Lesson 1

  1. Motive force
  2. Force and torque
  3. Spring-driven watches
  4. The Mainspring
  5. Terminology Review
  6. The Watch Going Train
  7. The Motion Works

1. Motive force

In this lesson, we will look more closely at the type of motive force used in mechanical watch movements which are known as spring drive movements.

Spring drive stores energy so that the movement only needs winding at intervals.  The stored energy generates  a torque, which is coupled to the gear train and drives the movement.

The motive force is required:

  1. to drive the escapement through the train, which provides the energy necessary to impulse the  balance and keep it oscillating.
  2. to drive the hands by means of suitable gear ratios in the train.

2. Force and torque

Before we proceed to far, you need to understand force and torque.

Force is measured in newtons. Take a 1 lb. bag of sugar and hold it out straight in front of you. The downward force you feel against your hand due to gravity is just under 10 newtons (the force varies slightly with latitude and altitude, but 9.8 newtons is a good approximation). This gives you some idea of how big a newton is. The newton is abbreviated to ‘N’, so normally we would write the above as 9.8 N.

We turn a shaft by applying force to it at a certain radius from its center, much like pulling on the end of a wrench. The turning force on the shaft is called the torque (Figure 1).

Figure 1

The greater the force we apply to the wrench, the greater the torque. Likewise, if we increase the radius at which we apply the same force (use a longer wrench), we again increase the torque.

The torque is directly proportional to both the force and the radius. Doubling the force doubles the torque, and doubling the radius also doubles the torque. If either the force or the radius is zero, the torque is also zero.

This means we can simply multiply the force and the radius together to work out the torque. Force, as we know, is measured in newtons. The radius is measured in meters. Torque, therefore, is measured in newton meters (N m).

Torque (in newton meters) = force (in newtons) x radius (in meters)

For example, a 2 N force applied at a 5 m radius generates a torque of: 2 N x 5 m = 10 Nm.

Similarly, we could apply a 5 N force at a radius of 2 m and generate the same amount of torque: 5 N x 2 Nm.

3. Spring-driven watches

The mainspring was invented around 1490. It is a long, thin strip of springy metal. Originally, mainsprings were made from carbon spring steel; nowadays, a rustproof steel alloy is frequently used.

A spring behaves according to Hooke’s Law, which can be expressed in various ways:

  • The extension of a spring is in direct proportion to the load added to it as long as this load does not exceed the elastic limit.
  • The deformation of a spring is in direct proportion to the load added to it as long as this load does not exceed the elastic limit.

For the watchmaker, this means that:

  • with a mainspring, the more you bend or wind the spring, the greater it will exert as it tries to straighten itself.
  • the driving force is proportional to the amount of winding.
  • the greater the effort required to wind up a spring, the greater the force it will exert during running down.

The force from a spring will vary, as anyone who has wound a clockwork toy, kitchen timer or a large clock will know. The force exerted during winding progressively increases as winding proceeds. Consequently, during running down (i.e., when the spring is driving the movement), the force exerted by the spring progressively   decreases. In order to maintain a uniform rate, it is essential that the torque exerted by the mainspring is as constant as possible. This is more readily achieved with a long, thin spring than a short, thick spring. (“Long thin” and “short thick” being comparative). The change of force from turn to turn is less with a thin spring than a thick one; a thin spring is also less likely to break.

There are other factors that influence the constancy of force provided by a mainspring, one of which is friction. During unwinding, there should be as little friction between the coils as possible, and what friction remains should be as constant as possible. The spring should therefore be uniformly tempered, well finished, of constant thickness, and lubricated with suitable oil or grease. Good quality mainsprings have rounded edges.

Imagine one end of a straight strip of metal is clamped to an immovable object. A round arbor is fastened to the other end, and forcibly turned so that the spring begins to wrap around the arbor in a spiral form. It takes a considerable amount of work to wrap the spring around the arbor fully, and the resultant energy is stored in the spring.

At the same time, the spring is trying to straighten and return to its relaxed (straight) form. If allowed to do so, it would release the energy stored in it. We could release this energy in two ways. Firstly, we could let go of the arbor, which would rotate freely in the opposite direction to that which we wound it as the spring straightened. Secondly, we could unfasten the outer end of the spring, which would then unwind itself around the arbor to unwind the spring.

