Which factors affect the timing accuracy of mechanical watches

The timekeeping accuracy of mechanical watches is influenced by many factors. Generally speaking, they are mainly the following 8 major factors:

1, External Influences

These are various influences originating from outside the timepiece, depending on its working environment. Commonly adopted measures include: shock-resistant design, water-resistant design, anti-magnetic design, and additional protective outer cases. Cardan joints are frequently used on precision marine chronometers to keep them level during rough motion.

2, Friction  
   
Friction generally has both positive and negative effects. It has positive aspects, such as friction-fitted minute wheels, friction between the automatic watch mainspring and the barrel wall, and screw self-locking. On the other hand, friction leads to a reduction in transmission efficiency and wear on components, thereby affecting timekeeping. Common solutions include: improving lubrication conditions by selecting different lubricating oils according to various requirements; using jewel bearings or shakers; and improving the tooth flank conditions of gears, including the adoption of scientific conjugate tooth profiles and enhancing surface finish. However, tooth flanks are generally unlubricated (in such cases, the resistance generated by the viscosity of the lubricating oil may be higher than friction).

3, Regulator

The regulator is an economical structure that facilitates time setting, but both theory and practice confirm that it affects the isochronism of the system and may also produce positional error. These timekeeping errors are relatively random and cannot be compensated for or cancelled out. Solutions include: minimising the distance between the inner and outer regulator pins; however, the best approach is to have no regulator and to adjust the rate by altering the moment of inertia of the balance wheel, such as Rolex's Microstella adjustment system.

4, Escapement

The influence of the escapement is primarily the effect exerted on the balance spring system during the energy transmission process. The balance spring system can only maintain a fixed oscillation frequency under conditions of free oscillation; clearly, the energy transmission process of the escapement will affect this oscillation frequency. Theory indicates that when the transmission process is close to the balance point of the balance wheel, this influence is reduced. Solutions include: employing a precision escapement, such as a detent escapement, where the energy transmission occurs near the balance point, the impulse angle is very small, and the impact is also relatively minor. Furthermore, its unidirectional impulse gives the balance spring system more room for free oscillation (in this regard, the relative error can be halved!). Of course, the Swiss lever escapement has the advantages of sound craftsmanship and ease of adjustment, making it the mainstream escapement in international mechanical watches today. Nevertheless, the impulse angle should be minimized as much as possible during design. To reduce the impact of the escapement on the timekeeping reference, Swiss Omega introduced the co-axial escapement, an invention by Dr George Daniels from Britain, though from a working principle perspective, it is a hybrid of the lever escapement and the detent escapement.

5, Temperature Influence

The effect of temperature manifests in two main ways: firstly, temperature changes alter the working length of the hairspring while simultaneously changing the moment of inertia of the balance wheel, which directly impacts timekeeping accuracy; secondly, temperature changes affect the viscosity of the lubricating oil and transmission efficiency, thereby influencing timekeeping. Methods to address this include: adopting a bimetallic split temperature-compensated balance spring system; using special alloy materials to manufacture the hairspring and balance wheel so that they provide a certain degree of temperature compensation within the operating temperature range (8°–38°); and employing movable regulator pins for temperature compensation. Standard lubricating oils are used, whilst for extreme temperature conditions, such as Omega's Moonwatch, unlubricated or solid lubrication is employed.

6, Magnetic Fields

Magnetic fields have the greatest impact on the hairspring, altering its modulus of elasticity and causing the hairspring to deform under the action of the magnetic field, generating additional stress. In severe cases, magnetic fields can cause the hairspring coils to stick together, seriously affecting timekeeping. The solution is: to use anti-magnetic materials.

7, Hairspring Equilibrium

With a standard flat hairspring, its center of gravity changes with the banking angle of the balance wheel, and under the action of gravity, it produces positional error. Solutions include: adopting an overcoil (Breguet hairspring) where the center contracts and the center of gravity does not change with the banking angle; using cylindrical hairsprings with Philips terminal curves employed symmetrically top and bottom; using straight hairsprings; historically, spherical hairsprings have been used which offer superior performance, but their craftsmanship is poor, so they are rarely applied in practice.  
 
    8, Balance Wheel Equilibrium

The balance of the balance wheel components directly affects positional error, and static balance of the balance wheel assembly is a fundamental requirement.

If the aforementioned factors are relatively ideal and the timekeeping of the watch is relatively stable, the timekeeping performance can be comprehensively improved through dynamic balancing of the watch. There is a very special method: the principle is that when the amplitude of the balance wheel reaches 220 degrees, various impulses transmitted to the balance wheel have no effect on frequency. Someone once used a constant force mechanism installed on the escape wheel to control the amplitude to remain near 220, which is also a viable method.