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From Spring to Spring – Mechanical Watch Movements Explained

Steve Mansfield
Steve Mansfield

Mechanical Watch Movement What is it?

Mechanical watch movement is a generic term that refers to the springs, wheels and gears that are inside a mechanical watch and maintain the time for the watch.

The classification of mechanical watches is further subdivided into automatic watches (sometimes referred to as self-winding watches) and manual watches. The difference between these two is the manner in which the mainspring is wound. Apart from the winding method the rest of the watch movement is effectively the same. 

Variations of a Mechanical Watch Movements

An automatic watch, as the name suggests, includes a mechanical system that automatically winds the mainspring. Most commonly this is driven by the motion of the wearer’s arm.

A watch that is mechanical and not wound automatically is generally referred to as manual watch. Manual watches need to be manually wound at regular intervals by the wearer to ensure accurate timekeeping. This is usually achieved by rotating the crown of the watch which winds the mainspring. Of course there are also hybrid movements which can be wound both automatically and manually.

Functional Units

Interestingly the principal functional units of a mechanical watch movement correspond to the major functional units of a motor vehicle drive train. Both start with an energy store, in a car the fuel tank or battery pack and in a mechanical watch movement it is the mainspring. The fuel in an engine flows into the motor, either internal combustion energy or electric motor, and creates rotary power, in a watch movement the mainspring unwinding creates the rotary power which is delivered through the barrel. This rotational power is then distributed through the gearbox, in a car the gearbox is set up to allow the vehicle to operate efficiently over a large spectrum of speeds (this is managed electronically in an electric vehicle). In a watch movement the wheel train (or gear train) the gears are selected to deliver various speeds (hours, minutes and seconds) simultaneously.

These three functional areas, power reserve, power release and finally, harnessing the power into utility are all housed between the mainspring and the balance spring in a mechanical watch movement. But how exactly do all those gears, springs and wheels work together to deliver an accurate time keeping instrument? Let’s have a look.

Mainspring

Without the mainspring there would be no power for the movement to operate. This is the starting point for keeping time. It is the energy store for the movement. When the mainspring is unwound it is like the fuel tank in your car being empty.

The mainspring of a movement is constructed from a thin ribbon of spring steel approximately 30cm (12 inches) long and is contained in the “barrel”. The ribbon is coiled around a strong spindle (called an arbor in watchmaking) creating a spiral spring. One end of the spring is attached to the arbor and the other end is fixed to the barrel.  When the spring is wound up the coils lie close together upon the arbor and as the spring runs down, the coils separate from the arbor and lie close to the barrel.

This is the basic process for storing power. But to drive the watch movement accurately there are a number of issues that need to be overcome. 

So what exactly are the issues?

The three biggest issues are first, how to allow winding of the spring while the mainspring continues to deliver power to the watch movement? Second, how to ensure that the power is delivered at a constant rate to the watch movement regardless of how wound (or not) the mainspring is? Third, how can the spring be protected to ensure that it is not over-wound or damaged?

3 Solutions To Make The Mainspring “Work”

Winding the mainspring, whether manually or automatically, is critical to ensure accurate timekeeping. The problem is, if the mainspring is being wound, how can it continue to deliver power to the movement as winding will, by definition, remove pressure from the mainspring?

Solution 1 – The Barrel

This problem is overcome by a remarkably simple but effective solution. The mainspring is completely enclosed within the barrel and the arbor is concentric with the barrel. When torque is applied to the arbor through the winding gears the arbor rotates independently of the barrel and increases the tension in the mainspring.  The other end of the mainspring, which is attached to the inside of the barrel, maintains a pressure on the barrel and energy into the movement by the barrel rotating. Thus the arbor can wind the mainspring without affecting the barrel rotation.

Solution 2 – Make the Mainspring Extra Long

The second problem is to ensure that a constant torque, or as constant as possible, is applied from the mainspring regardless of whether the mainspring is fully wound or 20% wound.  This was a technical challenge for watchmakers through history and there have been many innovative solutions.  The solution used today is only available because of technical advances in the production of spring steels.  How exactly?

The mainspring is designed to be significantly longer than necessary to retain sufficient energy to maintain the watch operational for the desired length of time. The mainspring is then placed in the barrel in a partially wound state, this results in the force being applied to the movement mechanism only coming from the “mid-range” of the coil spring and the torque delivered from the spring around this mid-range is even and consistent.

A key point to note is that this explains why it is dangerous to disassemble the barrel and mainspring – unless you know what you are doing. This is because even when the spring is fully unwound in the barrel it remains in tension and, if released, can fly out with remarkable force.

Solution 3 – Insert the Bridle

Finally, protecting the spring from over-winding is a constant concern of many but this concern has been addressed in every modern movement. The solution is commonly referred to as a “slipping mainspring” or an “unbreakable mainspring.”  This protection mechanism is achieved by the outer end of the mainspring not being permanently attached to the barrel but to a circular steel expansion spring, called the “bridle”.  The bridle presses against the inside wall of the barrel that is engineered with serrations or notches. The tension in the bridle against the serrations on the surface of the barrel are tuned to ensure that should the mainspring get to the limit of its tension the bridle will slip around the inside of the barrel. If the bridle is slipping it is not possible for the mainspring to be wound further thus protecting it from over winding.

