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The Mainspring

From Horology magazine, June, 1939

The Mainspring
By Rev. Francis Bimanski

A century ago the famous philosopher, Balmes, mentioned a rather simple way of improving our knowledge and wisdom. "Before reading a book or even a chapter," he says, "just sit down, gather your thoughts and test your knowledge on the subject, reviewing your past studies and experiences. Then only begin your lecture, correcting and enlarging your views and storing new treasures in your memory." This method is surely full of common sense. The philosopher possessed a deep insight of human nature. He knew how little we profit from all the long years of study. It would be well, just for relaxation, to try the method and test, for instance, our present knowledge of a subject of some importance, I mean the mainspring.

A spring, in general, requires a material that yields to a force and at the same time resists. This resistance is used as a new power. In due proportions, the resisting force is equal to the force applied. In great mechanics an axiom says, "The width of the spring increases the resistance power proportionately  the thickness cubes it." The reason for the last is obvious. The thicker the band, the greater is the exterior expansion and the interior compression. But, the axioms of great mechanics do not seem always to fit fully in the smaller mechanics of horology, according to the observations of some learned professors. The reason is that tests from which general laws were deduced and formulated were limited in their character and numbers.

Elasticity implies a resistance to force and the faculty of returning to the original form. It has it limits, a spring may be overtaxed and deform or break. The study of 15 used mainsprings shows a difference of .01 and .02mm between the center and two ends.

Mainsprings in watches:

A mainspring is a band of steel embedded in a barrel. Its duty is to furnish power to the watch. The process of making a first class spring is not so simple as we might expect. There are numerous operations, the choosing of the right material, the hot and cold rolling into bars and strips, the frequent annealing, the cutting into strips, the testing for hardness and flexibility and the equalizing of the tension while reversing the coils. Yet, each factory seems to keep its own secret, they say.

In late years much experimenting has been done. Different alloys have been and are at present proposed. Beryllium bronze is gaining in favor. The experiments with steel inaugurated by the auto factories will sooner or later produce an alloy that will satisfy the demands of the profession. Meanwhile, which ones are the best only time will tell.

The spring must have the right dimensions in width, length, and thickness. Width is determined by the barrel, about .lmm is granted for freedom of action.  The latest experiences seem to favor wide springs.

Length is limited by the size of the barrel. The spring must be long enough to furnish steady power for 36 hours. It cannot be too short, nor too long, although an excess of one centimeter in a 16 size watch would not exercise a great influence, according to a Swiss professor.

The rule is, the empty area must be equal to the area taken by the spring, or the area for winding and unwinding must be equal. The object is to obtain the maximum capacity of the spring. It is clear that the unwound coils are less numerous than the wound ones, and that the difference gives the number of active coils. Some find it difficult to divide the area reserved for the spring into equal areas. The theoretical problem belongs to geometry, the practice is easy. In the area reserved for the spring draw a circle and a line perpendicular to a radius passing through the center of the barrel. The two points at the periphery will give two points for the circle dividing the area into two parts. 

In practice 4 turns are usually required for 24 hours running and an extra turn is kept in reserve to last 36 hours. A Geneva rule says that out of 6 1/2 to 7 turns, 5 are useful, others say 5 1/5 to 6, of which 4 are useful. The reserve allows the best part of the spring to do the work. For this reason in railroad watches longer but thinner springs seem to be preferred.

Thickness is also called strength. Some require 1/32 of arbor; others 1/80 - 1/100 of interior diameter. It is somewhat surprising that the tables of the wholesale catalogs mention only the width and the strength, while little is said about the elasticity or resiliency. And yet, it seems elasticity does not depend on the width and strength only.

The elasticity of springs showing the same strength and width but made by different companies may vary. The subject is rather important because thin and highly elastic springs are in favor. Firms seem to realize this point. Thus, we see new alloys produced every year and praised to the skies. This is a proof that we are still in the progressive stage.  Soon we shall have tables indicating the elasticity.

A loss of power is naturally expected. The loss is due to two factors. First, the long way through the transmission wheels. Then the friction in the barrel, between the teeth of the wheels and in the walls of the bearings. Some calculations put the loss at 40, even 50 percent. It might be much more.

How to explain the occasional falling off of the power? This may occur after the first turn. This phenomenon is explained by the irregular tension of the spring. This fault has been recognized by the factories which remedy the evil in the process of equalizing the tension.

Choosing a spring:

Different tables and graphs have been published in the course of years, Grossman, Degallier, Gribi, Jaquet, Mathey, etc. But, will they meet the requirements of the American watch of today? It is clear that coarse watches will need strong springs to overcome the friction, which may vary from 2 to 20 percent. To reduce the friction rounded edge or double side bulged springs have been introduced.

It is a consolation for us to have such catalogs as those published by American wholesale houses. They satisfy every watchmaker.


Keep the spring clean, use mainspring oil and keep acid fingers away. In olden days winders were imperative because of the stiffness of the springs. Nowadays winders are used, when they fit the case, otherwise the use of fingers, though strictly forbidden, may prove good tools if properly used in case of dire necessity.


Why? Lots of suggestions and experiments have been proposed, and the imagination was helping too. The primary cause may be found in unavoidable defects in different processes of alloying, rolling, hardening, tempering, polishing. The secondary cause might be the overtaxing of the strength.

Two special features are the Maltese cross or Geneva stop, praised so much of old for allowing the best part of the spring to do the work and the English fusee, invented to produce a uniform force. Both will soon be forgotten in watches on account of the high grade spring with great elasticity.

Repairers are interested in the snailed arbor, which occupies 1/3 of interior barrel area, and in the interior attachment. Those attachments have various forms. The best is the one that allows concentric development. The reinforced tongue seems to have the best results, though some still use T ends in certain models. The tongue or simple hook might be consigned to the cheaper grades.

Men of learning tried to find some relation between the balance and the mainspring. But this study together with many others, seems rather to concern the constructor. 

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