MOTOR ORGAN
Definition
The power source of a timekeeping device provides the energy required for its proper functioning.
Different types of power sources
Weight / gravity – Weight-driven clocks
Spring / barrel – Wristwatches, pendulum clocks, alarm clocks
Battery / electrical power – Electric, electronic, and atomic watches and clocks
Weight-driven motor:
The driving weight of a clock is attached to the end of a cord or chain wound around a cylinder fitted with a gear. Through the force of gravity, the weight descends and drives the cylinder and its gear in a rotational motion. This rudimentary system produces a perfectly constant torque. The power reserve depends on the length of the cord or chain to which the weight is attached (and its available drop distance). Once the cord or chain is fully unwound, the clock stops. A crank or key mechanism, together with a ratchet, allows the cord to be wound back around the cylinder, raising the weight once more. It is from this type of mechanism that the expression “winding a watch” originates.
Unlike a barrel or battery, a weight-driven system does not store energy.
The barrel:
The barrel is a power source system found in some small clocks and pendulums, as well as in all portable watches (pocket watches and wristwatches). It consists of a spring wound inside a cylinder that stores energy accumulated during manual or automatic winding, and gradually releases it to drive the movement. This is the type of power source we will examine in detail in this chapter.
The Battery:
In electronic watches (tuning fork, quartz, etc.), energy (in electrical form) is stored in the battery, which powers the movement (oscillator, stepper motor, display). An external electrical supply (as used in electric or atomic clocks) does not store energy and is thus suited to fixed instruments only.
Description
The complete motor organ of a mechanical watch is called the barrel. The barrel consists of four components:
– Barrel drum: A cylindrical component housing the mainspring. Around the outer edge of the drum is a toothed rim (or ring gear) that meshes with the centre pinion, transmitting energy from the barrel to the rest of the gear train.
– Mainspring: A long, rectangular-sectioned steel blade. When wound (manually or automatically), it stores energy and then gradually releases it to power the watch.
– Barrel arbor: The central axis of the barrel around which the mainspring coils.
– Drum cover: Once the mainspring and barrel arbor are properly positioned inside the drum, the cover snaps into place, sealing the barrel and protecting the spring from dust.
Barrel assembly
The first step is inserting the mainspring into the barrel drum. When new, mainsprings are delivered fully coiled and tensioned, encircled by an aluminium retaining ring. To install, position the mainspring in the centre of the drum, press it to the bottom, and remove the aluminium ring. To reinstall a relaxed mainspring, it must be rewound and tensioned using a mainspring winder, which is also used to insert it into the barrel. Modern mainsprings are self-lubricating and require no additional greasing.
After lightly lubricating the pivots of the barrel arbor to allow it to rotate freely inside the drum, the arbor can be placed centrally in the barrel, ensuring it passes through the inner coil of the mainspring. Once properly positioned, the innermost coil must hook onto the arbor’s collet.
After confirming that both arbor and the full length of the spring are seated correctly at the base of the drum, the assembly is closed by snapping the barrel cover into place. This is done using a dovetail (or eagle-claw) type fit to securely fasten the cover.
Winding Phase
The ratchet wheel is the final component of the winding mechanism and is fitted onto the barrel arbor, to which it is connected via a square fitting. This square fitting allows the ratchet wheel to drive the barrel arbor in a rotational motion when the winding mechanism is operated.
During the winding phase, the barrel drum and its cover remain stationary while the arbor rotates freely at their centre. The inner end of the mainspring is secured to the arbor’s collet by a hook, while the outer end hooks into a notch in the inner wall of the barrel. As the arbor turns, it draws the spring tight, causing it to coil around the arbor’s collet at the centre of the drum. The click prevents the barrel arbor and the entire winding mechanism from snapping back due to the mainspring’s tension, which would otherwise result in an uncontrolled release of stored energy.
Unwinding Phase (Powering the Movement)
Once the spring is fully wound around the arbor (watch fully wound, mainspring tensioned), the energy it contains, unable to escape through the arbor, drives the barrel drum via the bridle pulling on its notch inside the drum wall. The barrel drum then rotates around the now stationary arbor. As it turns, the toothed rim of the barrel meshes with the centre pinion, driving it and delivering the energy needed to power the entire movement.
Watches with Multiple Barrels
To increase the power reserve and/or the available torque, additional barrels may be used. As in electrical circuits, connecting barrels in series (one after another in the gear train) increases the power reserve, while arranging them in parallel (both barrels driving the centre pinion simultaneously) increases the torque delivered to the movement.