Mainspring
The ribbon of steel which serves to produce the motive power for a watch, chronometer, or clock. It is said to be the invention of Peter Hele, a clockmaker of Nuremberg, about the year 1500.
It would appear that the mainspring, when first applied to the watch, was not enclosed in a barrel, but the outer end of the spring was bent into the form of a hook and fixed to a winding arbor, together with a ratchet wheel and click. A guard was attached to one of the plates in order to check the outer coil of the spring and prevent it expanding too far. The inner end was made fast to the axis of the great wheel, consequently it was wound up from the center. The re-expansion set the train in motion.
The motive force due to the tension of a spring is more or less variable. The causes of this want of uniformity, says Saunier, are as follows: The elastic reaction of a spring becomes greater as the spring is further wound up. A metallic blade is very rarely homogeneous, and worked with sufficient care to avoid different parts being of variable strength. Its energy alters with time dependent on the duration and intensity of the flexure, and this change nearly always occurs irregularly throughout its length. Its elastic force diminishes slightly on elevating the temperature, and lastly, a spring rubs against the bottom and lid of the barrel in uncoiling. The successive coils also adhere and rub together, either permanently or occasionally. All these resistances are from the nature of the case variable.
Various forms of mainsprings have been adopted from time to time. The cylindrical spring was one in which the central coils were made thicker with a view to diminish the differences in the pull of the spring when wound up to varying degrees, and to increase its energy when nearly run down. The spring, when fully wound up, rubbed together in the central coils, so that the motive force when it was fully wound was neutralized by the friction. These springs are very rarely seen now, as they were expensive to manufacture, and the advantages they possessed were more apparent than real. The taper spring was another form, which is rarely seen now. The thickness of the metal in these springs, gradually diminished throughout its entire length, the effect being to make the coils, when fully wound up, separate, and on this account the spring developed freely. This form was abandoned on account of the cost of manufacture. The third form is the ordinary spring in use to-day, the thickness of whose coils is the same throughout. The development is less uniform than with the tapered spring, as is also the separation of the coils, but it is cheaper of construction, and the variations do not exceed the limits that ordinary escapements can neutralize.
M. M. Roze, in a work on the mainspring, lays down and demonstrates the following theorems: 1. A mainspring in the act of uncoiling in its barrel, always gives a number of turns equal to the difference between the number of coils in the up and down positions.
For example, if 17 is the number of coils when the spring is run down, and 25 is the number when against the arbor, the difference between 17 and 25 or 8, will represent the number of turns in the uncoiling. 2. With a given barrel, spring and arbor, in order that the number of turns may be a maximum, it is necessary that the length of the spring be such that the occupied part of the barrel, (exclusive of that filled by the arbor), be equal to the unoccupied part; in other words, the surface covered by the spring when up or down must be equal to the uncovered surface of the barrel bottom.
The diameter of the arbor is not an arbitrary quantity, as it depends on the duration of flexure and thickness of the spring, and this depends greatly on the quality of the metal; if it is too small, it is liable to rupture the spring and deprive it of part of its elastic reaction, and if too large, part of this reaction will be wasted. M. Roze demonstrated that the thickness of the spring should be to the diameter of the arbor as 1:26 or 34, according as the rotation of the barrel takes place more or less rapidly. For example, 1:26 is best suited to watches; 1:30 for chronometers; and 1:34 for clocks or time pieces that are expected to go for longer periods. (If the reader is desirous of studying the subject at length, he is referred to Saunier's Modern Horology, pp. 661 to 673 inclusive, and a Simple and Mechanically Perfect Watch, by Moritz Grossman. Geo. K. Hazlitt & Co., Chicago.) Until within a very few years mainsprings were made by a method that had been in use, and never improved on, for years.
About 1885 the American Waltham Watch Company secured the services of foreman Logan, who for years had been engaged in making hairsprings, and was about to carry out a scheme for making mainsprings, which he had long experimented upon and secured patents on.
At the outset, Mr. Logan forsook the old methods of manufacturing springs, and adopted new and novel ways of producing better results at less cost. The experiments necessary to such a radical change were costly, but the improvement in the quality and finish of the springs was so gratifying that mechanical appliances in great variety have from time to time been put to work, so that to-day the product is double that of two years ago, while the number of employes necessary is about one-half, owing to automatic machinery. The steel used is manufactured expressly for springs, and comes in strips varying in length from one hundred to five hundred feet. As may well be supposed the best quality of steel adapted for the peculiar demands of a first-class watch mainspring was not found without much trouble, experimenting and expense. Steel made in England, France, Belgium, and this country were tried. After a series of trials, just the kind of steel desired was obtained. This steel is run through a machine which cuts it into numerous narrow ribbons, of widths suitable for the particular size of spring desired. These ribbons are simultaneously wound upon bobbins, and are next passed, individually, through specially designed rolls, to bring the steel to a more exact and uniform thickness than can possibly be obtained from steel makers. The next operation is that of rounding the edges, which is done by a new and unique machine. Following this the flat sides are ground and polished. Up to this point the steel is in the untempered condition in which it is received at the factory. Hardening and tempering is next in order; these operations are performed by new methods which are almost automatic. One of the elements which contributes largely to the very successful treatment of the steel in this important but delicate part of spring making is that of the fuel used for heating. In the earlier days of the manufacture a great variety of fuel was tried, but nothing has been found to equal the carefully purified water gas, which is now used.
Next in order the finishing polish is put on the sides and edges of the ribbons. Next the ribbons are cut up into exact lengths for individual springs. The ends are then annealed, preparatory to the punching for the reception of the barrel arbor hook and the tip. After punching follows coiling, when any faults in tempering are made apparent. If the steel has been overheated the severe strain of coiling will cause it to break. Failure to draw the temper sufficiently low will produce the same result, while too low a temper will cause them to "set," and thereby indicate their worthlessness. Too soft springs are seldom found and the breakage in coiling is less than one-third of one per cent. The springs returned from the finishing department of the American factory for unsatisfactory performance in any direction amount to less than one-half of one per cent.
After the springs have been coiled, the tips riveted on, they are care fully gauged and then oiled to prevent rusting. They are then either wound into capsules (from which they can be transferred directly into a watch barrel) or enclosed in packages containing one dozen each.
Dimensions of Mainsprings