A further advantage of the slipping mainspring is that it also protects the winding gear as the maximum torque that the winding gear will face is defined by the  friction between the bridle and the serrations inside the barrel.

Wheel Train / Gear Train

Wheel train and gear train are used interchangeably to refer to the wheels and gears that are situated between the watch barrel and the escapement mechanism in a mechanical movement.  If the main plate of the movement is considered the divider between the top of the movement (closest to the watch face) and the bottom of the movement (what can be seen if the case back is open) then the wheel train is generally located on the bottom of the watch movement with some of the wheels having axles that protrude through the main plate to the top of the movement.

Remember what we discussed earlier?

All the energy to operate the timekeeping mechanisms in a mechanical movement is delivered from the mainspring barrel rotating, which itself is driven by the mainspring slowly unwinding. As the mainspring slowly unwinds it transfers its energy through the barrel and transforms this very slow moving rotational force into progressively faster rotational movement that is metered into useful periods.

The gear train can have many different configurations (and nomenclature), but for our purposes we will use the Sellita SW200-1 as our reference movement. This is a high quality mechanical movement and is the movement we use in all our SNGLRTY OHI2 watches so I know if reasonably well.

The mainspring barrel has gear teeth on its exterior that engage with the next wheel in the gear train. The size of the barrel determines the number of teeth on the barrel and the following wheel. The wheel that comes immediately after the barrel is normally referred to as the center wheel (if located in the center of the movement) or the great wheel if offset. In the SW200-1 it is in the offset position so we refer to it as the great wheel. The great wheel and barrel are designed so that the great wheel rotates once per hour and each subsequent wheel in the gear train rotates progressively more quickly. The second wheel and the third wheel are designed in size (and number of teeth) such that the second wheel turns once per minute.  In the SW200-1 the second wheel is located in the center of the movement and its arbor extends to the top side of the movement – the second hand is attached to this arbor. The third wheel enmeshes with the escapement which is the beating heart of the timekeeping mechanism.

Did you know?

The enemy of accuracy in a mechanical watch movement, or any mechanical system, is friction.  In order to reduce friction to a minimum each wheel is very finely engineered with hardened teeth and pinions. The friction is further reduced by the axle of each wheel rotating within synthetic jewel bearings, typically these bearings are made of synthetic sapphire or ruby. The jewel bearings are very hard and extremely smooth and reduce the friction of each moving component the minimum possible.

Escapement

The escapement is the heartbeat of the watch, this is the beat that maintains the accuracy of any mechanical timekeeping instrument you may wish to consider.  History has documented a vast variety of escapement mechanisms in watches – vertical escapement, lever escapement, horizontal or cylinder escapement, duplex escapement, chronometer or detached escapement, lever chronometer escapement – to name but a few. Today the lever escapement is in common use throughout watchmaking and is the style used in the SW200-1 movement.

Purpose of the Escapement

The escapement is located on the bottom of the movement and consists of two moving parts, the pallet fork and the escapement wheel. The escapement has two specific purposes, first it stops the mainspring and the gear train from unwinding and releasing all the mainspring energy in an uncontrolled manner. Second,  it provides the regulation to the gear train to ensure it keeps time accurately. The escapement achieves this by allowing the energy transmitted from the mainspring through the gear train to be released in small metered increments.

The pallet fork pivots in its center and is “anchor” or “T” shaped with typically 2 jewels positioned at each end of the top of the “T”. These two jewels are referred to as the pallets and are the contact points with the escape wheel. The escape wheel has very specifically shaped teeth that allow the pallets hook, lock and release the escape wheel in sequence. This sequence of actions allows the escape wheel to rotate in metered impulse increments. The escapement operates very rapidly and the faster the escapement operates the more accurate the movement.

Regulation of the Escapement

Regulation of the operating speed of the escapement is governed by the balance wheel (details below), this wheel oscillates back and forth in an isochronal beat, or each beat takes an identical period of time. The balance wheel interacts with the smaller end of the pallet fork, unsurprisingly this end of the pallet fork is shaped like a fork. A protruding pin, called the impulse pin, on the balance wheel hits one side of the fork end of the pallet fork, this releases the diagonally opposite pallet jewel from the locked position against the escape wheel tooth. As the pallet jewel slips free from the escape wheel tooth a small impulse of power is delivered from the mainspring through the gear train. The pallet fork in turn pushes the impulse pin launching the balance wheel into another swing and the process repeats so long as the watch has mainspring power.

The characteristic watch ticking sound is made by the pallet jewels as they catch the escape wheel teeth. Each incremental escape wheel movement is called a beat and the higher the number of beats the more accurately the watch will keep time.   A common watch beat rate is 21,600 bph which is 6 beats per second or 3Hz, as there are 2 beats to each full cycle.  The Sellita SW-200 operates at 28,800 beats per hour or 4Hz which provides increased accuracy. Just for completeness the calculation is 8beats x 60 seconds x 60 minutes equals 28,800 beats per hour.

