Chapter III: The barrel and mainspring

24. An examination of how this section of the watch is implemented by modern manufacturers must lead us to the conviction that due care is not given to a part of such importance. This fact is all the more surprising as these days a large number of cheap lever watches are produced whose pinions and escapements are so badly made that they can be brought to a reasonable vibration only by an excess of power.

Figure 1. Barrel size.

25. When designing the barrel of a watch, the manufacturer should be driven by the principle that the width and thickness of the mainspring should be limited only by the height and diameter of the watch. It is of extreme importance that the barrel be made as high and wide as the size of the watch permits. For this purpose a good rule for the diameter of the main wheel is to multiply the outside diameter of the pillar plate by 0.47. This gives a main wheel with the largest diameter the size of the watch will allow.

It is even possible to go beyond this size by putting the toothed part of the barrel a little lower than usual, so that this largest part of the barrel is located in the hollow area of the case body where there is enough space, provided the case springs are arranged in an appropriate way in hunter watches.

In this case the diameter of the plate can be multiplied by 0.485 in order to find the diameter of the barrel.

26. The height of the barrel in three-quarter plate watches is the sum of the height of a pillar and the thickness of the pillar plate, from which we take off only a sufficient amount for free movement between the upper and lower faces of the barrel and the plates, and the necessary thickness of the bearing for the bottom pivot of the barrel arbor.

27. We can readily understand that a watch whose escapement and pinions are incorrect, and which is made without any care, will require a strong mainspring, while in a carefully made watch this excess is avoided. Further, by the astute use of the area available for the barrel, a long and thin mainspring can be used, which results in fewer breakages because of its pliancy; and, because of the number of turns which it makes in the barrel, it offers the advantage of being able to select the middle turns of the spring for daily running and so attain a greater uniformity of power.

Figure 2. Barrel size.

28. It is also advisable to limit the width of the toothed edge of the barrel to the smallest required by the length of the teeth. We often see a watch whose barrel is too small and which has a toothed edge of excessive width, so that too much of the area available to the spring is lost completely. It is apparent that a barrel of this kind loses power in two ways. Not only must the spring be thinner and weaker than we could otherwise make it, also the internal radius of the barrel (which defines the force lever) is smaller, while the radius of the toothed part, which is the resistance lever, remains the same. The same point shows that the wall of the barrel should be just thick enough to attach a solid hook.

29. If the barrel meets all these conditions, as it should, then a spring of a thickness equivalent to 1/80 of the internal diameter of the barrel is completely sufficient to produce a lively oscillation in a watch whose escapement and pinions are carefully made. Such a spring, if the core of the barrel arbor measures 1/3 of the internal diameter of the barrel, develops more than 6 turns, from which the middle ones can be reserved for the daily running of the watch.

30. The way in which the barrel arbor is constructed shows tremendous variations between different production countries. I want to openly express my disapproval of the Swiss system generally used in watches. In the greater number of these, the lower end of the barrel arbor has no bearing or support and the barrel is held in place only by the ratchet wheel, which is in one piece with the barrel arbor. This system shows clearly that the favour which it enjoys can be attributed essentially to blind habit. It offers neither saving of time in production or repairing, nor a better distribution of space in flat watches, but it is mainly from the point of view of firmness and durability that it is to be rejected. In all watches, both in the well made and in those of inferior work, the barrel arbor should be supported at both ends; in the former because of the greater firmness and additionally in the latter because of cheaper production.

31. There are two main ways to make the hanging barrel arbor. In the first the ratchet wheel forms a part of the barrel arbor, and it is recessed into the upper side of the barrel bridge and held in place by a cap secured with 3 or 4 screws.

These screws, which have just 3 or 4 threads in the metal of the bridge, are the only means to stabilise the barrel. Every repairer must know from repeated experience that this arrangement is an inexhaustible source of annoyance and that, if oiling the rubbing surfaces is neglected or the teeth of the ratchet are not rounded off at their points, the internal surface of the cap or the barrel bottom is rapidly worn by daily winding. The consequence of this wear is an increased shake of the ratchet and the barrel. Every error of this kind is very serious, because the barrel and the centre wheel are the largest mobile parts of the train and necessarily lie with their surfaces very close to each other.

Figure 3. Barrel arbor and ratchet wheel parts.

32. With the other method of construction, the ratchet wheel is attached by 3 screws to a shoulder which is formed on the part of the barrel arbor above the barrel.

Figure 4. Different kinds of barrel arbor.

This system is, from all points of view, even worse. There are only two narrow circular surfaces which hold the barrel. The top of the barrel arbor and the face of the ratchet gradually wear away the surfaces of the barrel bridge, and the screws are often loosened by the countless small impacts of the click when the watch is wound.

Besides, the ratchet wheel is exposed to faults during hardening and its ability to hold is weakened because the three holes with their sinks for the screw heads must be very near the edge. In these two cases the core of the barrel arbor is a separate piece which is screwed onto the arbor or, and we must watch out for this, it is held by a pin which goes through both parts. The stop finger has a square hole and is pinned on by a transverse hole bored through the end of the barrel arbor.

Figure 5. Barrel parts of old design.

33. The best method for the manufacturer and the repairer, and no less for durability and good service, is to make the barrel arbor with two pivots which run in holes. A barrel arbor of this kind is very easy to make. The ratchet wheel must fit onto the square of the arbor, and this easier to achieve than fitting the core of the Swiss barrel arbor. In addition there is no necessity to make a hole in the lower end of the barrel arbor to hold the stop finger in place, because this is done by the barrel bridge.

A barrel of this kind is much easier to take apart and assemble than a Swiss hanging barrel; we only need to remove the cover of the barrel and everything is done. But with the other the pin of the star finger must be taken out and, after opening the barrel, the pin which fastens the core to the arbor must be removed, or the core unscrewed, before we can clean the parts or put in a new spring; and afterwards these parts must be re-assembled.

34. In a frame whose pillar plate is turned off only 0.2 or 0.3 mm on the dial side, there will be a sufficient gap to attach a thin steel bridge which can take the lower pivot of the barrel arbor. The same space would be also necessary to accommodate the locking pin of the star finger if we wish to avoid the deplorable characteristic of so many flat watches, in which it is almost impossible to remove or insert these pins without splitting the end of the barrel arbor. From this we see that no saving of space worth mentioning is gained.