The balance wheel and hairspring assembly is the pendulum of a wrist watch - it regulates the entire mechanism. This is done through precisely measured back & forth rotations called oscillations. Most balance & hairspring assemblies turn anywhere from 270˚ to 315˚ if they are functioning properly, which is referred to as the movement's amplitude. When the balance wheel rotates in one direction it's called a vibration. Two complete vibrations in opposing directions equal one oscillation. The term beats per hour refers to how many vibrations the balance & hairspring assembly makes in 60 minutes. 18,000 b.p.h. equals 5 per second, which means the "sweep" function actually "ticks" 5 times per second. 36,000 b.p.h. equals 10 "ticks" per second...so fine you really have to study the motion very closely to see it. Theoretically, higher b.p.h. rates can make a watch more accurate, but that's only on paper as there are many more factors to the accuracy equation. Higher b.p.h. rates can shorten the lifespan of a movement due to increased potential for friction and require more frequent servicing.




As a basic example the diagram above shows the parts of the balance & hairspring assembly. The balance wheel (1) is mounted on the balance staff (the axel running through its center). The hairspring (2) is attached at its inner end to the balance staff by a collet (4), and to the balance bridge (3) at its outer end by a stud (5). The balance staff is attached to the balance bridge at one end using a hole jewel and cap jewel with shock protection, and to the main plate of the movement at it's other end. The stud is attached to a mobile stud holder (6) which is clamped to the balance bridge. The regulator (8) has two regulating pins (7) at it's end which the hairspring passes through. The position of the regulator determines how the watch keeps time by essentially making the hairspring seem a different size. By moving the regulator to a spot that makes the effective length of the hairspring shorter the watch will run faster. If it's moved to a position that makes the effective length longer, the watch runs slower. The spacing between the regulating pins also affect accuracy. The further apart they are, the slower a watch will run.

Another process used to make a watch run smoothly and accurately is called poising. This works similar to the idea behind balancing a tire, but not quite as easily accomplished. The more balanced a balance wheel is, the easier it is to regulate and the less stress is put on the hairspring, resulting in a more consistent oscillation.

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Early watches had threaded holes along the perimeter of the balance wheel where screws could be placed to compensate for weight imperfections. If extra weight was needed washers could be installed under the screws. The first photo above is an example of this from an Omega "bumper" from 1950. First the wheel was made entirely of steel, and then later with an outside layer of brass, and had two "spokes" that ran across the diameter of the wheel. It was found that dividing it into two pieces helped with tolerance problems in different temperatures due to expansion and contraction. The next advancement was the use of an alloy called Invar which didn't change size with the temperature. Modern watches use an alloy of beryllium, copper, and iron called Glucydur to fashion the balance wheel in a better quality piece. Lower priced watches use a nickel alloy for the wheel. The second photo above is an example from a Maurice LaCroix from 2006 with a 3 spoke design. Glucydur (or beryllium bronze) is non-magnetic and resists deforming from temperature changes or abuse much better than steel. After a few years of using the new alloy, watchmakers found the adjustment screws became obsolete if the wheel was poised properly initially. Any heavy spots on the wheel are pinpointed while the watch is running and are shaved off to achieve uniformity.

Nickel (left) and Glucydur (right) balance wheels with hairspring and stud attached:




Hairsprings went through changes as well although they still have the usual 12 to 15 turns in a spiral pattern. The strength of the hairspring governs the arc traveled and the elapsed time of the rotations of the balance wheel. As materials advanced over the years, hairsprings also improved. Early steel springs would have their characteristics altered significantly by temperature changes. An alloy named Elinvar was developed to work with Invar balance wheels, and it retained consistent elasticity in temperatures that would cause steel springs to stiffen. Modern springs are made from alloys like Nivarox - chromium, titanium, nickel, beryllium, aluminum, and iron. These new metals stay within tolerances much better than steel. Rolex uses Parachrom, a proprietary alloy that consists of niobium, zirconium and oxygen among other elements which is then blued. Parachrom is completely anti-magnetic and extremely resistant to temperature changes.

Parachrom blue hairspring:


photo & parachrom information courtesy ofJBHII / John Holbrook


The balance wheel is matched to the hairspring based on equations that factor many properties in. Several physical measurements including the diameters of both pieces, the elastic torque of the hairspring, the rotational inertia of the balance wheel and even the frequency of the movement are all taken into account. Since this assembly is meant to regulate the entire movement, it also must compensate for any faults or disturbances caused by all the other pieces so the watch will keep time accurately. This, plus the beating motion, is why the balance and hairspring assembly is called the "heart" of a watch.