3.1 Mainspring in a barrel

In most watch movements, the spring is coiled into a barrel. The outer end of the spring is hooked to the barrel wall. The inner end is hooked to the barrel arbor, which is free to turn in the middle of the barrel.

The spring is always trying to straighten itself until it is tight against the outside wall of the barrel. This is the normal state of an “unwound” spring in a barrel as shown in Figure 2.

Figure 2
Figure 2

3.2 The going barrel

The going barrel is the most common arrangement in a watch movement. The spring is wound from the center by the arbor, and the drive is taken from the outer end of the spring via the hook on the barrel wall and thus transmitted to the 2nd wheel’s pinon, which is turned by the barrel teeth. The 2nd wheel pinion then drives the rest of the train.

Since the spring is wound from its inner end and the drive taken from the outer end, a going barrel can be wound without interrupting the drive to the train. This is a big advantage of the going barrel. No additional power source is required to keep a going barrel in a watch functioning while it is being wound.

There is another arrangement which does not provide this advantage.  This is called the fusee, which we will learn about in another lesson.

3.3 Winding the spring

When we turn the arbor in the barrel, the spring gets wrapped tightly around the arbor. The spring must bend through a progressively smaller and sharper radius, which requires a certain force.

This force tries to rotate the arbor backwards or, if it cannot, to drag the barrel around in the same direction as we wind the arbor. After five to eight turns of the arbor, the whole spring is now tightly wrapped around it, with just the outer turn curving out to the hook on the inside of the barrel. At this point, the spring is fully wound (Figure 3). The entire length of the spring is now bent through a tighter radius than it was previously, so the “unwinding” force from the spring is considerable.

Figure 3

With a simple “going barrel” arrangement like this, we stop the barrel arbor from turning backward with a click (Figure 4), which is a mechanism that permits the barrel to rotate in one direction only. As the watch runs, the barrel rotates, driving the train via the integral 2nd wheel. As it does so, the spring inside gradually unwinds until it is resting against the barrel wall again and is thus prevented from straightening any further. At this point, the spring is unwound (figure 3).

Figure 4

In summary, we wind the spring in a going barrel by turning the arbor, which wraps the spring around the arbor in a tighter radius. This stores energy in the spring because we are bending it more tightly. The energy is released by letting the barrel rotate in the same direction as the arbor to unwind the spring. The force produced by the barrel drives the train. The spring is unwound when all the turns are pressing against the barrel wall (apart from the short loop inwards to the arbor).

4.4 Space in the barrel

In order to get the maximum number of turns from a mainspring, it must be of the correct length. For any particular barrel and spring thickness, there is a correct length of mainspring, which permits it to be wound up through the maximum number of turns and therefore produce the greatest time between winding the watch.

Calculating the ideal length of spring is surprisingly complicated and beyond the scope of this course. However, when the spring is of the correct length, the position of the inner surface of the inside coil of the spring, when run down, should coincide with that of the outer surface of the outside coil when wound up. This means that every bit of space in the barrel is being employed, and none is wasted.

This condition is met when the spring occupies half of the available volume in the barrel – that is, half of the volume between the arbor and the barrel wall. Such a spring takes up somewhat less than a third of the barrel diameter when fully unwound and slightly more than a third when fully wound up around the arbor (Figure 5).

Figure 5

Watches usually have a relatively large diameter arbor – about a third of the barrel diameter.

If a spring is too long, so it has more coils and takes up more space in the barrel, then there is not enough space to allow it to develop its full action after being wound.

Similarly, if there are fewer coils, the spring is too short; space that might usefully be occupied by giving the spring more turns around the arbor is wasted. Hence, a watch will run down too soon if the mainspring is either too long or too short.

With that said there are situations when working on vintage movements that you may not be able to find a mainspring of the exact manufactures specification and need to find a suitable replacement. Here is a video I made on how to do that.

4.5 The true shape of a spring

For ease of explanation, we have imagined the mainspring as starting out straight and then being forced into a spiral to fit in the barrel. In reality, it is not like this. Modern watch springs are normally ‘S’ shaped, which is intended to help equalize the force from the spring as it unwinds (Figure 6).

