The architecture of a tick: what does an escapement do?
As energy travels through a mechanical watch, its rhythm governs the whole movement. We take a closer look at the exchange between mainspring, gears, escapement and balance.
It all starts with winding
The ticking you hear when you hold a mechanical watch to your ear is the sound of a division of labour. The components that store energy, transmit it and set the pace of the movement work together to keep the hands advancing across the dial.
Energy is stored in the mainspring, inside a cylindrical housing called the barrel. In a manually wound watch, turning the crown tensions this spring through the winding mechanism; in an automatic watch, the rotor, turned by movements of the wrist, does the same. As the mainspring unwinds, it turns the barrel, and motion passes through intermeshing wheels and pinions to the escapement. Their tooth ratios determine the relative speeds of rotation at different points in the movement. But a gear ratio alone cannot create a minute. Another mechanism is needed to give the system a steady pace. Knowing a bicycle’s gear ratio does not tell you the cadence at which its rider will pedal.
A tiny gatekeeper
To understand this role, let us look at the Swiss lever escapement. Facing the escape wheel is a pallet fork carrying two pallet jewels. When one pallet locks a tooth, the wheel is held in place. The balance’s motion reaches the fork through its impulse jewel, releasing the lock. As the released tooth travels along the pallet’s sloping impulse face, it pushes the fork, which in turn delivers an impulse of energy to the balance. The other pallet then engages, locking the wheel again.
The same arrangement of contacts makes both waiting and advancing possible. The separate roles of the locking face and the impulse face are part of the escapement’s ingenuity. Imagine a doorkeeper who opens the way and, at the same moment, gives the next person a little helping push. In a watch, this encounter must happen in the right position every time. As the two sides of the fork work in turn, the wheel advances in small steps instead of releasing all the energy at once. The ticking we hear is the sound of these regular mechanical contacts.
The balance swings back
How, then, does the fork know when to unlock? The answer lies in the partnership between the balance and the fine spiral spring attached to it. As the balance turns in one direction, the spring changes shape; its elasticity draws the balance back. The spring’s properties and the balance’s moment of inertia together determine the duration of this back-and-forth motion. We can think of moment of inertia as the effect that the distribution of mass around an axis of rotation has on motion. The same total mass, distributed differently, can behave differently as it turns.
The balance spring has a different job from the mainspring inside the barrel. One stores energy; the other works with the balance to establish a rhythm. The small impulses delivered by the escapement replace lost energy and sustain the oscillation. The balance, in turn, determines when the escapement advances. Energy travels from mainspring to balance; the balance’s oscillation sets the pace. This reciprocal relationship makes the movement more interesting than a one-way chain of components.
The difference between winding and regulation
Winding a watch does not adjust its rhythm. It adds energy to the store. Regulation, which corrects the amount of time a watch gains or loses each day, changes the duration of its oscillation. A regulator that alters the spring’s effective length is one common way of doing this. Effective length means the portion of the spring that participates in its action; we are not talking about replacing the entire spring. That is why moving a small regulator arm produces an entirely different result from turning the winding crown.
In some movements, adjustment is made by changing the balance’s moment of inertia rather than the spring’s effective length. So we should not expect to find the same regulator arm in every watch performing the same function. The construction varies, but the aim remains to correct the duration of the repeating motion. This distinction is useful in everyday use, too: winding a stopped watch gives it the energy to run, but does not automatically correct the rate of a watch that consistently runs fast. Your winding routine and a watchmaker’s regulation address different needs within the same movement.
A rhythm tested by everyday life
Energy and accuracy are not entirely independent, however. The mainspring’s state of wind can affect a watch’s daily rate. In its instructions for mechanical watches, Grand Seiko explains that as the available energy falls, the components governing accuracy can become more susceptible to outside influences. This does not mean that winding a watch more will always make it run faster. An adequate, steady supply of energy helps the regulating system perform more consistently.
Temperature on the wrist, the watch’s position and the movements it experiences are also part of this little mechanism’s working conditions. Judging the entire movement by a single day’s gain or loss can therefore be hasty. The same manufacturer recommends assessing the average over a week to ten days of use. The ticking you hear over coffee is still that familiar sound, but behind every beat, energy, geometry and the surrounding conditions meet again. This is part of the escapement’s appeal: a contact almost too small to see contributes to the time we can read on our wrist.
Sources & further reading
This article was prepared for 1010 WatchClub using the sources below.