Pocketwatch 101^℠ – Learn about Vintage and Antique Pocket Watches

Going Barrel? Motor Barrel? What's the Difference?

What are the different types of mainspring barrels and winding mechanisms used in vintage watches?

The Mainspring Barrel

The mainspring is the source of motive power for a watch, and the mainspring "barrel" is the little can, sometimes with a geared rim, which holds and contains the mainspring. At the center of the barrel is an axle or arbor to which one end of the mainspring is hooked. The other end of the mainspring attaches to the inner wall of the mainspring barrel.

Since the mainspring is attached at both ends, then if the arbor is held while the barrel is turned, the mainspring is going to wind around the arbor, and will be under tension which will exert a pulling force on the barrel. Similarly, one could hold the barrel while turning the arbor to achieve the same result. The spring gets wound-up inside the barrel, and then supplies power to the watch as it slowly unwinds. The controlled release of mainspring power is goverened by the watch's escapement.

Understanding the various types of mainspring barrels encountered in vintage watches is all about understanding a) where and how the winding tension is applied to the spring, and b) how the power of the spring is transferred to the gear-train of the watch.

Simple Mainspring Barrel

The simplest mainspring winding arrangement is when the mainspring is attached to the center-arbor on one end, and attached to the barrel on the other end as described above. The watch is wound-up by turning the barrel which winds the mainspring around the barrel arbor. The spring-tension causes the barrel to turn in the opposite direction, imparting power to the gear-train of the watch.

Early mainspring barrel.

Actual diameter 3/4"

One of the main problems with this arrangement is what watchmakers refer to as isochronism. This is the tendency for the watch to run at different rates when under different amounts of spring tension. In this most basic barrel arrangement, the barrel applies a lot more force to the gear-train of the watch when the mainspring is fully wound than it does when the mainspring is run down. This causes the watch to run faster when the spring is fully wound, and slower as the spring winds down... an undesirable characteristic for accurate time-keeping.

The other problem with this arrangement is the lack of what watchmakers call "maintaining-power". While the watch is being wound, there is no driving tension on the gear train, so the watch essentially stops during the time it is being wound. Another significant problem is that if the mainspring breaks (and old steel mainsprings had a tendency to break), all of the power of the spring could be instantaneously released into the mechanism of the watch, often causing significant damage in the form of cracked jewels and sheared gear-teeth.

For all of these reasons, simple winding barrels weren't used much past the early-1800's, and are almost never found on vintage American pocket watches.

Mainspring with Fusee

An even earlier and quite successful approach to solving the problem of isochronism was the mainspring barrel and fusee combination. A fusee is another barrel of sorts, but it's conical in shape. It looks and functions similarly to the derailleur on your bicycle. In a sense, the fusee allows the mainspring to "shift gears" as it goes from full-tension to low-tension.

Typical Fusee.

About 3/4" diameter.

The fusee and mainspring barrel sit side-by-side within the watch movement. The fusee acts as a power-regulator between the simple mainspring barrel and the gear-train of the watch. The mainspring barrel is connected to the fusee barrel with a tiny chain, which looks almost like a microscopic bicycle chain. That's why fusee watches are sometimes referred to as "chain-driven" watches.

As the mainspring is wound, the chain is wound onto the conical fusee barrel, first winding around the wide end of the cone and progressing up the cone to the narrow end. So when the watch is fully wound, the lever-arm of the chain acting on the fusee is shorter. As the watch runs down, the fusee chain winds back onto the outside of the mainspring barrel, and the chain unwinds to the wide end of the cone. So when the mainspring has low spring-tension, the lever-arm of the chain acting on the fusee is longer. This ingenious mechanism, when properly implemented, can deliver a constant and uniform force from the mainspring to the gear-train of the watch.

Early woodcut of mainspring and fusee barrels.

The fusee mechanism was first used in clocks in the 15th century, and was widely used in watches and clocks through the 19th century. It came into use with the invention of the first spring-driven clocks, and was a major technological advancement for improving the accuracy of spring-driven timepieces. These days, fusees are most frequently encountered on European watches from the early-to-mid 1800s, both with verge and lever escapements.

Going Barrel

If you take a simple barrel, as described above, and attach a ratcheted winding-gear to the center-arbor, then the mainspring can be wound by turning the center arbor rather than turning the barrel. The winding power is applied to the center-arbor through some arrangement of winding gears, and the geared outer rim of the barrel imparts the mainspring's power to the gear-train of the watch. A big advantage of this arrangement is that the barrel always maintains spring tension, even while the watch is being wound; a clever solution to the problem of maintaining-power! Thus the name "going barrel"... because the barrel is always "going" even when the watch is being wound.

The going-barrel is the most commonly encountered mainspring mechanism on vintage American and European watches made in the past 150 years.

The Safety Pinion

About the same time as the going barrel became the dominant mainspring barrel arrangement, the "Safety Pinion" was introduced as a solution to the problem of broken mainsprings. The safety pinion didn't prevent the mainspring from breaking, but it prevented the damage that often occured when the power of the mainspring was instantaneously released into the gear-train of the watch.