Figure 6

4.6 Dimensions and strength

There are three dimensions of interest: the length, the height, and the thickness.

Length

The length is self-explanatory but is quite difficult to measure directly because you would need to straighten the spring along its entire length. Alternatively, you can feed a piece of cotton thread into the spring, wrapping it around each turn, and then pulling it out to measure it. The length is important because, along with the thickness, it determines how much space the spring occupies in the barrel. However, in most cases, you do not need to measure it directly, as springs are normally catalogued by barrel diameter.

The strength of the spring is inversely proportional to its length. That is, if you double the length of the spring, you halve the force it exerts for a given amount of bend. However, when a spring is contained in a barrel, the number of “free turns” can vary as the spring unwinds. Any turns that are tightly wrapped around the arbor or inside the barrel wall are not free turns, resulting in the effective length at that point of winding being significantly less than the overall length of the mainspring. Consequently, changing the overall length of the mainspring may not result in a directly proportional change in the force (torque) exerted on the barrel.

Height

The height of the spring must be just right for the barrel. To allow it to rotate, it must fit within the barrel with a small clearance between the edges of the spring and the barrel and its end cover. It must not be too loose, as this could allow the spring to buckle and jam between the barrel and its cover. The height is most often referred to as the width.

The strength of a spring is directly proportional to its height. That is, a spring with twice the height exerts twice the force for a given amount of bend.

Thickness

The thickness of the spring makes a big difference to the force it exerts.

The strength is proportional to the cube of the thickness. In other words, a spring twice as thick generates 2³ = 8 times the force for a given amount of bend. So, a small difference in thickness makes a big difference in the force generated.

It must be remembered that these theoretical comparisons cannot be exactly applied to a spring in a barrel. For example, part of the length of the spring is required for hooking to the barrel arbor and the barrel – it is not all “active”.

Barrel diameter

The material suppliers and spring manufacturers know how difficult it is to measure the overall length of a spring because of the need to straighten it out. If you know the thickness of the spring and the inside diameter of the barrel, it is possible to work out the length required to fill the barrel correctly, but most suppliers have already worked it out for you.

Almost all catalogs let you select a spring using the height, thickness, and inside barrel diameter.

In some cases, the supplier will list all four measurements, and occasionally you will see two springs with the same thickness and barrel diameter but different lengths. This will affect the space available in the barrel.

4.7 Spring friction

The coils of a mainspring have to slide past each other as the spring unwinds. Too much friction or a buildup of old, sticky lubricant can cause the coils to stick together, and then suddenly slip. This produces an uneven driving force and can cause bad timekeeping. This on one cause of irregular or inconsistent amplitude readings on the timegrapher.

To prevent this, mainsprings must be clean and properly lubricated. Mainsprings are very lightly lubricated with a thick oil or grease. Too much grease will have the same effect as old sticky grease. Lubricants with graphite or molybdenum disulphide additives are particularly effective in this role. Moebius makes a few different grease’s for lubricating mainsprings (Figure 7). Don’t get the grease for mainsprings mixed up with the grease for the barrel walls on automatic movements. They are different and will be covered in a later lesson.

Figure 7

4.8 Spring failures

Up until the 20th century, mainsprings were made from carbon steel. There are two main reasons why carbon steel mainsprings are likely to require replacement:

  • Breakage
  • Loss of strength (adopting a permanent set)

In addition, there is one common type of damage: “coning,” where the spring adopts a cone shape due to careless removal from the barrel or hand winding springs into the barrel.

When a mainspring is “coned” it forces the spring to drag across the barrel top or the bottom of the barrel, causing friction and lowering the possible power available out of the barrel which will lower the amplitude at the balance wheel. The other sign of “coning” is wear on the barrel lid or bottom of the barrel drum and fluctuating amplitude.

Breakages invariably occur at stress concentration points where cracks can initiate and then propagate through the remaining material until eventual breakage. Stress concentrates at sharp nicks, which can be seen at the edge of a spring or on the corners of the holes serving as hooking points or at points of distortion occurring when the diameter of the barrel arbor is small. Areas of corrosion, sometimes as small as pinpricks, can also create stress concentration.

