Power reserve indicators for nerds

(This is an old article of mine published in 2004 that I thought would be fun to share with those really interested in watchmaking.)

Wristwatches and power reserve indicators

Mechanical wristwatches with power reserve indicators have been hugely popular for years, and virtually every brand has at least one model with this particular function in their collection. No one is quite sure exactly when this type of indicator made its first appearance in the world of horology, but its usefulness has never been debated. It is actually one of the very few indicators on a dial that conveys something of the watch’s interior state of affairs to its owner. Although these days power reserve indicators are also supplied on automatic watches, there is something that is just perfection when a manual winding watch is outfitted with one. But for those collectors with several watches who alternate between their favorites regularly, it can also be useful to know how much time you have before one of your wristwatches has to be worn again in order to wind it!

The amount of power that a mechanical watch contains, its autonomy, or simply stated its ability to continue ticking correctly has been the subject of much research and development during the past years, as witnessed by the introduction of such watches as the Patek Philippe 10 days and the Chopard Quattro some years back. Both of these creations were at the vanguard of substantial increases in autonomy that in the standard wristwatches on offer today vacillates roughly between 48 and 60 hours. But springs and watch movements are fickle things; the last few hours of power reserve will never be the ideal ones for timekeeping purposes and watchmakers have developed all kinds of subtle tricks to equalize these issues. The reasons why most watches fall into the 48 to 60 hour power reserve zone is not a conspiracy, but just a fact of certain physical laws that are part of the way in which springs function.

The constraints on power

The winding energy supplied by your fingers or rotor to the winding barrel is held inside it by a spring. The going train- a series of wheels responsible for transferring the winding barrel’s energy- transmits this power to the escapement, which as its name implies, allows for the regular release of energy. Since the hands are in communication with the going train- they slowly turn with each release of energy. This is a simple description of how a watch functions, and we will forget about how temperature, friction, energy loss and a dozen other items affect timekeeping- and stick to basics.

A winding barrel must simply provide a sufficient amount of energy via a sufficient number of complete turns in order to allow the watch to function accurately. The spring itself has specific constraints: elastic modulus, length, width and thickness all will define how strong it will be and define its compatibility with the demands of a particular movement. Strength can be defined in a number of parameters. In length, it will work inversely: if we halve the length of a given winding spring, it will double in strength. Sounds great, but it also means the winding barrel will complete fewer turns before reaching its residual state, and therefore lower the watch’s autonomy severely, making it impractical. Increasing the width of the spring (a winding spring has a flat profile) will indeed increase the total power available proportionally, but also the total height of the watch movement. (This is part of the solution used by Patek Philippe in the 10-day watch and 10 day tourbillon. For the Chopard Quattro,  the winding barrels and total profile were kept closer to normal height, and therefore required the use of 4 barrels to accomplish 9 days autonomy).  Changes in thickness of the winding spring will result in proportional changes of force that are the cube of the original within the same conditions. But in this case however, the spring’s reaction and curvature will also be affected, and more space will be taken up within the confines of the winding barrel. Another inherent problem is that any such great increases in power require more strength from the movement structures and wheels in order to deal with the extra forces involved. It is indeed a real balancing act between several sets of parameters.

All this information means that quite simply, even in the space age, we still have roughly the same problems as did the watchmakers from hundreds of years ago: if you want longer autonomy- you just need to make a bigger watch able to accommodate a bigger winding barrel. It might be possible to make a wristwatch that you would wind once every month, but it would not be very pleasant to wear on your wrist!

