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What Puts The Automatic In Automatic Watches?

Steve Mansfield
Steve Mansfield

SNGLRTY WATCHAutomatic movements are what puts the “Automatic” in an automatic watch. Almost any reasonable quality mechanical watch these days is an automatic movement. The automatic refers to the automatic winding process.  We are all aware of the strange mass that rotates in the back of our watch to keep it wound up, but how does all that really work?   

Who Had The First Self-Winding Watch

Initially referred to as “self-winding watches” the first automatic watch was documented in the times of Napoleon I, he had a watch that wound itself up as he walked.  It operated by means of a weighted lever with a slight spring under it that bounced up and down on each step which then ratcheted the barrel on his pocket watch (there were no wristwatches in those days). This movement was designed by Abraham-Louis Perrelet.

The modern version of the automatic movement was invented by a British watchmaker in 1923.  John Harwood invented the current system of automatic winding that relies on an eccentric weight, or rotor, driving gears and ratchets.  There have been unidirectional automatic winding mechanisms which are simple – especially after understanding the bi-directional system (only one ratchet rather than two).  We are going to take a dive into the details of the bi-directional system and then look at how it is implemented into a watch movement in more detail. 

But How Does It Really Work?

To understand the automatic winding process it is important to understand the difference between a ratchet and a gear. A ratchet mechanism or a ratchet and pawl is used in mechanical transmission chains so that a rotational force, or torque, can only transmitted in one direction and will freewheel in the opposite direction.  A good example of this which most people have encountered is on a bicycle, when pedaling the gears engage and drive the gears, when not pedaling the rear wheel turns freely.

How The Ratchet Works

Internal schematic of a four pawl ratchet
A Schematic of the Operation Of A Ratchet and Pawl

An example of the ratchet and pawl is in the image to the right.  When the outside case is rotating anti-clockwise it is free wheeling and no  torque is transmitted.  When it is rotating clockwise the pawls (the 4 lever like pieces on the inside) engage with the gear teeth and then transmit a rotational force.  It is the pawls slipping over the internal teeth that creates the classic clicking noise on a bicycle when free wheeling.

And a Gear?

A gear, on the other hand, that is not attached to a ratchet, is always engaged in the transmission of  rotational force regardless of the direction of rotation. It is through the interaction between the ratchets and gears that the bi-directional movement of the rotor is transformed into a unidirectional rotational torque to wind the mainspring.

How The Ratchet Is Integrated Into the Winding Mechanism

The self winding mechanism is created from two ratchet gear wheels and then a number of reduction wheels (usually two) that wind the mainspring through the arbor.  The key to this mechanism is in the interaction between the two ratchet gears and the rotor driving gear.

The ratchet gear wheels have 2 levels to them.  The upper level engages with the drive gear that is attached to the mass or rotor and are indicated by the grey discs on the schematic diagram below.  The lower level of the ratchet gear engages with the reduction gears and drives the reduction gears and winds the mainspring arbor.  These are indicated in blue in the below schematic diagram. 

Schematic of The Power Transmission within an Automatic Watch Movement For The Rotor Rotating Couterclockwise
Schematic of The Power Transmission within an Automatic Watch Movement For The Rotor Rotating Counterclockwise

The upper and lower gear wheels are joined by a ratchet, so gear 1 and gear 1A are linked by a ratchet as are 2 and 2A linked by a ratchet.  The key difference between the two ratchet gears (indicated as 1 and 2 above) is that one ratchet gear will only transmit torque clockwise (1) and the other will only transmit torque anti-clockwise (2).   

The upper gears are interlocked either directly with each other and/or through the linkage provided by the driving gear which enmeshes to both the gears (1 & 2 above).  This results in the upper wheel always moving in unison, this can be clearly seen in the video above. If the rotor rotates in a clockwise direction then the upper gears rotate in an anticlockwise direction and vice versa.

