Internal Structure of an Automatic Mechanical Watch

　　The development of mechanical watches spans over a century. Today, mechanical watch architecture is remarkably precise. Today, Watch House presents an overview of the internal structure of an automatic mechanical watch!

　　I. Classification

　　Mechanical watches are generally divided into two types: manual-wind and automatic watches. Both types derive their power from a mainspring within the movement, which drives the gear train to move the hands; they differ only in how the power is supplied. Manual-wind mechanical watches rely on manual winding; the movement is slightly thinner than that of an automatic watch, making the watch relatively lighter in weight. Automatic watches use a oscillating rotor in the movement that swings side to side to generate kinetic energy and wind the mainspring, resulting in a slightly thicker overall watch profile compared to manual-wind models.

　　II. Structure

　　Watch gear train

　　The gear transmission train of a watch, particularly the main wheel train, widely adopts a so-called circular-arc tooth profile. This profile evolved from the line-contact tooth profile; because pure cycloidal profiles are very difficult to machine, circular arcs are used to replace the cycloid, also known as a modified cycloidal tooth profile. This allows the minimum number of teeth on the pinion to be 6, which achieves a large gear ratio without requiring too many teeth on the wheel, which is extremely advantageous for reducing movement diameter and in high-frequency watches. Transmission efficiency is relatively high, generally reaching around 95%. Due to the small size of watch movements, the energy stored in the barrel assembly is not large, and if energy loss is too great, it will directly affect the timekeeping quality of the watch. It is quite sensitive to machining errors, such as tooth profile errors and centre distance errors, both of which can cause changes in meshing characteristics. Because the tooth profile is determined by a pair of meshing gears and the module, different numbers of teeth and modules require different hobbing and milling cutters.

　　Escapement mechanism

　　The construction is relatively simple; Swiss watches contain few parts, primarily consisting of the escape wheel, the pallet fork assembly (including the pallet fork, entry pallet stone, exit pallet stone, banking pins, and pallet arbor), the balance assembly (balance wheel and impulse pin), and banking pins located on the main plate. However, some watches do not use banking pins, instead milling two raised platforms directly onto the main plate or pallet bridge to serve as limits. Others use a pin extending from the pallet fork assembly that inserts into a hole in the main plate, using the walls of the hole to limit movement. This type of escapement is known as a lever escapement, which is further divided into straight-line and side-lever types. In the former, the escape wheel arbor hole, pallet fork arbor hole, and balance arbor hole are aligned in a straight line; in the latter, the line connecting these three holes forms a specific angle. Although the two forms differ in appearance, their composition and working principles are identical. They are primarily used in mid-to-high-end watches.

　　Balance wheel and hairspring

　　The balance wheel and hairspring assembly is the component that generates a stable oscillation frequency. These two parts are organically linked through the gear train and escapement mechanism, forming the backbone of the watch movement. The oscillation of the balance wheel and hairspring assembly consumes a certain amount of energy, and this energy replenishment is provided by the mainspring system. Greater supply results in a larger amplitude of the balance wheel and hairspring assembly; conversely, a smaller supply results in a smaller swing angle, meaning a smaller amplitude. If the supplied energy remains constant at all times, the swing angle of the balance wheel and hairspring assembly will also remain unchanged, meaning the amplitude is constant. In reality, maintaining a constant energy supply is impossible. Because mechanical watches derive their motive power from a winding mainspring, as the mainspring unwinds, its torque becomes progressively smaller, and naturally the energy supplied decreases accordingly. Furthermore, this energy passes through the gear train and escapement, and the meshing characteristics of the gear train, the operating characteristics of the escapement, transmission efficiency, and escapement efficiency are all constantly changing. Therefore, the amplitude of the balance wheel and hairspring assembly varies at different times; when measured with an amplitude meter or amplitude recorder, the displayed values fluctuate continuously, and generally the average of the maximum and minimum values over a certain period is used to represent the amplitude for that period.