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Watch Movement Jewels by Roffensian
Watch Movement Jewels
So this week, let’s look at the history of watch movement manufacturing. Again many sources to help me, but a very big thank you to Gisbert Brunner and Christian Pfeiffer-Belli, the authors of a very good book called Wristwatches. I have learnt a lot from them.
It’s been a while since I wrote one of these technical articles, so thought it was about time I spewed forth some more pseudo wisdom, and let’s handle watch jewels as they are something that aren’t terribly well understood.
Let’s start with what we mean when we talk about jewels. Historically they were rubies or sapphires (which are actually essentially the same) or occasionally diamonds. In the last 100 years or so they have been artificial or ‘grown’ jewels technically called corundum. They are generally referred to as rubies these days due to the colour, although it should be noted that they are the same material as sapphire crystal.
Jewels have two properties that are useful for watchmakers – they are hard and therefore wear very slowly, and secondly they can be worked to a very smooth finish. These properties relate directly to their function in watchmaking – reducing friction.
As we all know, there are a lot of moving parts inside a mechanical watch (although analogue quartz watches will also have jewels), and the key to an accurate and efficient movement is to minimise friction. In order to achieve this, the tolerances on wheels and gears are extremely tight, ensuring that the teeth interlock as smoothly as possible. Clearly any flaw on the axle or stem of a wheel will be magnified on the edge of the wheel where the teeth are and it is therefore vitally important to have the arbours move as smoothly as possible. By fitting the arbour or shaft into a doughnut shaped jewel this smoothness can be maintained as the hole in the jewel can have smoother sides than if the hole had metal sides.
Additionally, if the metal shaft were fitted directly into a hole in a metal bridge or plate, the two metals would wear against one another over time and make the hole larger, thereby disturbing the balance of the shaft and increasing friction. It would also do damage that couldn’t be repaired. By setting the doughnut shaped jewel in the holes in the bridges and plates and then fitting the shaft or arbour into the hole in the doughnut the two metal surfaces never touch one another and therefore cannot wear.
These doughnut shaped jewels have their surfaces shaped and finished in such a way that the oil that is used on all of these moving parts is held where it is needed rather than spreading across the jewel surface.
Doughnut shaped jewels (technically hole jewels) are not the only jewels that are used in watches. There are also cap jewels (often referred to as end stones – especially if diamonds are used), which affix to the end of shafts but do not have holes drilled right through them. These are often used in conjunction with hole jewels and prevent movement of the axle up and down. An example of a cap jewel is the end of the balance shaft.
There are also pallet jewels – these roughly rectangular jewels fit on the pallet fork (one on each side) and are the part that actually engages with the escape wheel to control the rate of the movement. Finally there is a roller jewel (impulse pin) that is on the balance wheel and engages with the other end of the pallet fork to rock the fork back and forth.
Watches have different numbers of jewels, and the perception is that more jewels are better. Up to a point that’s true, a 15 jewel watch is better than a 7 jewel watch (you’ll see why when I explain the counts below), but it’s not always the case, especially in modern watches. The law of diminishing returns definitely kicks in – jewelling a date wheel that does one revolution every 31 days is somewhat less important than jewelling a balance wheel that will oscillate back and forth over 21 million times in that same 31 day period (in a 4Hz movement). Also, watches with more complications will need more jewelled parts as there are more moving parts to potentially generate friction and wear.
Jewels also indirectly play a part in shock absorption. The most common shock system in use today is the Incabloc system invented in the 1930s which mounts the balance cap jewels in springs to absorb any shocks that occur and prevent damage to the movement.
So let’s look at jewel counts for simple watches and what the jewels are:
- 7 jewels (common in older pocket watches, but unheard of now) – this is simply a jewelled escapement – two hole jewels and two cap jewels (one each for each end of the balance wheel shaft), two pallet jewels and a roller jewel.
- 15 jewels – the 7 from above plus hole jewels for the high speed elements of the running train (it’s not needed on minute or hour wheels).
- 17 jewels – about the minimum in a decent watch today add hole jewels to each end of the centre wheel
- 19 jewels – adds cap jewels to the escape wheel
- 21 jewels – cap wheels to the pallet fork
- 23 jewels – mainspring barrel jewels (more common in pocket watches than wrist watches)
Now jewels start to get a bit confusing, because Joe Public thinks that more jewels are a good thing, so marketing departments start coming in to play. Waltham once produced an automatic movement that had 100 jewels, unfortunately 83 of them were in a rim around the edge of the rotor mount and did nothing. Waltham claimed that if the watch got knocked the 83 jewels would prevent damage to the rotor mount – yeah right! Ironically there are 84 mounts for the jewels but only 83 are jewelled to take the 17 jewel movement to 100 jewels instead of 101!
More recently movement manufacturers have been drawing a grey line with jewels, largely because there is now an ISO standard for jewels that state that they have to be functional at reducing friction and wear. That means that movement manufacturers can add jewels to places that are theoretically subject to friction, but where the use of jewels is of questionable (or no) benefit. Many manufacturers (including ETA) use jewels like this in the automatic winding mechanism.
Barely scratching the surface again, but hopefully that helps a little!