Chapter V: Stopwork
40. The last of the barrel accessories is the mechanism to control the tension to which the mainspring is subjected when winding and to limit the turns which are used for daily running. This part of the watch has been the subject of a variety of opinions as to the best way to implement it. We find stopwork designed in the most diverse ways, from its complete omission to the most complicated and ingenious stopwork in some Swiss and French watches.
41. If we try to compare the advantages of different constructions then our judgement must be based on an important criterion. This is friction, and we can object to all stopwork whose parts move under the control of frictional resistance; because friction, as small as it may be, results in a useless loss of power. By the way, in all stopwork of this kind only one tooth or finger stops winding by pressing against the full part of the star wheel. This tooth or finger is in danger of breaking off under the force which is put on it by the careless way in which some people wind their watches.
42. With frictional stopwork, which is not seen very often in watches, a wheel with 3 or 4 teeth is usual, while the remainder of the wheel is left uncut. This wheel is screwed onto the plate by a shoulder screw and the end of the barrel arbor carries a finger or tooth, which meshes with the wheel teeth; and with each revolution of the barrel arbor it rotates one tooth. At the start and end of winding the finger presses against the full part of the wheel and prevents further movement of the barrel arbor in that direction. It is apparent that the wheel is not held during the whole time between two rotations of the finger, and an outside vibration could make it turn around on its axle if its freedom to move were not controlled by a spring washer producing sufficient friction. Sometimes the stop wheel is turned off to a thin rim and slit opposite the teeth, so that it can be sprung onto a slightly under-turned stud on the barrel cover, where it will hold itself without a screw or a spring.
Figures 11 and 12.
43. To the same class belongs a kind of stopwork with a type of internal gear. An eccentric circular groove is turned in the barrel cover and is undercut at its outside edge. This groove contains a circular spring on whose internal edge some teeth are cut, into which the stop finger meshes; when the stop finger comes into contact with the uncut parts of the spring, winding stops. The friction of this spring in its groove prevents unwanted movement. This arrangement has the same problems as the previous.
44. Of the other class of stopwork, which works without friction, a very clever design should be mentioned here, which we frequently find in the best Swiss and French watches of about 50 years ago. It consists of two wheels with identical teeth which mesh into each other; the one on barrel arbor having more teeth than the other, so that the same teeth of both wheels meet again only after a certain number of rotations determined by the winding. On the upper side of both wheels a stop piece of steel is well fastened and these two stop pieces, if they meet together, halt the movement by coming into contact at right-angles.
The mechanical perfection and reliability of this stopwork are without doubt; the only disadvantage is that it requires a greater height for the stop pieces, which sit over both wheels, and it follows that the width of the mainspring must be reduced by the same amount.
45. Stopwork with a Maltese cross, Fig. 15, is the most frequently used and preferred for watches. It is so well-known it does not need to be described. It is true that careless execution of this stopwork, which we very often find in inferior grades of watches, is a source of annoyance and trouble for the repairer and the owner. Do not be mistaken, Maltese stopwork will not stand poor execution or carelessness; but if it is well made it has the strength to withstand any test. With well designed tools its correct production is not difficult.
46. However Geneva stopwork, no matter how well built, is always a necessary evil; because it makes the mechanism more complicated, is always exposed to different kinds of disorders and errors, and because it takes up some of the space which otherwise could be used to increase the mainspring width.
For this reason it is not surprising that the question was seriously studied, whether it would be feasible to completely omit stopwork without endangering durability and normal running, and without a mainspring of a disproportionate strength. This question requires a careful discussion, because the advantages which we may get from the omission of stopwork cannot be underestimated. It will thus be necessary to examine whether these advantages are counterbalanced by serious drawbacks.
47. The omission of stopwork was tried in various ways. More than 20 years ago a mainspring for this purpose was suggested, to whose outer end a piece of the same spring was riveted, about as long as a third of the internal diameter of the barrel. This piece was fastened backwards to the direction of the spring and its free end supported against a hook in the barrel of the usual kind. This arrangement allows the spring to completely roll up to its outermost end, and the short piece riveted to it rises in a diagonal direction against the hook and prevents further winding.
This system is to be preferred to the simple omission of stopwork because the spring is far more protected against breaking; but it does not protect the other parts of the movement against the sudden force which results from imprudent winding. This is, however, a fault which occurs with all of the stopworks previously considered.
This method looks rather grotesque but it should not be summarily rejected. At the time I wanted to arrive at a correct judgement of its value, and about 24 years ago I installed barrels of this type in two small ladies watches, as these are intended to be very flat. These watches are in continual use by people who often work with me, and so I had them under observation during the whole time; they were satisfactory in their running and, up to now, have not broken a mainspring.
