Under the Loupe/The Wheels
Wheels, as referred to in horology, are essentially gears; round mobiles whose circumference is defined by a series of teeth. The ideal form for these teeth is defined by an Epicycloid. Typically, a wheel is riveted to an arbor, or shaft, the ends of which are formed into pivots on which wheel can turn. This arbor is also often host to the pinion of a wheel, which will drive or be driven by the wheel gear depending on the type of gear train in which it is employed. While there exist a number of ways to fix a pinion to the arbour, in most modern watches, the pinion is cut. directly into the material that forms the arbor. The purpose of a wheel is to transmit energy to another wheel by turning. A series of wheels (or gears) acting upon each other is referred to as a gear train.
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[edit] Types of Gear Trains
There exist three basic classes of gear trains:
- Multiplying Gear Trains
- Reduction Gear Trains
- Direct Gear Trains
The type of gear train most commonly used in watch and clockmaking is the multiplying gear train. A multiplying gear train uses large gears to drive smaller gears. In watchmaking, a multiplying gear train is one in which the wheels drive the pinions. It is called a multiplying gear train because the last wheel in the gear train rotates at a rate which is a multiple of the rate of the speed of the first wheel in the gear train.
A reduction gear train use small wheels to drive larger wheels. In watchmaking, it is one in which the pinions drive the wheels. It is called a reduction gear train because the last wheel in the gear train rotates at a reduced rate as compared to the rate of speed of the first wheel in the gear train.
In a direct gear train energy is transferred directly between wheels of the same size and number of teeth.
In all cases, it is of utmost importance that the teeth of all wheels be of the right fit and in proper engagement with one another (not too deep or too shallow) to allow for the minimum loss of motor force coming from the energy source.
[edit] Composition of a Typical Arbored Wheel
1. Arbor The arbor (or staff) of a wheel is generally made of steel, which is resistant to wear, moderate shocks, and, in certain alloyed forms, resistant to corrosion as well. Additionally, when polished, its pivots are well suited for turning in jewelled bushings. The lubrication used at the friction surface between the jewel and the steel has furthermore been fine-tuned to work with the properties of the two materials.
2. Pinion The pinion is a toothed cylinder which is cut into the rod stock along with the arbor when the arbor is being formed, or it is fixed later by soldering, welding, riveting, or gluing. By definition, the pinion is smaller of the two gears of which a horological wheel may be composed.
3. Wheel hub The center of the large gear in a wheel is known as the wheel hub. It is here that the arbor is fixed to the large gear, usually by means of riveting, though it may also be soldered, welded, or glued. Like the rest of the large gear, the wheel hub is normally composed of hard brass.
4. Arms The arms of a horological wheel are realized by the openwork created in the disc of the large gear. Their importance is to maintain the rigidity of the wheel and its perfect perpendicularity to the wheel arbor, while minimizing the inertia required to turn the wheel (its resistance to change in velocity as quantified by its mass). The arms are normally composed of hard brass.
5. Rim The rim of a wheel is the circular exterior part of the large gear, whose perimeter is cut to form the outmost teeth of the wheel. The rim is normally composed of hard brass.
6. Teeth The teeth are formed by small, regular projections around the rim of a gear or pinion, and are the most critical component of a wheel. It is the teeth which allow for energy to be transferred easily and efficiently from one wheel to another. The teeth are composed of the same material as their host, typically brass or steel in a mechanical timepiece.
[edit] Composition of an Intermediate Wheel
An intermediate wheel (also called an idle wheel) is a flat disc with a toothed perimeter and a hole bored in its center. Like a typical wheel in a watch, it has the ability to transmit energy, however, it rotates around a stud and not between bushings as it does not have an arbor. In most implementations, this type of construction loses more energy to friction than a wheel with an arbor, however it is advantageous in some uses to limit the thickness of a movement.
[edit] How it Works
The transmission of force through a gear train is done by the mobiles (arbor, pinion, hub, arms, rim, and teeth) and must be accomplished with a minimum loss of energy to friction.
In a simple, time-only watch, the first wheel in the gear train is fixed to the same arbor as the Mainspring. This wheel drives the pinion of the second wheel, which is often referred to as the center wheel of a watch. The center (or second) wheel, driven by its pinion, engages with the pinion of the third wheel, in turn driving it. In order to dispel confusion, as irony would have it the third wheel is also often referred to as the seconds wheel, as it is typically geared to turn one revolution per minute, thus making it ideally suited to carry the seconds hand of a timepiece via an extension of its pivot through the dial side of the movement. The third wheel engages with the pinion of the fourth wheel, or escape wheel, in turn driving it and completing the multiplying sequence of the timekeeping gear train.
The power remaining at the end of this multiplying sequence is transferred from the escape wheel to the Regulating organ through the escapement. For accurate timekeeping, optimal and regular power must be available to the Regulating organ. Thus, the freedom of rotation and correct penetration of teeth between each wheel in the gear train is of utmost importance to meet this end. If a wheel is held too tightly between the plates of a watch or the depthing of gear teeth is too extreme unnecessary loss of energy due to excess friction will occur. Likewise, if the wheels are too loosely held or the depthing of gear teeth is too shallow, skipping or galloping of the teeth within the gear train may occur.
[edit] External Links
- The Scientific Property of Inertia as defined on Wikipedia
- The Gear Train on WatchmakingBlog.com