When working with steel mainsprings, it is important to clean them thoroughly, check for corrosion, and then lubricate them immediately. Do not handle them with your bare hands, as the sweat from your fingers will lead to corrosion.

There is a theoretically ideal proportion between the inside diameter of the barrel arbor and the thickness of the spring. The thinner the spring, the smaller the curve around which it can be safely bent without risk of breaking. For a thick spring, a larger arbor diameter is needed. If the barrel arbor is smaller and, particularly if the hooking projects too much, the spring is more likely to break.

The eye at the outer end will sometimes tear out where it is attached to the barrel, and if the hooking projects too much, there will be a hump causing distortion and uneven winding. Watch mainsprings made from carbon steel sometimes break as a result of temperature changes – if a fully wound watch was placed on a cold surface, the contraction caused by the change in temperature of the spring caused the spring to break. The new steel watch mainsprings do not break so easily.

A permanent set occurs when the steel has lost some of its elasticity (also referred to as “fatigued,” “tired,” or “set”). A spring with a permanent set often has several loose coils in the center (see Figure 7.1). If the loss of power is severe enough, the movement will not run be able to deliver power satisfactorily. The fault can be difficult to diagnose without proper inspection.

Figure 7.1 Loose center coils in mainspring

New springs in old movements

Modern alloy springs for watches are often stronger than the old carbon steel springs of the same dimensions in original condition. For this reason, some people advocate using a spring slightly thinner than the original steel one if the mainspring needs replacing, in order to avoid over powering the movement. I believe that in vintage movements that usually have significant wear to parts, that a stronger mainspring is better to use as long as it doesn’t cause knocking. For watch springs, there is usually a wide range of thicknesses, and it is often recommended that a spring is selected of the same length and height, but the next size down in thickness.

Modern springs are usually only available with a loop-end or a hole-end. Check the old spring and be prepared to modify the new one with the appropriate hooking if necessary.

A mainspring breakage can cause severe damage to the movement, stripping teeth and breaking pivots. There is an argument for protecting a valuable movement against such a failure by replacing its existing springs with modern alloy ones before the steel ones break. This is a controversial topic, as it goes against the principles of conservation to replace parts unnecessarily. On the other hand, protecting a watch movement against damage is also a good conservative practice. If you do decide to replace the existing, unbroken, steel mainsprings, place them in a sealed labeled plastic bag and store it so it can always be refitted in the future if needed.

You should use a proper mainspring winder when replacing mainsprings to protect the spring from distortions like cupping. We will look at mainspring winders in upcoming lessons.

4.9 Spring Attachment Points

Inner Attachment Points

All springs are hooked onto the barrel arbor in the same way. The end of the spring is annealed and a hole, or eye, is made near the end of the spring – see Figure 8, and this fits over a hook-shaped projection on the arbor (Figure 8.1). The softened end is easily shaped to conform to the circumference of the arbor.

Figure 8
Figure 8.1

Often times when replacing the mainspring on a vintage watch with a new modern mainspring, the inner coil that fits onto the arbor is too small. This is a video that will show you how you can correct this problem.

Outer hooking

There are various ways of hooking the spring to the barrel. (Figure 8)

Figure 8-Common Mainspring Ends

Hook and Eye (older watches)

The hook and eye method is basically the same as that used for the arbor. The end of the spring is annealed, and a hole is made near the outer end of the mainspring (Figure 8 Type H). The hole should have rounded, rather than square, internal corners in order to avoid stress concentration points that can cause the spring to break at the eye. The eye fits over a hook formed on the barrel wall. The hook may be formed by stamping a lip out of the barrel wall or by screwing or riveting a hook to it.

The hole made in the spring weakens it, and if too much force is used when winding the movement, the spring may crack at the sides of the hole. This can also happen with poor quality springs. Nevertheless, the hook and eye system was widely used in some watches, to good effect. Modern alloy springs almost never break.

When properly made, the hook and eye will tend to anchor the spring flat against the barrel wall, rather than acting as a hinge and allowing the spring to pivot inwards. Anchoring a spring in this way helps keep the coils separate, minimizing friction between them.

The end of the spring is annealed to prevent breakage arising from the high bending angle.