Reviewing the function of the escapement it is clear that it does not consume power except for minute friction losses. It is the pallet fork that transfers power to the balance wheel, regulates the gear train and dissipates the energy from the mainspring. The gear train’s ability to transform the tiny movement of the mainspring into thousands of oscillations of the escapement allows a watch to keep time over an extended period of time with a single winding.

Balance Spring Wheel

The balance wheel is the most delicate part of the mechanical movement. It also creates the accuracy we need for timekeeping in a mechanical movement.  Positioned on the bottom of the movement in the Sellita SW-200, it is mounted in a shock absorbing system complete with jewel bearings. The balance wheel and the balance wheel spring, a tiny spiral hairspring, interact to create an isochronal movement.  The balance spring is fixed at its outer end to the watch main plate and the other end to the arbor or axle. It is assembled in the movement so that when the hairspring is in its neutral position the impulse pin is sitting in the middle of the fork of the pallet fork when the pallet fork is in its neutral position.

The principal function of the balance wheel is the same as a pendulum in a grandfather clock but with 2 important changes. First, and most obviously, the mechanics are shrunk to fit into a watch. The second is a bit more subtle.

The objective of a pendulum in a grandfather clock and the balance wheel in a watch movement are identical – to create isochronal (uniform in time, having equal duration. recurring at regular intervals) beats. The key difference is that the pendulum is regulated by gravity, it slows and then accelerates the pendulum around its arc. The balance wheel, on the other hand, works independently of gravity. In fact a watch can be in any spatial orientation to the force of gravity and regardless of this orientation the movement of the balance wheel must be regular. As the balance spring accelerates and slows the movement of the balance wheel it is independent of gravity and the spatial orientation of the watch movement. 

Regulation

The balance wheel and the balance spring are designed as a unit so that they oscillate at the frequency the escapement is designed to operate at. For example, the escapement in the Sellita SW200-1 is designed to operate at 28,800 beats per hour which is 4Hz, so the balance wheel must go through a complete cycle in one quarter of a second.  Alternatively you can think about the impulse pin interacting with the pallet fork 8 times per second.  You can see that the balance wheel is moving quickly, especially considering that generally the arc of the balance wheel is anywhere between 100 and 400 degrees.

Ensuring that the balance wheel and the balance spring deliver the regular beat is a delicate balance between the two. The mass and therefore the inertia of the balance wheel needs to be carefully proportioned with the balance spring. The forces exerted by the balance spring is governed by Hooke’s law that states that the force exerted by a spring is proportional to the movement away from its neutral position. Alas Hooke’s law is not accurate enough for mechanical watch movements operating in the real world. This is where the skill of the watchmaker is tested as the selection of the balance spring and balance wheel are so critical. In the early years of watchmaking the combinations were determined by experimentation.

The oscillation frequency can be modified in two ways. The first is by adjusting the moment of inertia of the balance wheel, if the moment of inertia is reduced the wheel will rotate faster and vice versa. The second is by adjusting the length of the spring, if it is made shorter the balance wheel will speed up and if it is lengthened it will move slower. 

Adjustment of the inertia of the balance wheel is the method used in watches that demand extreme accuracy. More commonly the the balance spring is adjusted. This adjustment is achieved by a very elegant solution. The hairspring has regulator pins that allow the watchmaker to adjust the active length of the spring, this alters the force the spring will impart for a given displacement. If the spring is applying a different force for a given displacement the balance wheel swing rate will be changed and therefore the speed of the entire watch. This process is referred to as  regulating the watch – adjusting the balance spring so that the watch keeps time accurately.

End of Timekeeping

For a mechanical watch movement this completes all the components that are dedicated to the process of keeping time.  Everything that is included in a watch after this point is either for the display of time or for accumulating energy into the mainspring. At this point it is worthwhile to make a few observations and comparisons.  When I look at a mechanical movement it always surprises me how similar the basic operations of a quartz watch are to a mechanical watch. The main difference is that the quartz movement is working at an atomic level, but keeping time comes down to managing oscillating systems, the mechanical system is regulated at the physical level by the balance spring, the quartz movement is regulated digitally in the clocking digitization.

The other, and somewhat more curious observation, is that a mechanical watch movement both starts and finishes with a spring.  The spiral mainspring starts the process by accumulating and then releasing energy through the gears and escapement. Then, at the other extremity, the balance spring, another spiral spring, is regulated to ensure the movement is keeping time as accurately as possible.  It brings a beautiful, elegant symmetry to every mechanical movement.

The mechanical technology that we have just read about is based on the accumulation knowledge from centuries of experimentation and diligent innovation.  To really appreciate the magic of today’s watch engineering take a walk through the history of timekeeping with one of the finest watchmakers of his time. Endmund Beckett, Lord Grimthorpe who, in 1883 wrote the “A Rudimentary Treatise on Clocks, Watches & Bells” is so good we are giving it away for free! His observations from over 135 years ago will give you a new appreciation of time and timekeeping mechanics

 

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