To drive timekeeping in mechanical watches, a watch escapement converts continuous rotational motion from the spring into a harmonic back and forth motion.
A watch escapement includes three main parts:
- Balance wheel
- Balance wheel staff
- Pallet lever
Each of these main parts contains jewels, which serve different important functions.
There are 4 types of jewels:
- Hole Jewels, donut shaped jewels that go over the gear axles
- Cap Jewels, flat jewels on the ends of the axles
- Pallet Jewels, brick shaped jewels on the pallet fork that engage and release the escape wheel
- Roller Jewel, which is on the large oscillating balance wheel and engages with the pallet fork on the end opposite of the pallet jewel
Typically, the jewels used in watches are synthetic or natural, such as rubies, sapphires, garnets and diamonds. More jewels often means a higher watch cost, but not because of the value of the jewels. Rather, this is because the jewel count is an indicator of quality.
There are 2 distinct advantages of watch jewels that lead to increased quality:
To increase accuracy, watchmakers make reducing friction an important goal.
They can use jewels, which have 2 properties that help with this goal:
- Smoothness, which lets the metal parts slide easily
- Hardness, which prevents quick wear
Specifically, precision-made watch jewels hold oil in pivots, which reduces friction on pivots. This allows the gear train to run with greater ease. A reduction in friction reduces the amount of power required from the mainspring. This allows the watchmaker to use a longer, finer mainspring that allows the watch to run for 40 hours on a single wind. As a result, some watches even feature a power reserve indicator that displays the amount of power left (in hours) on the face of the watch.
Most modern watches use synthetic sapphires, which are very hard and durable. Sapphire surfaces can maintain smoothness over decades of use, reducing friction variability.
Therefore, a watch with jewels will usually not wear out as quickly as one without jewels. Sapphire also helps accuracy, because the coefficient of friction of brass-on-steel is 0.35, but sapphire-on-steel’s is about 0.1.
While watches with more jewels tend to be of higher quality, some watch manufacturers have taken advantage of the general public’s connection of jewels to luxury by adding non-functional jewels.
However, a typical fully-jeweled watch has only 17-21 jewels. Don’t be fooled by companies who claim that their mechanical watch with 50 jewels trumps one with fewer jewels.
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