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The Hairspring and Balance Wheel Assembly of the HAMILTON MARINE CHRONOMETER

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Hamilton Watch Company

The Hairspring and Balance Wheel Assembly of the Hamilton Marine Chronometer

By

E. W. Drescher

Assistant to Chief Engineer

Hamilton Watch Company

Lancaster, Pa.

Reprinted from H. A. A. Journal August 1946.

 

One of the very latest achievements in the persistent progress of horology is the hairspring and balance wheel assembly of the Hamilton marine chronometer.  This assembly has contributed to the history of the marine chronometer records of performance never previously attained,-- records of performance which have been repeated not just once or twice, but thousands of times.  Its success was the result of an intensive, modern engineering analysis which depended,  for its execution, upon the traditional craftsmanship and skill of the horological industry.  A detailed description of this balance unit and an explanation of the many features which contribute to its remarkable properties are being given her for the first time.

Before the days of Christopher Columbus and down through the years until the latter part of the eighteenth century, whenever man ventured out of sight of land he had at his command only crude methods of dead reckoning to determine his location at sea.  This method of navigation involved computation of the course of a ship on the basis of the known location of the starting point and estimates of direction, speed, drift and so forth.  It was only as precise as the observation of these variable factors.  Its inaccuracies not only caused endless inconvenience, but introduced hazards of shipwrecks on voyages of long or short duration.

Following the period of discovery, as colonial expansion and sea commerce continued to develop, the need for a better solution to the paramount problem of navigation became even more critical.  Early mariners, for the determination of their latitudes (or north-south positions) at sea, had employed a method satisfactory in principle but only approximate in execution.  About 1730, by the invention of the sextant, this method was defined to nearly its present state,   Determination of longitude, however was found to be a much more difficult problem.  Its solution was regarded so important that during the seventeenth and eighteenth centuries many leading nations offered immense financial rewards for the "discovery of the longitude."  The most classical of these was that posted by England through the especially created Board of  Longitude.

At that time, various methods of determining longitude were being advocated.  The most feasible of all appeared to be that one requiring these three factors:  an almanac, a sextant, and an accurate source of time.

By means of the almanac and sextant, both already at his disposal, a navigator could readily determine his local time at sea.  If he could only know by how much hi local time differed from that at the zero meridian (Greenwich) at the moment of observation, he could readily establish his longitude through the relationship: 24 hours equals 360 degrees of longitude.  Hence, the development of an accurate, portable timepiece became a challenge to the horologists of the period.

The man who successfully met this challenge was John Harrison.  His was a long struggle.  He face many obstacles in proving the excellence of his chronometer.  But finally, by his genius and persistence, he qualified for the prize of 20,000 pounds sterling offered by the Board of Longitude.  He had created a timepiece that would assure determination of longitude within half a degree,--thirty nautical miles.  Through this achievement marine navigation became an exact science and an invaluable instrumentality in man's ever-growing conquest of the seas.

Succeeding generations of horological craftsmen refined and improved the chronometer.  They contributed steadily to its value and importance to the sea-faring men of all nations, making it the most treasured and carefully guarded piece of equipment aboard ship.  And even now, in our own era of advanced scientific development, the marine chronometer remains an indispensable implement of marine navigation.  This is a tribute to the art and science of horology.

However, down through the years until the entry of this country into the recent war, the production of chronometers was conducted on a small-volume basis.  Skilled chronometer makers, individually or in small groups, produced and assembled the  parts and adjusted the instruments.  Sources of supply were confined almost entirely to England and Switzerland.

As the participation of the United States in the war became imminent in 1940 and 1941, plans were laid for the building of history's greatest navy.  And out of this program sprang history's greatest demand of marine chronometers.  Existing sources of supply were hopelessly inadequate.  Some were threatened with extinction, others with virtual isolation.  How was the United States Navy to get these chronometers-- vast quantities of them--in the shortest possible time?  Could dependability of performance be assured?  Could greater accuracy be attained to meet the precise requirements of the modern warfare?  This was the challenge issued by the United States Navy through the United States Naval Observatory.

The Hamilton Watch Company accepted that challenge and, with the fine cooperation of the Naval Observatory, designed and produced a marine chronometer which fully met the Navy's requirements.  The success of that venture is attested by the performance records of 10,000 Hamilton chronometers delivered to the Navy and the Maritime Commission since early in 1943, sometimes at the rate of 500 per month! 

