Having spoken to watchmakers with many more years at the bench than myself, I came to realize that the confusion around jewels in decades past was a hotly debated topic.
My personal favorite is the story about the customers who sternly informed their watchmakers they would be counting the jewels once the watch was returned from repair.
A friendly warning, just in case they were thinking about pocketing them. While some customers may have believed their watches made use of precious stones that in some way contributed to the value of the watch, the reality is that watch jewels are practically economically worthless. Friction is the enemy of the watch movement as watches are required to work on a scale that most of us can not even comprehend. When manufacturing pivots for train wheels and balance staffs, tolerances are generally 5 microns either side of the actual number.
Five microns is equal to 0. Yes, of one millimeter. So reducing friction is necessary to ensure top performance for a watch movement. And this is done by setting such watch components in bearing jewels instead of having metal rub metal.
The jewels that we use in watches today and decades past are synthetic, the most common being synthetic ruby.
These jewels are grown in a controlled environment as something called a boule , the French word for a cone-shaped chunk of the material. The ruby jewels must then be milled, sawed, and polished into the desired shapes, which is time-consuming and difficult, necessitating the use of diamond-tipped tools. Where the natural rubies would have impurities called inclusions that made them difficult to work with as a bearing jewel, when grown in a laboratory setting inclusions do not occur: the grain in the jewel is minimal and they can be polished to a very high standard.
On the Mohs scale of hardness both synthetic and natural rubies rate at 9. Diamond is the hardest material on the Mohs scale, coming in at a rating of 10, making synthetic rubies a logical, cost-effective choice as a bearing jewel.
Jewels today are friction-fit into main plates and bridges, however that process only began around the s. The great disadvantage with this style of jewel setting is the time and effort needed to replace them.
Modern friction fit jewels are just pressed in and out with ease, but with a rubbed-in jewel great care and time must be taken to burnish the new setting. Contemporary watches utilize jewels in a variety of areas, including as pivot bearings for wheels, automatic winding components, and calendar mechanism as well as pallet stones. The gear train wheels of a watch are the means of transmitting power from the mainspring to the escapement.
In order to make that process as efficient and friction free as possible, jewels are used as bearings for the pivots of those wheels. Steel or brass bearings would cause excessive friction, thus consuming unnecessary power from the mainspring. The use of jewels in combination with the highly polished steel of the pivots drastically reduces that friction. Balance pivots use jeweled bearings, though their setup is slightly different: they utilize a standard-style train wheel jewel bearing except that it has metal seating around it.
That metal seating holds another jewel in place that is positioned on top of the balance pivot, keeping the lubrication for the jewel in place but also greatly reducing friction. Watches of a higher quality, such as those receiving C. C certification or higher are manufactured to a higher level of precision. One area this particular aspect stands out is the spring barrel. These jewels are functional - they are used as the bearings for the wheel trains and in high wear parts such as the escape lever and impulse jewel.
A lower-end movement from before would typically use 5 or 7 jewels; this end of the market has pretty much been taken over by quartz. Nowadays, most manual wind watches will have a standard complement of 17 jewels. Jewels are the small red or clear circles that are sometimes visible the faces of various watch movements.
In watch movement design and engineering, the axels and pins of numerous rotating cogs, wheels, and levers are set into holes and indentations referred to as pivots.
These pivots are normally lightly oiled for the parts to run smoothly, but in addition to oil, small Rubies and Sapphires are sometimes utilized.
Since it's so hard, the surface of a ruby serves as a sort of natural lubricant and essentially becomes a bearing. In an ordinary plain jewel bearing without a cap jewel, the outer face of the jewel bearing is dished to form a reservoir for oil. When a cap jewel is added, a much better reservoir for oil is formed, capillary action causing the oil to form a globule around the pivot in the cavity between the cap jewel and the jewel bearing.
When filling this reservoir it is important not to overfill it because if it touches the plate the oil will penetrate between the plate and the cap jewel setting and be dispersed by capillary attraction.
There are two ways of introducing oil into this reservoir. One way is to place a drop of oil onto the cap jewel before it is put in place, which I find is difficult because the jewel can move about while you are trying to secure it, and the oil can get onto places it shouldn't.
The other way is to introduce oil into the assembled setting through the jewel bearing. This is done with a fine piece of wire, or with a special oiler which makes the job easy and is my preferred method. It is sometimes said that the pivot of the balance staff will push the oil through so there is no need to lead it through by hand, but the setting should be examined after oiling to make sure that there is the right amount of oil in place.
