Mechanical principles › How the mechanical Movement works

THE MOTOR ORGAN (POWER SOURCE) (the Barrel)

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Figure 1: Exploded view of a Barrel (manual winding)

Figure 3

Kinematics of the winding mechanism

(accelerated)

Click on the animations to enlarge them

1. Definition

The motor organ (power source) of an object designed to measure time provides the energy necessary for its proper functioning.

2. Different types of motor organs (power sources)

The driving weight:

The driving weight of a clock is attached to the end of a cord or chain wound around a cylinder bearing a toothed wheel. Under the effect of gravity, the weight descends and drives the cylinder to which it is attached, as well as its toothed wheel, in rotation. The torque developed by this rudimentary system is perfectly constant.

The Power reserve depends on the length of the cord or chain to which the weight is attached (and on its range of travel). When the cord or chain is fully unwound, the clock stops. A crank or key mechanism together with a click system allows the cord to be wound around the cylinder and the weight to be raised again. It is from this type of mechanism that the expression “winding one’s watch” comes.

Unlike a barrel or a battery, a weight-driven system does not store energy.

Le barillet :

The barrel is a motor organ system present in certain small clocks and pendulums as well as in all worn watches (pocket watches or wristwatches).

It is a spring coiled inside a cylinder that stores the energy accumulated during the winding of the watch (manual or automatic) and gradually releases it to the movement. This is the type of motor organ that we will detail in this chapter.

The battery:

In electronic watches (tuning fork, quartz, etc.), electrical energy is stored in the battery, which powers the movement (oscillator, stepper motor, display). An external power supply (electric or atomic clocks) does not allow energy storage and is therefore suited to fixed instruments.

3. Description & components

The entire motor organ of a mechanical watch is called the barrel. Four components make up the barrel (Figure 1):

–The Barrel drum. This is a cylinder in which the Mainspring is housed. Around the periphery of the Barrel drum there is a toothing which allows the Barrel drum to mesh with the Centre pinion and drive the entire going Train. FIND OUT MORE

–The Mainspring is a long Blade of steel with a rectangular cross-section. It is by relaxing after having been pre-stressed (wound or Winding) that it provides the energy necessary for the watch to function. FIND OUT MORE

–The barrel arbor. It is theAxis of the barrel around which the mainspring winds. LEARN MORE

–The barrel cover / lid. Once the mainspring and barrel arbor are correctly positioned inside the barrel drum, the barrel cover / lid closes the assembly, thus protecting the mainspring from dust. LEARN MORE

4. Barrel assembly

The first step consists of inserting the mainspring into the barrel drum. When new, mainsprings are delivered coiled and pre-stressed, encircled by a ring made of Aluminium. It is therefore sufficient to position the spring at the centre of the drum, push it to the bottom and remove the aluminium ring. To reposition a relaxed spring, it must be wound (pre-stressed) using a mainspring winder, then used to reposition the spring inside the drum. Modern springs are self-lubricating and require no lubrication.

After first lubricating the pivots of the barrel arbor, which allow it to pivot freely in the drum, the barrel arbor can be positioned at the centre of the drum, ensuring that it passes through the centre of the first coil of the mainspring. Once the barrel arbor is correctly positioned, the first (inner) coil of the mainspring must wind around the core of the arbor.

After ensuring that the arbor and the entire length of the spring lie flat against the bottom of the drum, the assembly can be closed by the cover, which clips onto the drum with a dovetail fit.

5. Operation

Winding (Figure 3)

Last element of the Winding mechanism, the Ratchet wheel is positioned on the Barrel arbor to which it is connected by a square fit.

This square allows the barrel arbor to be driven in a rotary motion by the ratchet wheel when the winding mechanism is rotating.

During the winding phase, the Barrel drum and its Barrel cover / lid remain fixed, while the arbor rotates freely around their center. The mainspring is held by its inner end to the Core of the arbor by a hook, while the strap on its outer end hooks onto the inner wall of the drum. By rotating on itself, the barrel arbor pulls on the blade of the spring, which then wraps itself around the core of the arbor, at the centre of the drum. The winding click prevents the barrel arbor and the entire winding mechanism from suddenly turning back under the force of the mainspring and thus letting its energy escape to no purpose.

Running (letting down)

Once the spring is wound around the barrel arbor (watch wound, spring tensioned), the energy it contains cannot escape through the barrel arbor. Although the spring seeks to drive the arbor in rotation (Figure 2-1), the latter, firmly linked to the ratchet wheel, is blocked by the action of the winding click (Figure 2-2). From then on, the spring will drive the drum in a rotary motion by pulling on the spring’s hook in its housing on the inner wall of the drum. The drum then pivots itself around the arbor, which is now fixed. In its rotary motion around the barrel arbor, the drum’s teeth mesh with the wings of the Centre pinion, driving it and thus providing all the energy required for the watch to function (Figure 2-3).

Watches with multiple barrels

In order to increase the Power reserve and/or to obtain more torque, the number of barrels can be multiplied. As with an electrical circuit, connecting the barrels in series (the two barrels follow one another in the kinematic chain) will increase the power reserve, while operating them in parallel (the two barrels simultaneously drive the centre pinion) will increase the torque transmitted to the movement.

6. Calculations of the motor organ (power source)