is not necessary for the Lavet motor to convert electric impulse into mechanical movement EVERY SINGLE SECOND, and so the impulse is ‘chopped’ into larger chunks, pulsing maybe once every 20, 30 or even 60 seconds in order to reduce the strain on the battery.

Here’s a neat little definition for you:

The length of the impulse is varied (chopped) according to the load on the motor, optimising power consumption. Basically, the watch only uses power when necessary. It is common for more power to be released during the date change phase of a quartz watch as it takes slightly more energy to power the movement during this period. This whole process is controlled by the Integrated Circuit (the IC), which reacts to the feedback it receives from the movement and distributes power accordingly.

The stator is made of a permeable soft iron that does not hold on to magnetism. I is magnetised by the coil, which passes an electric current through it. The current is reversed with every impulse, meaning the polarity of the stator alternates, affecting the rotor, which sits in a gap in the middle of the stator. The Rotor is a cobalt disc, which is always strongly magnetised. The rotor pivots in jewelled bearings and drives the train. It is moved itself by the stator which, thanks to ‘air gaps’ cut into the walls of the hole in which the rotor sits, positions the rotor correctly so that each impulse will turn it in the right way and prevent it from every flipping backwards.

The coil creates a temporary magnetic field in the stator when current is applied. The polarity depends on the direction of the current.
The coil, under influence from the Integrated Circuit (IC), alternates the polarity of the stator so that the rotor, which sits in the cut out of the stator, can rotate and thus drive the gear train of the watch.

The stator is made of a soft permeable alloy, which conducts the alternating magnetic field from the coil to the rotor. The shape and slots in the stator hold the rotor in the correct position, ready for the next impulse.
The Rotor is a cobalt magnet disc, pressed on to a pinion. This magnet rotates in the stator and drives the train.

The bipolar motor, designed by Lavet, has a fixed polarity rotor (Green/Red disc in above picture), surrounded by a varying polarity stator of soft iron (Silver bit in above picture).
The bipolar motor relies on the stator made of a permeable alloy, being alternately magnetised by the current of a coil (Orange bit in above picture). The stator has special air gaps cut into the rim of the recess in which the rotor (a cobalt magnet that drives the gear train) sits.
The air gap around the rotor is designed to hold the rotor at a specific angle to the stator. When the coil receives an impulse of electricity from the Integrated Circuit (IC) it magnetises the stator for a number of milliseconds. The rotor then moves to align itself with the magnetic field. When the electric impulse is turned off, the rotor continues through 180 degrees and comes to rest aligned with the air gaps. There it waits for the next impulse, which will be of different polarity to the last.
The Rotor only rotates in one direction thanks to the air gaps in the stator. The direction in which the current is passed from the IC, through the coil will determine the polarity of the magnetism.

To understand how the Quartz watch works, you need to the basic principles behind the timekeeping element of the watch, then get your head around how these principles translate into mechanics.

First things first, what is a Quartz Watch? A Quartz Watch is so named because of the sliver of genuine quartz crystal that, remarkably, plays the same role as the hairspring would in a mechanical watch. When a quartz crystal is properly cut (normally in the shape of a tuning fork) and mounted, it can be made to distort in an electric field by applying a voltage to an electrode near or on the crystal, distorting the crystal. This property is known as piezoelectricity. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The result is that a quartz crystal behaves like a circuit composed of an inductor, capacitor and resistor, with a precise resonant frequency.

This frequency (32,768 Hz) is equal to 2 to the power of 15 cycles per second. The frequency is ideal due to the fact that it can be stepped down cleanly to 2, thus allowing the wheels in the watch to easily translate that impulse into one tick per second.

Now you understand how the quartz crystal acts as a timekeeping element, we can move on to the motor that receives the impulse from the Integrated Circuit, which is under the instruction of the quartz crystal itself.

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A Pivotal Notion

The fundamental problem in watchmaking is friction. There are loads of ways to reduce the friction in a watch, such as jewelled bearings, correct oiling, new materials such as silicon etc. but one of the most straightforward is the shape of the pivots themselves.

In wristwatches in particular, there are two main shapes of pivot: conical and straight.

Straight pivots are used for the workhorse parts of a movement, such as the gear train and the barrel arbour. At ‘larger’ sizes, a straight pivot is perfectly functional and enables the end-shake of meatier parts to be adjusted accurately. But on the finest parts of the watch, which require pivots of an incredibly small size, a straight pivot would be too brittle – it could snap at the shoulder – and so conical pivots are used instead.

Conical pivots are used on Balance Staffs because they are stronger and can be manufactured smaller than straight pivots, which reduces friction. Also, the end shake of a conical pivot is decided by the tip of the pivot, rather than the shoulder on a straight pivot. This also reduces friction. Additionally, some watches use conical pivots on the escape wheel.

