Analyzing the Watch Train

From The American Horologist magazine, February 1939

Analyzing the Watch Train
By W. H. Samelius

QUITE often we find a watch that stops at regular intervals and by understanding the watch train as to relationship of gear ratios to one another, we can readily tell where to locate the cause of the trouble. Let us take the accompanying diagram of an ordinary watch train where the balance wheel vibrates 18,000 per hour.

The barrel has 72 teeth and the center pinion 12 leaves, so for each revolution of the barrel the center pinion makes 6 revolutions, or, for each turn of the mainspring the watch would run 6 hours and in order to have the watch run for 36 hours it would be necessary to have as many turns of the mainspring as 6 is contained in 36 or 6 turns of spring driving the mainspring barrel 6 revolutions.

The mainspring must not be too long or too short, for in either case it cannot drive the barrel the full number of turns required. Since we find the barrel revolves one turn each 6 hours, it is obvious where a watch gives trouble or falls off motion each 6 hours, the barrel may be the cause. It may be out of flat, out of round or may have a damaged tooth.  

Following up we find the center pinion makes one revolution per hour and naturally, a watch that stops or shows poor motion hourly tells us to look for our trouble at the center wheel. This wheel may be out of flat, rubbing a plate, or, it may be out of round, or, like the barrel, have a damaged tooth.  We also might look to the cannon pinion which is attached to the center arbor, making one revolution per hour. It is possible the cannon pinion may be out of round, binding on the plate or have a broken tooth. A cracked center pinion, or center pinion out of round may also cause trouble. If the center pinion has 12 leaves, one leaf passes through the barrel each 5 minutes so we can look to the center pinion should the watch stop each 5 minutes or the motion of the balance fall off.

Sometimes we find the center arbor out of upright causing the hands to rub on the dial on one side and directly opposite the minute hand will rub on the glass, in both cases create friction enough to slow up the motion of the balance, or possibly stop the watch entirely.

Continuing further through the train we find the center wheel has 64 teeth and the 3rd pinion 8 leaves. If the center wheel makes one revolution per hour, the 3rd pinion will make 8 revolutions per hour or one revolution every 70 minutes so a watch stopping or falling off motion each 7 1/2 minutes tells us to look for our trouble at the 3rd wheel, which like the other wheel, can be out of round, out of flat ·or have a damaged 3rd pinion.

The third wheel has 75 teeth and the 4th pinion 10 leaves and while the 3rd wheel is making one revolution, the 4th pinion is making 7 1/2 revolutions and while the 3rd wheel is making 8 revolutions per hour the 4th pinion will make 8x7 1/2 or 60 revolutions per hour or one revolution per minute, so a watch falling off motion each minute or stopping each minute shows our trouble lies in the 4th wheel or pinion.

The 4th wheel has 60 teeth and the escape pinion 6 leaves, so when the 4th wheel is making one revolution, the escape pinion is making 10 revolutions per minute, the escape wheel having 15 teeth and as each tooth of the escape wheel delivers two impulses, each revolution of the wheel then will give 30 impulses and as the escape wheel makes 10 revolutions per minute there will be 10x30 or 300 vibrations per minute the balance makes and multiplying this by 60, the number of minutes in the hour we will have 60x300 or 18,000 vibrations per hour, a watch that is commonly known as a quick train.

As was stated above, the cannon pinion has 10 leaves and we find the minute wheel has 30 teeth so as our cannon pinion makes one revolution per hour, the minute wheel would make 30 divided by 10 or one revolution in three hours.  Continuing we find the minute wheel pinion has 8 leaves, and the hour wheel has 32 teeth which is a gear ratio of 4 to 1 so while the cannon pinion is making one revolution, the minute wheel is making 1/2 revolution, and the hour wheel is making one-fourth less or 1-12th revolution, so it is possible to find trouble in the hour wheel and minute pinion when the watch stops each hour, or, should it stop each 12 hours it would indicate the hour wheel is out of round, out of flat, or may be binding between the plates. It might also indicate the dial being off center, the hour hand binding on one side of the opening each 12 hours.

I trust these suggestions will be helpful to our readers.