At first glance, the Hamilton marine chronometer is conventional in appearance.  However, a detailed examination will reveal many innovations.  Some were created for the benefit of volume production, but other were deemed necessary for the sake of excellence alone.   Characteristic of the former are the essential interchange-ability of the parts and the accessibility and service conveniences.  Of the later, the most outstand are those incorporated in the hairspring and balance assembly.  These also constitute the most significant departures from previous chronometer construction.  Just how these improvements came about can be understood best by  first analyzing the problem that existed before their development.

An appraisal of the various elements of a chronometer movement revealed that although the mainspring, fusee, train and escapement all contribute to the performance and serviceability of the instrument, these elements had already been developed to a stage bordering on perfection; that the inherent qualities of the hairspring and balance wheel were the limiting conditions affecting its precision of performance.

From analysis of an extensive series of tests on a variety of chronometers, the principal faults in rating were summarized as follows:

1.  Instability of rate.  After the balance wheel has been stopped or after a change in in temperature, the recovery of rate is not immediate or not exact.  After service or overhaul, an appreciable running-in or settling period is required before the rate becomes stable.

2.  Large isochronal error.  Slight changes in amplitude of balance wheel motion caused by variations in the driving force are usually accompanied by a significant change in rate.  As a rule, the rate increases when the motion decreases.

3.  Temperature compensation error.  This is appreciable and definite middle temperature errors exist, particularly when wide ranges of temperature are encountered. Compensation devices are varied and complicated in construction.  Adjustment of compensation error is rather difficult with these constructions and usually affects other performance characteristics.

Hence, in the design of a new marine chronometer balance unit, the prime objectives were maximum stability of rate and minimum isochronal and temperature  compensation errors.  Furthermore, it was imperative that these characteristics be achieved, in so far as possible, through inherent qualities of the various components.  Arduous and time-consuming application of personal skills in adjustment could not be tolerated because of the necessity for rapid and large volume production and the limited supply of highly experienced personnel.

In considering how these objectives were accomplished let us first view the balance as it appears in the assembled movement (Figure 1) and then discuss the various components in the following order:

1.  The hairspring,

2.  The hairspring collet and stud,

3.  The balance wheel, and

4.  The balance wheel hub assembly.

Figure 1, Hamilton Model 21, movement.

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The Hairspring (Figure 2)

Figure 2, Hamilton Model 21, Hairspring.

To contribute its part toward solution of the problem the principal requirements demanded of the hairspring were high elasticity, proper thermo-elastic properties and minimum isochronal error.  Other desired characteristics were resistance to corrosion and magnetism.

Fortunately, a material satisfying these requirements had already been developed at Hamilton just prior to the war.  It was the product of fifteen years of research.  Facilities and experienced personnel and been developed to permit processing this alloy at Hamilton from the pouring of the raw materials to the final heat treating of the finished spring under conditions of strict, scientific laboratory control.  This alloy practically defies corrosion and processes unusually high and stable elastic properties.    Hairsprings made of the alloy when combined with a solid (uncut) monometallic balance wheel yielded rates which varied only slightly with temperature, and then in a manner that was practically linear over a wide range of temperature.  This characteristic was readily compensated for by certain provisions in the balance we, as we shall see later.

For best isochronal properties it was necessary to employ the conventional, cylindrical, helical form of spring.  Past experience had demonstrated that this form possessed the best potentialities.  However, anew and unique approach to the design of the end terminals was necessary in order to assure that the spring would remain perfectly cylindrical and upright, that the center of the spring would coincide with the center of the balance staff during the entire period of oscillation of the balance wheel.  Terminal curves which satisfied this condition were found to be identical in shape for both the stud and the collet ends of the spring.  Of course, a definite angular relationship of the terminals to each other was also necessary.  The final result was a spring which appeared to "breathe" in a perfect manner as the unit vibrated.  This symmetry of action contributed to an essentially zero change in rate with changes in motion.

To eliminate drift and instability of rate, the physical proportion of the length, width and thickness of the spring blade were so selected that normal stresses were well within the elastic limit of the material.  The influence of any cold working stresses was also eliminated.  This was accomplished by forming the hairspring complete with both terminals while the material was still soft.  The springs were then heat treated in a specially constructed furnace which preserved the original high finish of the wire.  During this operation the springs were retained on special, individual forms so that they would "set" to exact dimensions.  This process also eliminated the difficulties of cold forming by hand and the nicks or mars there were no focal points for local stresses which might, over a period of  time, cause drift in rate or eventual spring failure.

