Mainspring
A mainspring is a spiral torsion spring of metal ribbon—commonly spring steel—used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it unwinds, until the next winding is needed. The adjectives wind-up and spring-powered refer to mechanisms powered by mainsprings, which also include kitchen timers, metronomes, music boxes, wind-up toys and clockwork radios.
Mainsprings appeared in the first spring-powered clocks in 15th-century Europe. The mainspring replaced the weight hanging from a cord wrapped around a pulley, which was the power source used in all previous mechanical clocks.
Modern mainsprings
A modern watch mainspring is a long strip of hardened and blued steel, or specialised steel alloy, 20–30 cm long and 0.05-0.2 mm thick. The mainspring in the common 1-day movement is calculated to enable the watch to run for 36 to 40 hours, i.e. 24 hours between daily windings with a power-reserve of 12 to 16 hours, in case the owner is late winding the watch. This is the normal standard for hand-wound as well as self-winding watches. 8-Day movements, used in clocks meant to be wound weekly, provide power for at least 192 hours but use longer mainsprings and bigger barrels. Clock mainsprings are similar to watch springs, only larger.Since 1945, carbon steel alloys have been increasingly superseded by newer special alloys, and also by cold-rolled alloys. Known to watchmakers as "white metal" springs, these are stainless and have a higher elastic limit. They are less subject to permanent bending and there is scarcely any risk of their breaking. Some of them are also practically non-magnetic. Proprietary alloys include SPRON made by Seiko and Nivarox by Swatch Group.
In their relaxed form, mainsprings are made in three distinct shapes:
- Spiral coiled: These are coiled in the same direction throughout, in a simple spiral.
- Semi-reverse: The outer end of the spring is coiled in the reverse direction for less than one turn.
- Reverse : the outer end of the spring is coiled in the reverse direction for one or more turns.
Operation
The mainspring is coiled around an axle called the arbor, with the inner end hooked to it. In many clocks, the outer end is attached to a stationary post. The spring is wound up by turning the arbor, and after winding its force turns the arbor the other way to run the clock. The disadvantage of this open spring arrangement is that while the mainspring is being wound, its drive force is removed from the clock movement, so the clock may stop. This type is often used on alarm clocks, music boxes and kitchen timers where it doesn't matter if the mechanism stops while winding. The winding mechanism always has a ratchet attached, with a pawl to prevent the spring from unwinding.In the form used in modern watches, called the going barrel, the mainspring is coiled around an arbor and enclosed inside a cylindrical box called the barrel which is free to turn. The spring is attached to the arbor at its inner end, and to the barrel at its outer end. The attachments are small hooks or tabs, which the spring is hooked to by square holes in its ends, so it can be easily replaced.
The mainspring is wound by turning the arbor, but drives the watch movement by the barrel; this arrangement allows the spring to continue powering the watch while it is being wound. Winding the watch turns the arbor, which tightens the mainspring, wrapping it closer around the arbor. The arbor has a ratchet attached to it, with a click to prevent the spring from turning the arbor backward and unwinding. After winding, the arbor is stationary and the pull of the mainspring turns the barrel, which has a ring of gear teeth around it. This meshes with one of the clock's gears, usually the center wheel pinion and drives the wheel train. The barrel usually rotates once every 8 hours, so the common 40-hour spring requires 5 turns to unwind completely.