Many years ago I saw some American watches whose barrels were designed in exactly the same way; the only difference was that the piece riveted to the end of the spring had two pivots at its free end which sat in holes in the bottom and the cover of the barrel.
48. More than 10 years ago a system was invented for omitting stopwork and it was completely free of the disadvantages just mentioned. This is the free spring of A. Philippe. An investigation of its advantages and the objections raised against is appropriate here.
This free spring is made in such way that it is held in the barrel without the usual hook, but instead by the greater tension and strength of its outermost coil which is left, for this purpose, approximately double the thickness of the working coils of the spring. The comparative thickness of these two parts of the spring must be arranged in such a way that the outside coil, gripping the barrel by friction, can follow the winding action, but slip if the spring suffers a certain higher tension. In this way the tension of the spring cannot go beyond this highest amount, even if winding were continued for a long time.
49. Springs of this kind are made in two different ways. With one the thicker part of the spring is in one piece with the rest; while in the other a spring of the ordinary kind is purchased with a special piece of greater strength, whose length equals the circumference of the barrel; it forms a flexible brace for the mainspring and is attached to it by a hook.
The effect of both systems is naturally the same.
50. It is not easy to give an opinion for or against free springs in a few words; because if we judge their advantages fairly then we must also consider the disadvantages and the objections made by watchmakers and repairers, and weigh them against the advantages which can be expected. The latter are:
1) Larger height of the barrel that allows, in a watch of the same size, a wider and thinner mainspring which is therefore less exposed to accidents and results in a more even rate.
2) Savings with the production of the barrel. However, this advantage is balanced to some degree by the higher price of the free spring; but this price would be very much reduced if the free spring became a regular commercial product.
3) Complete removal of all disturbances of the watch which come from faults and disorders of stopwork.
4) Protection of the work against damage which can result from imprudent and rough winding.
5) An extended running period in one winding, because the free spring is usually made in such a way that it produces 6 rotations or more.
These advantages, especially those of 3 to 5, are of great importance; and in particular the full importance of 4 has not yet been determined.
51. The disadvantages of the free spring are:
1) The lack of a clear indication that the end of winding has been reached. This objection can be overcome by making 3 or 4 vertical cuts on the inside wall of the barrel and giving the end of the spring a weak outward bend, so that it penetrates a little into one of these recesses. When the highest tension is reached the end of the spring is no longer held back by the recess and it slides into the next, making a quiet but audible noise indicating that winding should cease. At the same time this sudden small movement can be felt.
2) The large inequality of rate, which must necessarily occur between the two extremes of the spring development. At first sight this objection seems to be serious because the watch, if it is not wound regularly, continues to run until the tension of the spring is exhausted, and there can be no doubt that in the last hours of running the watch will show some deviation in rate from that which it has with regular winding. But it also must be said that we cannot expect a perfect rate from any watch if it is treated carelessly. In addition, we should ask what the consequences of irregular winding would be if stopwork were provided? Then the watch would simply stop, a very unpleasant occurrence for travellers, and it is at just these times that it is most likely we will forget to wind the watch. In such a case the owner of a watch with a free spring must realise how advantageous it is for his watch to continue to run, even if with a deviation of a quarter or half minute; which however is hardly likely to occur with a good watch, even under such unusual circumstances.
52. It seems that these two main objections against the free spring are of no importance. But there are probably practical difficulties which make most watchmakers averse to their use. So, to be added to the weight of the above are the inconveniences of having to stock an assortment of free springs in addition to the usual supply of springs (in case of breakages), and the higher price of the free springs. In contrast, springs of the usual kind are easy and cheap to get.
These circumstances got me thinking, whether it is possible to enjoy the indisputable advantages of the free spring without having to forego the ease with which we can replace a broken spring in a conventional system. It seems to me that I found a method that can be used at least in an emergency. I take an ordinary spring of suitable width and thickness for the barrel and break a piece off the end, long enough to go round once inside the barrel.
At the end of this piece I file a hook, into which the spring is hooked in the usual way, so that the loose piece extends backwards in the direction of the length of the spring. (4)
This arrangement causes the pressure of the piece against the inside of the barrel to increase with the tension of the spring, whereas with Philippe's method winding the spring decreases the friction of outside piece; and this is why in his method the outside piece must be stronger than the spring. With the changes just mentioned a piece of the mainspring is adequate, and its resistance can be increased by recesses in the barrel and a protuberance punched in the end of the tension piece; on the other hand, the resistance can be reduced if the tension piece is shortened. I believe that a spring made in this way would quickly win friends because it offers all the advantages of the free spring without its difficulties for the practical repairer. Anyhow, it provides the means to repair a watch in which a free spring has broken with a suitable new spring, without having to increase the stock of springs.
4 If Grossmann was the first person to propose this then he is the inventor of the mainspring used in all automatic watches. [Trans]