Watchmaker’s Bench: What Makes It Tick? (Part 1) – The Wheel Train

Throughout the centuries, watchmakers have strived to increase the accuracy of mechanical watches. Even as things were looking oh-so-dire during the 1970s, the industry managed to cling to the traditions of mechanical watchmaking, and today we’re seeing more technologically-forward movements than ever before. In the last few years, we have seen advancements with silicon hairspring and escapements, optimized tooth geometry in wheel trains, greater power reserves and more efficient winding, among other improvements. But in order to understand all these great advancements, one must first have a solid grasp of the fundamentals. To make sure our readership is armed with that knowledge, we’re continuing our series, Watchmaker’s Bench, with a three-part installment taking a close look at exactly what makes a mechanical watch tick.

A standard mechanical watch displaying only the time can be broken up into three sections: the wheel (or gear) train, the keyless work and hand setting mechanism, and the escapement. In today’s article, we will look at the role of the wheel train.

For the sake of simplicity, we’ll be referring specifically to a manual-wind mechanical movement with no added complications, unless otherwise stated.

The Parts

A wheel train consists of the following components: the barrel (with the mainspring inside), the center wheel, and the third and fourth wheels. There is also the “escape wheel,” but we’ll cover that in the next article, which will focus on the escapement. The purpose of these wheels is simple—to ultimately display the time on the dial via the hands—or sometimes via other means—and each of these wheels plays a distinct role in ensuring that happens.

The gear train wheels and the barrel are all specific to each movement and not interchangeable, and they’re machined to incredibly tight tolerances to ensure the optimal running of the watch.

First, let us look at the barrel.

The Barrel Complete

In a manual-wind watch, the barrel complete is a fairly simple construction. It has four parts in total: the barrel drum, the lid, the mainspring, and the arbor. The mainspring sits inside the barrel with the arbor at its center. The lid is then held on by friction. When all these parts are assembled, it is called the barrel complete.

A wound mainspring provides a mechanical watch with power, much like a battery does in a quartz watch. How does it work?

In an unwound watch, the spring is relaxed, and the bulk of it sits toward the sides of the barrel wall. conversely, when the spring is wound and under tension, the bulk of the spring sits toward the arbor. The mainspring has a small rectangular cut-out in its center for the arbor to attach. As the watch is wound, the mainspring coils more tightly around that arbor, and it forces the barrel to turn and give power to the train wheels. Once the mainspring is fully wound, the end attaches to a hooking point on the barrel wall, and it stops the spring from winding any further.

Automatic barrels work in a similar way, but instead of attaching when fully wound, the mainspring slips around the side of the barrel wall to ensure the watch can continue to be wound via the automatic work.

The Wheels

All the wheels in the train have teeth, pinions and pivots. Generally, the teeth from one wheel interact, or mesh, with the pinion of another wheel, thereby allowing the power to be transferred.

The center wheel sits next to the barrel, with teeth from the barrel interacting with the pinion of the center wheel. This wheel makes one revolution every hour. The teeth from that wheel then interact with the pinion on the third wheel, and the teeth from the third wheel then interact with the pinion on the fourth wheel. The fourth wheel makes one full revolution every 60 seconds.

It is no coincidence that the center wheel and fourth wheel make one full revolution every 60 minutes and 60 seconds, respectively. Those two wheels directly drive the minutes and seconds hands. The fourth wheel actually has an extended pivot (or post) that protrudes out of the dial side of the movement. The purpose of this post is so that the second hand of a watch can then be attached directly to it. This, however, is only the case for watches that have an off-set second hand.

In the case of watches that have a central second hand, there are a number of ways this can be accomplished. The traditional way is through “gearing” the watch back to the center. Here, the fourth wheel would have the extended pivot on the movement side instead of the dial side. Then you’d have another wheel friction-fitted over the fourth wheel. This new wheel has an extended pivot that protrudes through the middle of the center wheel. A second hand attaches to that wheel.

The center wheel works in a slightly different way than the fourth wheel. The center wheel also has a long pivot, but it is much greater in diameter than the one on the fourth wheel. A pinion, also known as the cannon pinion, is friction fitted onto the pivot of the wheel. That pinion also makes one revolution every 60 minutes because it is directly driven by the center wheel. The minute hand is then attached to that cannon pinion.

The reason the minute hand cannot be directly attached to the center wheel is twofold. The first is the need to set the time. When the time is set the hands are forced around the dial at a speed far greater than the gear train can handle. Therefore, the cannon pinion needs to “slip” around the shaft of the center wheel so that nothing is forced that shouldn’t be, which would result in a breakage. The second is because the hour hand advances around the dial once every 12 hours. Therefore, a wheel has to be inserted to make that happen, as there isn’t a train wheel that directly drives the hour hand, and so a pinion needs to engage with the wheel that drives it. This will be explained in more detail in the third part of this primer.

In the next installment, we will cover the components of the escapement and how it regulates time-keeping.