Hazards
The mainspring contains a lot of energy. Clocks and watches need to be serviced, cleaned and lubricated periodically, and if precautions are not taken during disassembly the spring can release suddenly, causing potentially serious injury. Before servicing, mainsprings are “let down” gently by pulling the click back while holding the winding key, allowing the spring to slowly unwind. However, even in their “let down” state, mainsprings contain dangerous residual tension. Watchmakers and clockmakers use a tool called a "mainspring winder" to safely install and remove them. Large mainsprings in clocks are immobilized by "mainspring clamps" before removal.History
Mainsprings appeared in the first spring-powered clocks, in 15th-century Europe. It replaced the weight hanging from a cord wrapped around a pulley, which was the power source used in all previous mechanical clocks. Around 1400 coiled springs began to be used in locks, and many early clockmakers were also locksmiths. Springs were applied to clocks to make them smaller and more portable than previous weight-driven clocks, evolving into the first pocketwatches by 1600. Many sources erroneously credit the invention of the mainspring to the Nuremberg clockmaker Peter Henlein around 1511. However, many references in 15th-century sources to portable clocks 'without weights', and at least two surviving examples, show that spring-driven clocks existed by the early years of that century. The oldest surviving clock powered by a mainspring is the Burgunderuhr, an ornate, gilt chamber clock, currently at the Germanisches Nationalmuseum in Nuremberg, whose iconography suggests that it was made around 1430 for Philip the Good, Duke of Burgundy.The first mainsprings were made of steel without tempering or hardening processes. They didn't run very long, and had to be wound twice a day. Henlein was noted for making watches that would run 40 hours between windings. The 18th century methods of making mainsprings are described by Berthoud and Blakey
Constant force from a spring
A problem throughout the history of spring-driven clocks and watches is that the force provided by a spring is not constant, but diminishes as the spring unwinds. However, timepieces have to run at a constant rate in order to keep accurate time. Timekeeping mechanisms are never perfectly isochronous, meaning their rate is affected by changes in the drive force. This was especially true of the primitive verge and foliot type used before the advent of the balance spring in 1657. So early clocks slowed down during their running period as the mainspring ran down, causing inaccurate timekeeping.Two solutions to this problem appeared in the early spring-powered clocks in the 15th century; the stackfreed and the fusee:
Stackfreed
The stackfreed was an eccentric cam mounted on the mainspring arbor, with a spring-loaded roller that pressed against it. The cam had a 'snail' shape so that early in the running period when the mainspring was pushing strongly, the spring would bear against the wide part of the cam, providing a strong opposing force, while later in the running period as the force of the mainspring decreased, the spring would bear against the narrower part of the cam and the opposing force would also decrease. The stackfreed added a lot of friction and probably reduced a clock's running time substantially; it was only used in some German timepieces and was abandoned after about a century.Fusee
The fusee was a much longer-lasting innovation. This was a cone-shaped pulley that was turned by a chain wrapped around the mainspring barrel. Its curving shape continuously changed the mechanical advantage of the linkage to even out the force of the mainspring as it ran down. Fusees became the standard method of getting constant torque from a mainspring. They were used in most spring-driven clocks and watches from their first appearance until the 19th century when the going barrel took over, and in marine chronometers until the 1970s.Stopwork
Another early device which helped even out the spring's force was stopwork or winding stops, which prevented the mainspring from being wound up all the way, and prevented it from unwinding all the way. The idea was to use only the central part of the spring's 'torque curve', where its force was more constant. The most common form was the Geneva stop or 'Maltese cross'. Stopwork isn't needed in modern watches.Remontoire
A fourth device used in a few precision timepieces was the remontoire. This was a small secondary spring or weight which powered the timepiece's escapement, and was itself rewound periodically by the mainspring. This isolated the timekeeping element from the varying mainspring force.Going barrel
The modern going barrel, invented in 1760 by Jean-Antoine Lépine, produces a constant force by simply using a longer mainspring than needed, and coiling it under tension in the barrel. In operation, only a few turns of the spring at a time are used, with the remainder pressed against the outer wall of the barrel. Mathematically, the tension creates a 'flat' section in the spring's torque curve and only this flat section is used. In addition, the outer end of the spring is often given a reverse curve, so it has an "S" shape. This stores more tension in the spring's outer turns where it is available toward the end of the running period. The result is that the barrel provides approximately constant torque over the watch's designed running period; the torque doesn't decline until the mainspring has almost run down.The built-in tension of the spring in the going barrel makes it hazardous to disassemble even when not wound up.