Under the Loupe/The Balance Wheel
A balance is essentially a fly-wheel. It may be circular or any other shape. It may run true or not. The one essential is that it must be in perfect poise; that is, it must have no part that is out of equilibrium with another. Practically stated, when turned along on the straight edges of a Poising tool it must have no tendency to settle in any one position. It is an advantage to have as much of the weight of a balance as possible in its outer rim. A balance may be plain or compensated, and may or may not be outfitted with timing screws or washers.
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[edit] Temperature Error
The rate of oscillation of a balance is controlled by the hairspring it is attached to, and the time of each oscillation depends on the strength of the hairspring relative to the weight and breadth of the balance wheel. A strong spring will cause a balance wheel to oscillate more rapidly that a weak spring. Both the balance wheel and hairspring are affected by changes in the temperature. As with most matter, both expand as they are warmed, and contract as they are cooled.
When the molecular structure of the balance wheel is excited by an increase in thermal energy, the arms lengthen and the rim increases in size, removing the weight further from its centre and causing the balance to move more slowly. Much in the same way that a figure skater spins more slowly when their arms are spread wide and more quickly when they are tucked in. As a spring is warmed, the tension in its molecular structure is diminished and it loses some of its acting force, further causing the balance to move more slowly at higher temperatures.
In early pocket watches, these physical properties served a source of critical errors in timekeeping. The combined effect of a change in temperature of only 7 or 8°C (from 21°C at room temperature to the warmth of wrist or waistcoat pocket, say 28°C) could cause an uncompensated watch to lose as many as several minutes a day. Conversely, moving from a very warm environment to a cold one, such as stepping outside on a cool winter's day, would cause the watch to gain time - quality of the oil permitting.
Temperature also has a small, though measurable, effect on the depthing of gear teeth, by causing the wheels to expand; it also alters the strength of the mainspring, and affects the fluidity oil and other lubricants. Nowhere does temperature have a greater impact on the integrity of timekeeping in a watch than at the hairspring and balance wheel. The combined effect of all these variables is referred to as temperature error, and varies with each individual watch. To discover the source of this error (now common knowledge to most any school aged child today) proved the bane of many a watchmaker in the 18th and 19th centuries. Modern advances in horology, however, have made it possible to standardize and contain these errors to within acceptable limits in even the most mass produced mechanical timepieces.
[edit] Compensated Balance Wheels
To combat temperature error at the balance, late 19th century watchmakers (particularly Chronometer makers) began using bimetallic, split-balances - which, when properly adjusted, are referred to as compensated balance wheels. A compensated balance is a balance wheel whose Moment of inertia is increased as the thermal energy applied to it increases, and vice versa, so as to compensate for the effect of variation in temperature not only on the balance wheel itself but on the hairspring as well. This is achieved by moving the Centre of gyration towards or away from the Center of rotation as the temperature decreases or increases respectively.
A compensated balance is composed of a circular rim with a steel crossbar. The rim is bi-metallic, being steel inside and brass outside, and cut through at two opposite points. Brass is more affected by heat than steel, and in a rise of the temperature the outer brass will lengthen more than the inside steel. The effect of this is to curve each half of the rim inwards and bring the weight of the balance as a whole nearer to its centre. This causes the watch to go faster, and, if the amount of the inward movement of the rim is exactly sufficient, will compensate the tendency of the watch to lose. These balances are weighted by screws fitted in a series of tapped holes all round the rim. By moving the screws nearer to the free ends of the two segments of the rim the effect of the balance is increased ; by moving them towards the fixed portions the effect is diminished. Therefore adjusting for temperature consists in trying the watch in cold, then in heat, and moving the screws according to the performance of the watch, until its rate in cold (40 to 50°F) is equal to its rate in heat (80 to 90°F).
A watch with an uncut compensation balance is no better than one with a plain brass or steel balance; but if the balance be cut and trued as described it will be greatly improved. A watch with a cut compensation balance, not specially adjusted, like the great majority of ordinary watches, is cured of most of its temperature error, and may generally be depended upon not to vary more than 30 seconds from its rate in one day between 40°F and 85°F. A masterly finished watch, with a cut compensation balance and the screws arranged by trial under varying temperate can be adjusted to reduce error to about 2 seconds per day or less.
[edit] Plain Balance Wheels
The majority of balance wheels in watches today are plain and made of Glucydur, although there also exist plain balances made of Steel, Brass, Gold, Palladium, or Titanium (the lattermost having the advantage of being both non-corrosive and non-magnetic). They generally have two or three light arms and an ideally true, circular rim. The use of the term "true" means that when held by a poising tool, the balance wheel does not roll nor settle; a balance wheel is said to be "true" when its center of gravity always falls on the midpoint of the axis of the balance staff.
In modern times, plain Glucydur balances fitted with Nivarox hairsprings have been engineered to compensate for temperature error comparable to the degree of accuracy achievable with a split, bi-metallic balance wheel.
[edit] Timing Screws
While all compensated balances employ a series of screws for temperature compensation, most are outfitted with four screws at equal distances from each other, with notably longer taps as well. These are the Quarter screws. They are never moved for temperature adjustment, but are for poising the balance or for small timing alterations. Drawing one out a little makes that part of the rim heavier. Drawing out an opposite pair will slow the watch. Turning a pair in will make it go faster. In the best balances Quarter nuts are fitted instead of screws. A Quarter nut is a gold nut, which turns on a fixed steel screw, and is not so liable to work loose the way a plain Quarter screw is liable to with frequent turnings. A Quarter nut is split, and slightly sprung on to its screw to move firmly and not get loose.
Timing screws can also be used for poising and timing plain balances as well.