Mainspring End Options

Mainspring for barrels in automatic movements

The mainspring for automatic watches is not attached to the barrel wall and is designed to slip inside the barrel when the torque reaches a certain point. This prevents it from breaking. (Figure 9) The outer end of the spring is bent sharply back on itself to form a hook. In order for the mainspring to slip around the inside of the barrel, grease such as seen on the Moebius oil chart (Figure 7) under barrel walls is used. Lubrication will be covered in future lessons.

Figure 9- Mainspring for Automatic Movement

4.10 Barrel faults

End shake

The arbor should be free to turn in the barrel with a small amount of end shake (free play in the end-to-end direction).

If the end shake is too much or too little, it can be adjusted by bowing the barrel cap inwards or outwards slightly. We will look at how to do this in lesson 4.

Wear

The holes in the barrel bottom and cap can wear, causing too much side shake (free play of the arbor in a side-to-side direction, i.e., when the hole is larger than the arbor pivot). The arbor can also wear away at the arbor holes in the main-plate and bridge causing too much side shake.

The big problem with side shake is that it allows the barrel to move bodily away from its original intended position as well as tilt over. This can have two adverse effects:

  • It may alter the depth between the 2nd or center wheel gear teeth and the next pinion in the train.
  • The barrel can rub against the plates, damaging them and causing power loss – watches are especially prone to this (Figure 10).
Figure 10

 

 

4.11 Recoiling Click

It is undesirable to leave the coils of the spring wound tightly against each other after winding. This can expel the lubricant and cause the coil to stick together. It can also produce so much force in the train that a watch with a lever escapement suffers from “knocking”. The balance swings through too large an arc (to much amplitude) so that the impulse pin knocks the lever against the banking pins too hard. This causes a severe gaining rate and rapid wear. We will look at watch escapements in Lesson 8, when these terms will become clear.

To avoid this, most watches have a recoiling click. This works much like a normal click, except that it allows the ratchet to recoil (turn backwards) slightly before locking it (Figure 11).

Figure 11

There are a number of ways of achieving a recoiling click. One approach is to extend the pivot hole in the click into a slot. The click is drawn to the forward end of the slot by the pull from the lifting surface of the ratchet tooth when the arbor is rotated in the winding direction. When the arbor is released, the click is pushed backwards until the rear end of the slot meets the pivot, allowing for some reverse travel of the ratchet.

In watches, the ratchet wheel normally has a standard tooth form. This is because the ratchet wheel must be driven by another gear.

Two types are shown here, although there are other variants. Figure 12 shows the most commonly used type in modern Swiss watches. It rotates anticlockwise to the position shown here during winding. The click spring is mounted beneath the click and is engaged with it, holding it in engagement with the ratchet wheel. When the winding mechanism is released (Figure 13), the ratchet wheel recoils anticlockwise, carrying the click with it until the outer diameter of the click locks against the ratchet wheel. The screw head on the ratchet wheel shows the amount of recoil achieved.

Figure 14 shows a type used on lower grade watches. The click acts on the transmission wheel, which lifts it upwards during winding. When released (Figure 15), the click descends and locks between the two wheels. Again, the screw head shows the recoil achieved.

5.0 Terminology Review

Before we proceed, I just want to remind you of the terminology. The train consists of wheels (large) and pinions (small) mounted on arbors. Each arbor has a wheel and a pinion rigidly mounted on it, so on a given arbor, the wheel and pinion rotate together. Wheels have twenty or more teeth, and the word “leaves” is used instead of “teeth” when we are talking about pinions.

The complete assembly of an arbor with pivots, wheel, and pinion is called an arbor assembly.

 The Basic Principle

Figure 16 shows a wheel meshing with a pinion. In the watch train, the wheels generally drive pinions. The point of engagement (or “meshing point”) is where the teeth on the driving wheel mesh with the leaves on the driven pinion, pushing them round. You will note that the direction of rotation is reversed with a meshing pair — a wheel driving a pinion.

Figure 16

Each time a tooth on the driving wheel goes through the meshing point, it engages a pinion leaf and turns the pinion by one leaf.

