Precision timekeeping - Part One

Precision timekeeping - Part One of Two

For generations, great minds have sought to create timepieces that deliver peerless chronometric performance. Over the years, a variety of mechanisms have been employed to enhance precision, however, such complexity may be nullified if the timepiece is not correctly adjusted and regulated. By way of explanation, this feature looks at an array of terms employed in watchmaking and how these factors influence the performance and operation of a timepiece.

The flow of time

Energy from the barrel, the fuel tank of the watch, is sent via the gear train to the escapement and regulating organ. The regulating organ consists of a hairspring (not to be confused with the mainspring) positioned at the centre of a wheel known as the balance wheel. Collectively the hairspring and balance wheel are termed the ‘balance’.

The escape wheel serves an impulse to the balance via a pallet lever, causing the balance wheel to oscillate to and fro. This rotational behaviour momentarily unlocks the escape wheel causing the hands to advance before locking again. This repeated action of locking and unlocking the escape wheel controls the flow of time.

It is the resonant frequency of the hairspring that determines the locking/unlocking period. The type of hairspring, its length and stiffness, as well as the balance wheel’s moment of inertia, determine the period of oscillation.

Poising

Prior to a watchmaker regulating a movement, they will ensure that the balance (hairspring and balance wheel) ‘runs true’. Effectively, they are seeking to identify any parts of the balance which are unduly heavy. This process, called poising, is very similar to a tyre fitter adding weights to the rim of a car wheel to ameliorate ‘wobbling’. In this instance, infinitesimal amounts of material are removed from the balance wheel manually, eliminating any heavy points.

Regulation

Once the balance is poised and fitted, the movement can be regulated (see Part Two). This involves placing the movement on a test instrument, an electronic device that effectively listens to the movement by employing a powerful microphone. Various information is shown on the test instrument’s display panel, including the beat rate, the amplitude, any beat error and the rate variation, expressed in seconds per day. But what do all of these terms mean?

Beat rate

The frequency of a movement is the number of oscillations performed by the balance (regulating organ) over a period of time. The frequency or beat rate is normally expressed as vibrations per hour (vph) or hertz (Hz). For example, the balance found on the Zeitwinkel caliber ZW0103 has a frequency of 28,800 vph or 4Hz.

One swing of the balance in one direction is termed a beat (tick), whereas the combination of one clockwise and one counterclockwise swing is termed an oscillation (tick-tock).

Amplitude

The swing of the balance wheel in either direction is expressed in degrees. The term amplitude is used to describe the magnitude of each swing or beat. In the case of the Zeitwinkel caliber ZW0103, the amplitude remains quite stable. Typically, the amplitude is lower when the movement is held in a vertical position due to additional friction.

When the amplitude is outside its operational range (ie too high or too low) it can indicate there is a problem with the movement.

Beat error

The clockwise and counterclockwise swing of the balance wheel should be the same. When there is a difference between the two (ie the swing in one direction takes slightly longer to complete than in the opposite direction), this is termed a beat error and is measured in milliseconds (ms).

Ideally, the resting position of the impulse pin (ie no power coming from the barrel) should be equidistant between both banking pins. In reality, this is very difficult to achieve as there is often a small inconsequential bias to the left or right of this point.

Unlike some mass-produced movements, the Zeitwinkel caliber ZW0103 features a mobile stud holder. This provides the watchmaker with a simple means of relocating the entire balance in order to make subtle changes to the position of the impulse pin. The ultimate goal is to achieve ‘anchor symmetry’, negating any significant beat error.

Rate variation

The rate of a mechanical movement can vary depending on a number of reasons. One such variable is ‘the positional influence’, a situation where gravity influences the behaviour of moving parts. Because this factor varies in magnitude based on the orientation of the movement, the rate is shown as a range, expressed in seconds per day (s/d), along with (+/-) denoting whether there is a small gain or loss.

The Zeitwinkel caliber ZW0103 is regulated in five positions and adjusted to run at +8/-2 seconds per day.

How is a movement regulated?

In the second instalment of this two-part feature, the behaviour of the balance, how the movement is regulated and post regulation influences on the rate are discussed.