Enter the power reserve indicator

Depending on the design and type of indicator being used, it is possible to show the wearer three different aspects of power held within the winding barrel:  too much energy (the watch is over wound), operation within correct limits, or operation at extreme low limits (with the danger of inaccuracy). Over winding is generally a sign of other, more serious problems, as all watches have some kind of built in stopping system to prevent over winding. This means that if you are able go deeply into the red area of your power reserve indicator scale when winding, there is likely to be another problem on hand. The correct operation area is harder to define, and many manufacturers do not even supply it. Basically stated, the winding barrel and spring must match the escapement within different parameters during periods of high energy (just wound) and low energy (almost unwound.) For instance, should the amount of energy left get below a calculated level, the balance wheel will not travel through even the minimum amount of necessary arc, and this will affect accuracy. This is called the power curve of the winding spring, the way in which it ‘spreads’ forces out over a longer time period. The trick all manufacturers want of course is to have as even a spread of energy release as possible over the longest time frame possible. This varies from model to model and maker to maker, but generally speaking, it would not be completely wrong to say that keeping your watch wound as much as possible a bit below fully wound is a good place fro accurate functioning. But it must also be said that with modern spring materials and the calculations available these days the spread of hours in which timekeeping will function accurately is quite broad, much more so than it was in previous centuries.

Two major types

With the unending inventiveness of the watch industry there have been some new inventions regarding power reserve indicators and there will surely be more in the future. But it safe to say that there are two major classes of system in use, of which only one is used in the vast majority of cases. The one based upon a cone shaped structure is the most widely used; the other uses wheels in conjunction with pinions. Both types use a form of differential technique to accomplish their goal; indeed since the winding up and winding down of the barrel must be shown, the gearing, whichever kind it is, will have to work equally well in both opposing directions, so this is more or less de facto.

Cone based power reserve with differential spindle

The central part of this system could best be described as a cone with the point sawn off. Through the center of the cone is a screw, best described as a threaded spindle. This threaded spindle turns easily and is attached to gears that engage with the ratchet wheel. As the barrel is being wound up, this motion is transferred to the spindle, whose turning slowly raises the cone. Added to this is a finger mounted on a pivot that can move only laterally touching this cone continuously. As the cone rises during winding, it literally pushes the finger outwards. This outwards, lateral movement is easily transferred via simple levers to an indicator on the dial. Unwinding works similarly. There is another layer of teeth along the edge of the winding barrel communicating with another wheel. This wheel has a small pin set in it that is attached to the same cone. As the barrel unwinds, the cone gets turned back down through this action. (Winding turns the threaded spindle in the cone to move it upwards; unwinding turns the cone itself back down the spindle, which is stationary during unwinding). It’s a simple and useful system. Due to the limits on the cone’s angle of slope as well as the upward and downward movement within the movement’s height considerations, this system is only useful for power reserve scales on the dial that use a short arc shaped segment normally under about 70˚.

Differential gearing power reserve

This type affords the display of a greater arc than the abovementioned variety and is also somewhat more accurate. Whereas the cone type version will always be in some state of sliding friction with the finger touching the cone during its upwards and downwards movements, the geared version is simply translating the movement of the barrel directly though a gearing system that will function in both directions. Several versions of differential exist. Whichever one it may be, the basics are that one wheel engaging the ratchet wheel will turn during winding and a pinion with some form of secondary wheels or a contrate wheel at 90˚will transfer this information to an indicator on the dial. Teeth located along the edge of the barrel located on the other side engage a very similar set-up to show the unwinding of the barrel. It is a much more elegant and direct system, but is only used in a small percentage of wristwatches today. This is partly due to the space constraints of movement height, as well as the abovementioned smaller arcs used for most power reserve indicators.

 It must be remembered that a power reserve indicator works from the outside of the winding mechanism and is not in direct contact with the spring itself. What is happening is that the mechanism is, in one form or another, counting ‘turns’ of the barrel, nothing else. If all functions as it should this information will still be beneficial and accurate enough to use of course. But at the outer edges of the very low values or the very high ones, their efficacy is not to be trusted 100%. This is especially true when we account for things like friction, oiling and loss of energy through the movement that takes place at a later stage in the chain of events after energy leaves the winding barrel. In any case, an owner of a fine watch who keeps his eye on the power reserve indicator, and supplies the extra energy when needed will likely be rewarded with a higher degree of accuracy from their timepiece.

Reply
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Great article !! Thanks for posting.

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tempus

Great article !! Thanks for posting.

Thanks!

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Very interesting read, thanks for sharing!

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This was great to read, you certainly have a knack for explaining three dimensional mechanics in a clear and effective way!  I can check this off of my "I've always wondered" list!