There are sixty teeth on the driving wheel, so for a full turn, sixty teeth pass through the meshing point. There are only ten leaves on the driven pinion, and when all ten have passed through the meshing point, the driven pinion has completed a full turn, and the first leaf is back at the meshing point again. Therefore, the driven pinion must go round several times while the driving wheel turns just once.

In fact, as the driven pinion has only ten leaves, but sixty teeth pass through the meshing point when the driving wheel makes one turn, the driven pinion must go round six times (10 leaves x 6 turns = 60 leaves through the meshing point).

So, we have an important relationship between the wheel and the pinion, which can be stated in several ways.

When a wheel drives a pinion:

  • The pinion goes round faster than the wheel.
  • The pinion makes more turns than the wheel.
  • The pinion takes less time to make one revolution than the wheel (i.e., it has a shorter period of rotation than the wheel).

It is important to grasp these principles because they are fundamental to the train calculations on the following pages.

How much faster? How many more turns? How much shorter is the period of rotation?

The answer is always the same: it is the ratio between the number of teeth on the wheel and the number of leaves on the pinion. The only thing that changes is which way round we express the ratio.

This Lesson will present two approaches to train calculations; one relies on a logical understanding of what happens in watch trains, which we will call the intuitive approach. The second approach uses an equation.

5.1 The Intuitive Approach

At the risk of seeming repetitive, we just want to state once more that whether we are talking about:

  • The turns ratio,
  • The speed ratio, or
  • The period of rotation ratio (where “period of rotation” is the time taken to complete one revolution),

… between two gears, the relationship is always the same: it is the ratio between the wheel teeth and the pinion leaves. Indeed, this is also true when we are talking about the torque ratio, the ratio between the torque on the arbor with the wheel and the arbor with the pinion.

Figure 17 shows a center wheel with 60 teeth, driving the third pinion with eight leaves.

Figure 17

The ratio between them is therefore 60/8.

We have also drawn in the third wheel for completeness, even though it does not figure in the calculation.

Let us try some simple questions to see how intuitive you are:

If the center wheel turns once per hour (once in 3600 seconds), what is the period of rotation of the third wheel?

The third wheel is connected to the third pinion, so…

… firstly, we must look at our ratio. Should it be:  60/8 or 8/60?

By inspection, the third wheel turns faster than the center wheel, so its period of rotation must be shorter. That means we need to multiply the center wheel’s period of rotation by 8/60 in order to produce a smaller number. Multiplying it by 60/8 would produce a larger number.

So the ratio will look like this:   8/60

Using this in our calculation gives us:

The third wheel period of rotation = center wheel period of rotation x 8/60

                                                                                  = 3600 x 8/60

                                                                                   = 480 seconds (i.e., 8 minutes)

For one turn of the center wheel, how many turns does the third wheel make?

By inspection, the third wheel makes more turns than the center wheel, so the ratio will be this way up: 60/8

Using this in our calculation gives us:

The third wheel turns = 1 center wheel turn x (60/8) = 7.5 turns

The center wheel has a torque of 0.45 Nm. What is the torque at the third wheel?

By inspection, the torque at the third wheel is less than the torque at the center wheel, so the ratio will be this way up: 8:60

Using this in our calculation gives us:

The third wheel torque = center wheel torque x (8/60) = 0.45 x (8/60) = 0.06 Nm

(I have ignored frictional losses in this calculation.)

The intuitive approach uses the ratio between the wheel teeth and the pinion leaves; all you need to do is decide which way up the ratio should be. If you decide whether the answer should be larger or smaller than the information you are starting with, it will become clear which way up the ratio should be.

6.0 The Watch Going Train

Watches have three characteristics:

  • They use a going barrel.
  • They run for at least 30 hours.
  • They use a lever escapement. (With the exception of much older watches)

Based on this information, we can make some immediate predictions:

  1. Although a going barrel will only run for around six-eight turns, the watch only needs to run for about 30 hours, so no intermediate wheel is required.
  2. The high-speed escapement with its small, fast-turning escape wheel necessitates the use of a fourth wheel.

In all other respects, the train is much like a clock train, only much smaller (see Figure 18). Many mechanical watches use a center seconds hand, but we will explore that in later lessons.