Theoretically, you could use conical pivots all over a watch, but this would require every jewelled bearing to have a cap-stone. Not only would it be a servicing nightmare, it is also totally unnecessary.

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Useful Facts and Definitions

Amplitude: Maximum displacement of any point from the dead point.

Vibration: Maximum displacement to maximum displacement, passing through the dead point.

Oscillation: One complete cycle of the escapement (2 vibrations).

The name of the rectangular slot in the mainspring (inner end) is the eye or eyelet and the bit attached to the outer end (it looks a bit like a spot-welded tail) is called the (slipping) bridle or hook.

A Mathematical Pendulum is a theoretical construct which imagines a weight (the pendulum) swinging as if it were attached by an arm, or bar, which is assumed not to exist. It is therefore a physical impossibility but a useful concept for the analysis of a pendulum’s theoretical effectiveness.
In other words: a theoretical or simple pendulum consisting of a mass suspended from a fixed point by a fine thread without mass, which is rigid and non-extendable. For experimentation only.

Recoil is not only a type of escapement, but also an action that occurs during the operation of the escapement.
Recoil is the slight backwards motion of the escape wheel just after the escape wheel tooth hits the locking face of either pallet.

If an escapement releases excessive energy to the balance, the impact can be reduced by using a weaker mainspring. If a weaker mainspring is unavailable, it would be acceptable to increase the depth of lock.
In really exceptional cases, you could try using a thicker oil on the escapement to reduce the amplitude (but this is unadvisable).

In a standard watch the balance spring is roughly 50% of the diameter of the balance wheel. Having fitted and timed a spring of known CGS (centimetre, gram, seconds system) one can calculate the correct spring for the given balance.

If you are working with a screw balance (that’s a balance wheel that has little screws sticking out of the sides for the purpose of poising and regulation), there are a few cheeky ways that you can speed the balance up if it not running at a sufficient tempo without changing the position of the pinning point. You could either move the screws in (imagine a spinning Ice Skater, drawing their arms into their chest to increase the speed of the spin), remove material (making the wheel lighter), or regulate it using the curb pins to make the active length of the hairspring shorter.

To establish the maximum variation, the watch must be tested in all five positions when fully wound and then again after 24 hours. The results are pooled and the maximum variation is the difference between the biggest gain and the biggest loss (effectively a ‘super-delta).

To check the isochronism of a watch, you must test the watch in all five positions when fully wound, and then again when half wound (often 24 hours later). The readings are then totalled (separately) and an average rate is found. The difference between those rates is then referred to as the isochronous error.

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Inventions and Facts

Here are some cool inventions and their inventors for you to remember:

In the eighteenth century (around 1702), Nicolas Fatio de Duillier and Pierre and Jacob Debraufre invented jewel bearings and pioneered their use in watches, greatly improving the isochronism and longevity of the movement. Initially, real jewels, such as ruby, garnet and diamond (and sometimes, in cheaper watches, glass) were used before the invention of a synthetic alternative in the early 20th century. Jewels reduce friction due to an incredibly low coefficient with steel (0.10-0.15 as opposed to 0.58 steel-on-steel).

In the late 1700s, Abraham Louis Perrelet invented the automatic winding weight for the pocket watch, which operated much in the same way as a pedometer. Although a theoretical improvement, practically it was largely ineffective due to the static nature of a pocket watch. However, in the 1920s, English watchmaker John Harwood patented his design for an automatic winding weight in wristwatches. Harwood’s design meant that the notion of Perrelet could now be applied functionally to wrist watches and thus increase the accuracy of the timepieces by reducing isochronous error caused by a drop in amplitude that can be avoided by the constant winding of the mainspring by an automatic weight.

Rumour has it that a new brand bearing Harwood’s name is on the horizon. How do you feel about new incarnations of defunct companies piggy-backing their forebear’s efforts to immediate heritage? Please let me know!

In the late 19th and early 20th century, Charles Edouard Guillame invented the revolutionary materials, Invar and Elinvar, which would later win him a Nobel prize in 1920 and go on to be used for the manufacture of watch components, most notably hairsprings. In particular, Elinvar (which comes from the words ELasticitie INVARiable) possess a very low temperature coefficient and is almost nonmagnetic. The consistency of timekeeping vastly improved thanks to Guillame’s invention and his original intention – to perfect an alloy to eradicate secondary error – was closer to realisation than ever before.

And one last fact that might come in useful:

The Greenwich Meridian is an axial line drawn from North to South, passing through Greenwich, London, England. This is zero longitude and is the beginning of the first time zone (Western European Time Zone).

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