This examination is complicated if the balance assembly is in place, and if the quantity of oil is wrong, removing the balance can result in oil getting where it shouldn't. Some movements have end stones on the escape wheel pivot bearings. As the escape wheel is the second fastest turning component after the balance this would be a logical place to enhance the bearing arrangement to reduce friction. Cap jewels are usually used with conical pivots, without the square shoulder that is needed to control end float on normal parallel pivots.
If the escape wheel pivots were made as fine as those of a balance staff to minimise friction, a downside of this would be that they were as fragile and prone to breakage as the pivots of the balance staff itself.
However, the escape wheel turns much more slowly than the oscillating balance, so its pivots do not need to be made so fine and can be more robust. Sometimes end stones or cap jewels use a Kif Duofix setting, where the cap jewel is held in place by a spring that looks like the spring of a shock protection system.
The spring is simply a convenient alternative to tiny screws to hold the cap jewel in place, allowing it to be easily removed and replaced during cleaning. Kif Duofix is not a shock protection system. It is often seen on the escape wheel pivots of Rolex watches. Train arbor pivots are parallel, with a shoulder that keeps them in the right place, stopping them dropping through their bearing.
However, when the watch is moved about this shoulder moves into and out of contact with the plate or jewel bearing. This causes a difference in friction, when the shoulder is in contact with the bearing the friction is higher. The oil flows along the parallel pivot surface by capillary attraction, and can get onto the shoulder of the pivot causing it to stick to the plate.
A cap jewel replaces the function of the shoulder in keeping the arbor where it should be, and eliminates the problem of the shoulder of the pivot touching the plate. These two factors, the additional oil reservoir and control of arbor end float, mean that the fact that cap jewel on train pivots are rare even in top end modern jewelled watch movements is surprising. Several companies produced endstone settings for use with train wheel pivots with square shoulders.
There was "Giracap" made by Universal Escapements Ltd. How many jewels are necessary? Although jewels are often said to be used to reduce friction, this isn't essential and many watches were made without any jewels at all, a good strong mainspring ensuring that the watch ran.
I have never seen an analysis of the effects of jewelling on the timekeeping properties of a watch, and I suspect that they are not large; timekeeping is determined by the characteristics of the balance and balance spring. A balance in a watch with a going barrel has to cope with a far greater variation in torque from the mainspring between fully wound and nearly run down 24 or so hours later than it would ever get from variations due to friction in the train.
Because jewels are hard, a jewel bearing can be shorter than a brass bearing. This is useful because it means there is a shorter film of oil between a jewel and a pivot than there is between a longer brass bearing and a pivot. A longer oil film increases drag on the pivot. The first bearings to be jewelled, at the beginning of the eighteenth century by Nicolas Facio or Fatio de Duillier, were the balance staff bearings.
These are the most important bearings in a watch because the quality of timekeeping depends on the balance oscillating with as little loss of energy as possible so that the impulse that keeps it swinging can be small.
Each impulse necessarily disturbs the timekeeping of the oscillating balance, so the smaller the impulse the better. This is why the balance staff pivots are made so small in diameter, and consequently are so easily broken. Balance staff jewels are usually regarded as essential in a good quality watch, although many successful cheap watches have been made without them.
Because the balance of a lever escapement is highly detached it is fairly well immune to small fluctuations in torque due to friction in the train. Jewelling of other parts of the movement is more a case of reducing wear and increasing longevity at a certain cost, rather than significantly improving the timekeeping of the watch.
It might be thought that the escape pallets must surely be jewelled because of the amount of sliding friction they experience, but the Roskopf pin pallet escapement, which has no jewels at all and lasts reasonably well for a cheap watch, belies that. In most jewelled watches, train jewels are more value in reducing wear and lengthening the life of the movement than for any effect on timekeeping. Counting jewels can be more difficult than it appears at first sight.
You can't simply count the number of jewels visible on the top of the movement and double this to get the total. The reason for this is that pivots were not always jewelled in the bottom plate in mirror image to the top plate.
If the centre arbor bearing in the top plate is jewelled, the bearing in the bottom plate usually isn't jewelled. Cap stones were often fitted only to bearings in the top plate, and sometimes even only the top bearings were jewelled. This was obviously to make the movement appear jeweled to a customer, but to halve the cost of jewelling by not putting jewels in the bottom plate.
This was very common practice throughout the American pocket watch industry, and also some English watches. I am not sure about Swiss practice but I am sure it would have been done by some. The picture here shows in red the jewel bearings and end stones cap jewels for the balance staff. To reduce friction the pivots of the balance staff are made very fine, only a few hundredths of a millimetre in diameter.
In addition, the holes of the two jewel bearings that the pivots pass through are made with convex rather than parallel sides, so that the pivots only touch them over a short distance.
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