Figure 18

Summary: The great wheel (mainspring barrel) in a watch train drives the center pinion directly but requires a fourth wheel (because it uses a high-speed escapement).

7.0 Motion Work

The “Motion Works” is a short train of wheels and pinions that ensures the hour and minute hands move at the correct relative speed, in the right direction, and on the same center.

The “right relative speed” is a ratio of 12:1 — that is, twelve turns of the minute hand produce one turn of the hour hand. This is a reduction ratio, whereas the going train in mechanical clocks and watches usually involves step-up gearing.

“The right direction” means both hands moving clockwise. As you know, a single meshing pair reverses the direction of drive. Therefore, we need two meshing pairs of pinions and wheels between the minute and hour hands. Note that in motion work, as it is a reduction ratio, the pinion is driving the wheel.

Finally, it must allow the two hands to be mounted concentrically.

7.1 Two-Stage Reduction

Remember that we will have two meshing pairs of wheels and pinions in our motion work, thus reversing the direction twice because we want both hands to move in the same direction.

We could, therefore, achieve our 12:1 reduction in two stages, which is the most common arrangement used in all modern watches.

A typical tooth count would be:

  • 14-leaf cannon pinion: 42 teeth minute wheel = 3:1 reduction
  • 12-leaf minute pinion: 48 teeth hour wheel = 4:1 reduction

This gives a total of 12:1 reduction.

The friction drive arrangement in the cannon pinion pipe ensures that the cannon pinion turns with the extended center arbor.  The hour wheel turns freely on the cannon pinion pipe. Figure 19 shows the layout.

Figure 19

7.2 Friction Drive in Watches

We must be able to move the hands (and thus the motion work) independently of the train in order to set the time. To allow this, the motion work is driven via a friction drive. The friction must be sufficient to drive the hands reliably in normal use but free enough to allow the hands to be moved without damaging the train or escapement when manually winding the watch.

The part of the cannon pinion which extends outward to carry the minute hand is called the pipe. Figure 20 shows the arrangement.

Figure 20

In watches, this is achieved by making the cannon pinion attach by snapping onto the part of the center arbor that extends in front of the plate. The walls of the cannon pinion pipe are reduced to form a snap groove, which is “pinched” inward or indented to create “pips” that provide friction between the cannon pinion pipe and the back taper on the center arbor.

The center arbor has a groove cut in it called the back taper. The snap groove in the cannon pinion fits tightly over the back taper on the center arbor and tends to ride down it, effectively “pushing” the cannon pinion against the shoulder on the center arbor. The friction points are where the cannon pinion presses against the shoulder and where the snap groove presses against the back taper.

This provides sufficient friction to drive the cannon pinion from the center arbor in normal use. While the hands are being set, the cannon pinion slides around the center arbor.

7.3 Centre Seconds – Watches

There are various methods for driving a center seconds hand. We will look at the approach used in modern Swiss watches.

It is helpful to contrast the modern Swiss system with the layout of a simple train without center seconds. We show this in Figure 21; Figure 22 gives a close-up view of the relevant parts of the train in a center seconds movement.

Figure 21

Referring to Figure 22, the arbor for the center wheel and pinion (red) is hollow. The bottom end of the hollow arbor runs in a jewel in the bottom plate, as usual (we will talk more about jeweled bearings in a later Lesson). This hollow center arbor extends beyond the bottom plate for the cannon pinion to be fitted as usual. The arbor is short: the top end runs in a jewel that is mounted in a special sub-plate, about halfway between the top and bottom plates.

Figure 22

The center wheel drives the third pinion and wheel (green) in the usual way.

The sub-plate carries another jewel, concentric with the center wheel jewel, but above it and of a smaller bore. This holds the arbor for the fourth wheel and pinion (orange), which therefore sits above and concentric with the center wheel. The fourth pinion is driven by the third wheel, as usual.

You can see that the fourth arbor goes through the hollow center arbor.

The end result is that the center arbor turns the cannon pinion directly, and the fourth arbor turns the seconds hand directly.

The rest of the train is conventional. The great wheel (not shown) drives the center pinion; the fourth wheel drives the escape pinion as usual. The only difference from a basic train is in the position of the fourth wheel, which is concentric with, and above, the center wheel.