The Hairspring Collet and Stud (Figure 3)

Figure 3, Hamilton Model 21 Hairspring Collet and Stud.

The collet and stud employed to couple the hairspring to the balance wheel assembly and to the balance cook, respectively, are illustrated with the hairspring in Figure 2.  The method of attachment of the hairspring to the collet is the same as to the stud.  Figure 3, for which the collet was chosen, show clearly all details of the arrangement.

In the methods most commonly employed for attaching the hairspring to the collet and stud, the extremity of the spring is retained in a round hole by means of a tapered pin.  This causes deformation of the spring blade adjacent to the point of attachment.  In the design adopted by Hamilton the hairspring is held between flat surfaces of the collet and clamp, both made of steel. and secured together by the pressure of the wedge pin.  This eliminated all deformation and consequently furthered the objective of keeping the spring entirely free from any cold working stresses.  In addition, all dimensions were accurately controlled in order to bring the edges of the clamping surface of both collet and clamp "in line".  By this provision the active length of the spring was the same for both the winding and unwinding directions of deflection.  This constancy of effective length contributed toward reduction of isochronal error.

A long hub was provided on the collet to keep the collet square with the balance staff.  The collet was also slotted through to the center h ole.  This imparted sufficient resilience to the collet to facilitate assembly to the balance staff.

Chronometer are generally considered as being "one position" instruments and are mounted in gimbals to assure operation in the dial-up position.  In this position the balance wheel oscillates in a horizontal plane and close attention to poise is therefore not usually considered essential.  However, perfect stability of position is never fully realized.  Disturbances encountered in actual service frequently cause temporary deviations.  Though the effects of these deviation might be small, it was concluded that they must not be ignored.  Consequently, the collet assembly complete with a predetermined element of the hairspring was statically balanced.  It was this consideration which led to the odd wedge like shape.

After being assembled to a collet, a hairspring must be trued.  This involves bending of the terminal.  To keep such adjustments to an absolute minimum it was essential that the clamping face of the collet be accurately controlled for position.  To assist in achieving this result a small work hole was provided in one end of the collet.  This hole served in conjunction with the center hole to locate the piece accurately during all manufacturing operations.  Aside from the features it possesses in common with the collet, the hairspring stud is of interest because of the means by which it is attached to the cock.  It is made of hardened steel and provided with two precision dowel pins to assure accurate alignment and position on the balance cock.  It is secured to the under side of the balance cock by means of a single screw passing downward through the clock and into the stud.  The fact that the stud was not mounted on the top of the balance cock, as is commonly done, was the result of other design consideration not affecting the performance characteristics of the balance unit.

The Balance Wheel (Figure 4)

The balance wheel, shown in Figure 4, is one of the most novel features of the Hamilton marine chronometer.

Figure 4, Hamilton Model 21 Balance Wheel.

In most balance wheels used heretofore, temperature compensation adjustment is provided by split bi-metallic rims, bi-metallic affixes or other complex arrangements.  Adjustments are difficult to make and the effects of centrifugal force impair the performance of the instrument.  And sometimes the bi-metallic element is found to gradually drift from its initial properties.

As mentioned before, the alloy used for the Hamilton hairsprings possesses very good thermo-elastic properties.  Its performance with a solid (uncut) monometallic balance balance wheel indicated that only small adjustments for compensation had to be provided in the balance wheel.  To accomplish this result, our Research Department conceived the idea of making a balance wheel with a solid, uncut, stainless steel rim and an Invar spoke silver soldered securely together.  Then, to eliminate any undue stresses, the wheels were "seasoned" by an accelerated process of alternately heating and chilling.

As the thermal expansion coefficient of Invar is practically zero, the radii of the balance wheel at each end of the arm remain constant for all temperature changes.  Those portions of the rim between the ends of the arm move outward with increases in temperature and inward with decreases in temperature, causing the balance wheel to assume a very slight5ly elliptical shape.  This movement was utilized for making temperature adjustments by providing a series of uniformly spaced tapped holes around the entire circumference of the rim, by employing conventional balance screws, and moving these screws either toward or away from the spoke of the wheel, as required.  The total range of the adjustment possible with this construction was relatively small, being only about 0.25 seconds per day per degree Fahrenheit, but it was entirely adequate.