In a going barrel, the outer rim of the mainspring barrel has gear-teeth, which drive the pinion on the center-wheel of the watch. That's the point of connection between the mainspring barrel and the first wheel in the gear-train. If the watch has a "safety pinion", the center wheel pinion is threaded and attached to the center wheel arbor with a reverse screw-thread. Under normal operation, the force of the spring just keeps the pinion screwed on more tightly to the center-arbor, but if the spring breaks and reverses the direction of the force, the pinion simply unwinds... which instantly disconnects the mainspring from the rest of the gear-train.

If your American pocket watch watch is marked "Safety Pinion" then it has this unwinding pinion arrangement. The safety pinion can no doubt be credited with saving countless watches from serious damage caused by a broken mainspring.

The Safety Pinion, disassembled and assembled for comparison.

The Safety Pinion. Left: wheel and pinion disassembled, as it would be after being "unwound" by a broken mainspring.
Right: wheel and pinion assembled for comparison. Enlarged to show detail.

Safety Barrel

The terms "motor-barrel" and "safety-barrel" were used by various watch companies to describe the winding mechanisms on their watches. While these terms are sometimes used interchangeably, they are actually different implementations of a similar idea. The Safety Barrel was an arrangement used by Waltham and Elgin in a lot of their mid-grade watches. In a safety-barrel watch, the barrel arbor and the barrel are "attached" by means of a square hole at the bottom of the barrel, and a matching square shoulder on the arbor. Because of this rigid coupling of barrel and arbor, when a mainspring breaks the force of the broken spring is transmitted through the barrel to the barrel arbor where the force can be harmlessly expended by free-rotation of the ratchet wheel.

Motor Barrel

Another attempt to address the problem of damage done by broken mainsprings was the "Motor Barrel". In a true motor-barrel, the functions of the barrel and arbor are reversed: the barrel is turned to wind the watch, and the barrel arbor turns as the watch runs down. Since the power is delivered through the center arbor, instead of through the geared barrel, any instantaneous release of power resulting from a broken spring is transmitted primarily into the winding gears (similar to the safety barrel), which can be made much stronger than the delicate gear-train and escapement of the watch. Because the barrel only turns when the watch is being wound, the motor-barrel allows for a more efficient transfer of power to the gear-train, which allowed for the use of longer and thinner mainsprings to achieve the same power, thus increasing run-time.

Note that some watches made by Illinois and Hamilton which are labeled as "motor-barrel" watches actually use a different arrangement that does not behave as described above. In this watchmaker's opinion, these watches are not true "motor-barrels" even though they are marked as such. In the Hamilton-Illinois motor-barrel the outer end of the mainspring is attached to the geared barrel which drives the center pinion... exactly like a going barrel. Any advantage of a safety barrel is lost, and a broken mainspring would result in a shock to the gear-train unless a safety pinion (or some other safety arrangement) is employed.

The jeweled motor-barrel escutcheon

with its 3 small screws is clearly

visible in the lower right.

Motor-barrel watches appeared in the late 1800's, and continued to be produced through the mid 20th century. Many of the highest-grade watches produced by American watch manufacturers were models which featured motor-barrels or jeweled motor barrels (see below). A motor barrel can often be recognized by the ratchet-wheel escutcheon which is typically attached with 3 small screws rather than one center screw as shown in the photo.

Jeweled Motor Barrel

A jeweled motor barrel just completes the jeweling of all the wheel-arbors in the watch. Adding jewels to the mainspring arbor of a motor barrel watch further reduces friction and improves theoretical efficiency of power transfer. While it's certainly likely that the effect of jeweling the motor barrel is negligible, it nonetheless carries functional jewelling of the gear-train to its logical conclusion, and allows the manufacturer to claim a higher jewel-count in their advertising! A jeweled motor-barrel was the standard for the highest-grade American timepieces.

Automatic Winding

While commonly only found on wristwatches, the autowind mechanism is an important variation of the mainspring barrel that deserves mention. First developed in the 1950's, the "self-winding watch" uses the oscillation of a small weight attached to the winding gears as the winding force. The movement of the wearer's arm causes the weight to swing back and forth, and a reverser-gear arrangement translates this oscillation in either direction into a winding force applied to the mainspring.

The problem with this arrangement is that the watch never stops winding, even when the spring is fully wound. If the outer end of the mainspring is rigidly attached to the mainspring barrel wall, the spring will continue to be wound until it either breaks or detaches from the mainspring barrel. As such, there is a need for a clutch mechanism to prevent over-winding the watch. This is achieved by attaching the outer end of the mainspring to a "bridle", a small piece of spring which presses against the inner wall of the barrel, but which can slide against the barrel wall if needed.

During normal winding, the friction of the bridle against the barrel wall is sufficient to hold the mainspring in place, but continued winding after the spring reaches full tension will overcome the bridle friction, and the lubricated bridle will slide along the inner wall of the barrel, thus preventing over-winding and damage to the watch.

Pocketwatch 101℠ – Learn about Vintage and Antique Pocket Watches