The challenge to understanding this mechanism is how two gears (1 & 2) that always travel in unison bi-directionally manage to transform this movement into a single gear wheel that progressively travels in a single rotational direction.  Here the elegant simplicity of the ratchet linkage solves the problem.

When The Rotor Rotates Counterclockwise

The simplest way to think about the process is to understand that for each direction that the top gear rotates torque can only be transmitted by a single lower gear.  This means that regardless of the direction of travel for the upper gears the first intermediate reduction gear can only travel in one direction.

Consider wheel 2A as being unable to transmit torque (indicated in light blue in the schematic diagram above) when rotating in a counterclockwise direction then gear 1A will be transmitting torque (indicated in dark blue) to the first reduction gear (indicated in red) and rotating the intermediate gear in the clockwise direction. 

When The Rotor Rotates Clockwise

For the mass moving in the clockwise direction lets consider the schematic diagram below. Gear 1A is unable to transmit torque in the clockwise direction (indicated in light blue below) then the movement of 2A in the anti-clockwise direction will send the first reduction gear in the clockwise direction. 

You will notice that regardless of the direction of travel of the rotor we end up with the first reduction gear being rotated in the clockwise direction.

Schematic of The Power Transmission within an Automatic Watch Movement For The Rotor Rotating Couterclockwise
Schematic of The Power Transmission within an Automatic Watch Movement For The Rotor Rotating Clockwise

Now To The Mainspring

The final piece of this puzzle is to transmit the torque (that is now being transmitted in a single rotational direction) into the mainspring.  The challenge now is that the rotor can spin quite freely but the mainspring is held at quite a high tension.  This means that the low rotational force of the rotor (low torque) needs to be transformed into a high rotational force (high torque) on the winding mechanism.

This is overcome by a series of reduction gears, these are called reduction gears as they reduce the speed of rotation of the gear.  The rotation from the two ratchet wheels is transmitted through usually two reduction steps, this reduces the speed of rotation but, crucially, increases the torque. 

Slow Down, Torque Up

Taking this one step further into detail a low tooth count gear will enmesh with a high tooth count gear. For example a 10 tooth gear driving a 100 tooth gear would reduce the speed of rotation by 10:1 thus the driving wheel needs to rotate 10 times for the large gear to rotate once. The equation for the transmission of torque is not quite so simple as it relies on the radius of the gear wheels (we can delve into that at a later date if you are interested, please just put a comment below), but as you reduce the speed of rotation the torque increases (this is due to the laws of conservation of energy).

For an idea of the degree of gearing that goes into the automatic winding mechanism you can look at the technical sheet of any automatic movement. For example the Sellita SW-200 movement, from fully unwound requires 27 rotations of the crown to wind the mainspring to its operating window.  The same process for the rotor to wind the mainspring requires over 1,250 rotations of the rotor to achieve the same result.  A quick calculation indicates that one rotation of the crown is approximately 46 rotations of the rotor.

See It In Action

The first stages of the automatic movement can be seen in the short video of the open case-back of one of our SNGLRTY OHI2 watches.  In the video we can see the upper level (grey discs in the above schematic) of the two top gears interact with the rotor and produces the synchronous movement of the two gears prior to the transmission of the torque through the ratchet.

One Final Thought

Taking a step back and looking at the collection and transmission of energy in the watch movement it is interesting to consider that the automatic movement takes a low torque movement and then, through gears, amplifies the torque sufficiently so that it can rotate the arbor in the mainspring and store the energy in the spring.

The other side of this equation is the exact opposite.  As we detailed in our blog From Spring To Spring – Mechanical Watch Movements Explained the gears that power the escapement takes a powerful but slow moving rotational force (the mainspring) and then speeds up the movement sufficiently so that the escapement can oscillate at the desired speed. In the case of the Sellita SW-200 that would be 28,800 beats per hour or 4Hertz. 

Power to the Spring!

And that is it.  If you want to understand more about how watches work do sign up for our regular updates or join us on our next webinar.

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