Final regulation of a chronometer to time must be performed through adjustment of balance wheel inertia since no regulator is permissible.  Usually one pair of time weights is provided.  These must be large to have sufficient capacity for adjustment and, consequently, are not very sensitive.  As an improvement over this condition Hamilton balances were provided with an additional pair of timing weights, very small in size, size, to permit extremely close adjustment to time.  One  complete turn of this pair of vernier weights will alter the rate of the chronometer only 2.8 seconds per day, as compared with 40 seconds per day for the larger weights. Hence, by means of a special screwdriver incorporating a liquid level, regulation to within 0.1 or 0.2 seconds per day was easily obtained.

In addition to the temperature compensating qualities previously described, several other important benefits resulted from the use of a solid, uncut balance wheel,  First, the effects of centrifugal force usually encountered in cut bi-metallic balance wheels were completely eliminated.  In a split wheel, as the amplitude of oscillation increases, the mass tends to move outward inducing an isochronal error.  For this reason, the motion of such balances is seldom made to exceed one turn.  With a solid balance wheel it was  possible to employ a motion of 1 3/8 to 1 1/2 turns.  This greater amplitude, with attendant increase in velocity, reduced the effects of external influences and improved the stability of rate.  Second, the balance wheel being uncut and free from centrifugal effects, it was possible to increase its diameter and reduce its weight without sacrificing moment of inertia.  In fact, from a comparison with other conventional chronometers, it is estimated that for the same total inertia, the weight of the Hamilton balance wheel is approximately 35% less.  This condition reduced pivot lad and friction, and thus contributed to improved rate characteristics.

The Balance Wheel Hub Assembly (Figure 5)      

Figure 5, Hamilton Model 21, Balance Wheel Hub Assembly.

When Hamilton chronometer were first produced, the balance wheel was assembled to the hub and staff in the conventional manner.  The  wheel possessed an accurately sized hole which fit the staff very precisely, and was fastened to the shoulder provided by the face of the hub with two flat head screws.  The heads of these screws seated in the countersinks on the top side of the balance wheel arm.  It was found that this assembly introduced lateral stresses between the staff and the balance arm.  Although small, these stresses caused detectable irregularities in performance.  Analysis indicated that this trouble was due to the fact that the shape of the under side of the screw head and the countersinks in the balance arm could not be maintained in line with the tapped holes in the hub.  To isolate these lateral stresses from the balance wheel, fillister head screws could have been used, but these would not possess the self-locking security of flat head screws.  The problem was finally solved by interposing a flat cap between the screw heads and the balance arm.  This is shown in figure 5.  The center hole in the cap was made large enough to prevent contact with the staff and the holes in the balance wheel were increased slightly in size to assure clearance with the screws.  By this means the lateral forces were isolated from the balance wheel and another refinement realized.

Performance

Now, in the application of this long series of refinements, what were the practical results?  Expressed briefly, the results were phenomenal.  Stability of rate was amazing.  No running-in period was required.  No settling period between test as different temperatures was found necessary.  If stopped momentarily and restarted, a chronometer readily resumed its original rate.  When "uncorked" after shipping, resumption of original rate was almost exact, being usually within tenths or hundredths of a second.

Isochronal properties were excellent.  Over the normal running time between windings (24  hours) the isochronal error never exceeded a few hundredths of a second.  In fact, routine production tests conducted on all Hamilton chronometers over a range of balance motion from 3/4 to 1 1/2 turns--a range far exceeding that ever encountered in operation--seldom exposed an isochronal error exceeding one second.

Temper5ature compensation qualities were also most gratifying.  Over the specified range from 55 degrees F. to 90 degrees F. compensation errors were well under one-half second.  Middle temperature errors, long a classical problem, could not be detected.

Incidentally, it might also be interesting to relate that on several occasions all chronometers on test would show a minute change in rate and changes would be  in the same direction.  A thorough analysis of this phenomenon revealed that, invariably, it was associated with  a change in barometric pressure.

Thus, it can justifiably be concluded that the careful analysis, the attention given every minute detail, and the workmanship required for their execution were all well rewarded.  A higher excellence of performance had been achieved!  Another stage of progress had been added to the glorious history of marine chronometers!  

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