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The Mechanics of Canon Pinion Friction

From Horology magazine, June 1938

The Mechanics of Canon Pinion Friction

In making repairs the horologist is by far more concerned in the perfection of the so-called "most important items" such as the mainspring, poise of the balance or the adjustment of the escapement, than in the trifling matter of hand friction. In fact, if there is something wrong with the cannon pinion, it is seldom discovered before the watch is ready for casing. At that stage, one is apt to assume that the job is actually complete and, because the annoying action of the cannon pinion was not anticipated in the first place, feel that he is not justified in spending too much additional time.

In some instances, the carrying of the hand is left altogether to luck. If the dial train is free in every respect, even the slightest amount of friction on the pinion will be enough to make it turn the minute and hour wheels. Occasionally it fails, but will resume its duties after the customer has, in the process of setting, turned the pinion to a more favorable position with a better friction. Thus, we so often come across a beautiful repair job but worthless from the standpoint of the customer, because the watch although ticking without stopping, still fails to indicate the correct time.

Repair jobs in which the hands have been left without friction are due only to negligence. Mutilated cannon pinions on the other hand, are due to nothing else than lack of understanding of the mechanical principles involved. One might even intimate that some manufacturers would do well to brush up on this subject. For instance, there is nothing more vexing than to observe the finest of workmanship in a marine chronometer and to look at the same time at the cannon pinion, whose friction consists of a filed crescent with a punch mark in the center. Similarly, one often finds a beautiful watch which contains a cannon pinion, improperly designed for satisfactory service.

The cannon pinion should receive attention before the watch is taken apart for cleaning. In many instances it is the center post which is at fault and no matter how near perfect the cannon pinion  may be, if it is forced over the sharp end of the post, the depression inside the pinion will be either shaved off or pressed outward. It is for this reason that one finds so many pinions squeezed around the neck, almost to the point of separation. but still too free to carry the hands. If this fault is observed before the watch is assembled, the sharp edge of the center post may be taken off in the lathe, but there is hardly any remedy if it is detected after the watch is finished.

What takes place inside of the small cannon pinion can perhaps be better explained through the sketches in Fig. 1. On the posts are three bushings, each of which has been provided with a spring finger whose action is similar to a cannon pinion. We find that at A the bushing remains away from the shoulder and one can see at a glance that every time it will be pressed back, it will return to its original position. At B, on the other hand, the bushing remains firmly seated against the shoulder and will snap down quickly should it be slightly raised. As shown at C, the bushing is without any friction and can be moved slightly up or down, although it would require some force to pull it off the post.

With these examples in mind, one may readily see why the cannon pinion shown in Fig. 2 will rise up after a few turns.  The spring pressure of the pinion is at such a point that it is bound to be forced up the incline of the arbor. In Fig. 3, on the other hand, the pressure is at a point which must force the pinion toward the shoulder of the center arbor, a condition which is sought after in cannon pinion adjustment.

Just as at C in Fig. 1, the combination illustrated in Fig. 4 cannot possibly give good results, for the point of pressure in the pinion just coincides with the center of the cone of the arbor and does not come in contact with the arbor itself. There is no other remedy but to change the point of contact to a position higher on the center staff.

Although the combination in Fig. 3 is shown correctly matched, it can not be called ideal, for after all, it is a spring pressure that is required for the pinion friction. But how can there be much spring in the short stubby section of the neck of the pinion in Fig. 3. In addition, if the cone of the staff should happen to be slightly out of round, and it is not at all unusual, the friction then might be irregular if not altogether absent in spots.
In order to have a smooth friction the neck of the pinion should be more slender, like that illustrated in Fig. 5. It is to be noted that not only is the neck thinner, but it is also longer.

For the purpose of altering the groove in a cannon pinion it is necessary to have a thin flat graver. The one shown in Fig. 6 is made from a so-called "escapement" file. Nearly every horologist will find around his bench a worn out file which he can easily grind to shape. The actual turning is done on a small hardened and tempered arbor, on which the pinion is forced. Of course, one must be careful not to cut the pinion in two during this operation. It is a good plan to measure the arbor and then the neck of the pinion with a Grossman millimeter gage. One can then tell easily the amout of stock which may be safely removed.

What is the right tool for tightening a cannon pinion? Gadgets for this purpose have been and are being continually placed on the market. Some of these may probably be altered and used, others are totally worthless and their construction shows plainly that their makers failed to understand their use. Cannon pinion tightening, from a mechanical standpoint, consists of squeezing the neck of the pinion at a particular point, until a notch is pressed into the wall of the pinion.

One can produce such a notch with a punch and stump in the taking tool or with other devices. However, the easiest tool to manipulate is a cutting pliers. An optician's pliers is probably best suited for this purpose. Its jaws are thin and it has a screw stop which prevents it from cutting and object in two. By removing the extremely sharp edges with an oil stone slip, it can be converted into the most perfect cannon pinion tightener.

It has already been mentioned that the point of pressure on the pinion must be at a point properly related to the cone of the center staff. It is not always possible to place the pliers at this particular point, for no matter how thin the jaws of the pliers are, they are still a bit too clumsy when applied to the small pinion of a modern baguette watch. The sketches in Figs. 10 and 11 show the possible choice of positions by merely turning the pinion around. The same illustration also explains why an hour wheel suddenly becomes so tight on the cannon pinion that it is necessary to broach it out. The danger of the cutting pliers or other instrument raising a burr on either corner of the groove is always present, and one should hesitate before deciding to broach the hole of the hour wheel instead of removing the burr from the pinion.
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