4: Hamilton Model 21 Escapement

12 04 2013

Note that figures may be enlarged by left clicking on them. Return to the text by clicking on the back arrow at top left.

At the end of the First World War, the United States of America had the largest of any navy, with nearly half a million personnel. A period of retrenchment followed the Treaty for the Limitation of Naval Armament of 1919 but in 1934, as world tensions seemed to be increasing, a program of ship building began to bring the US Navy up to the maximum size allowed by the London Naval Treaty of 1930. The Naval Act of 1938 contemplated a 20 % increase in building and the Two Ocean Navy Act led to a further 11 % increase, but this was soon followed by a plan to increase tonnage by 70 %, equivalent to 200 more ships. At that time, there were no manufacturers of chronometers in the USA, though there were firms that assembled them from imported parts and movements. The total annual world output of chronometers was probably in the region of 300 a year, principally from Britain and Switzerland. At the outbreak of the Second World War on 2nd September 1939, these sources became unavailable. Britain needed all her own chronometers while Switzerland, surrounded by belligerents, was soon to find that de facto limitations were place upon whom she could supply.

When WWII began, the US Navy had 394 ships of all types and by the war’s end she had 6,768, many of which  required chronometers, two or more for larger ships, one of more for smaller ones and navigational watches for vessels like patrol boats. The Hamilton Watch Company of Lancaster, Pa. was, among others, invited to commit to manufacture of the necessary chronometers and expressed interest in a letter of 2nd July, 1940. The Company was then provided with two Swiss Ulysse Nardin chronometers to examine. E W Drescher  said he thought Hamilton could make the instruments provided that certain design modifications could be made to allow mass production and the first order was placed on 16 May 1941. This accounts for the “1941” that appears on the face of the chronometer. It is not the date of manufacture but of the design. The first deliveries began on 27 February 1942 and 58 had been delivered by the year’s end, rapidly increasing thereafter, so that 8902 had been delivered by the end of the war. In most respects, the Model 21 chronometer closely followed the Ulysse Nardin design, except for a revolutionary balance design and the use of a pre-formed Elinvar-type balance spring, to obviate the time consuming and somewhat intuitive spring adjustments previously necessary.

Thus it came about that the escapement design is that of Nardin and uses a slightly more complex detent than that of the standard Earnshaw spring detent used by practically every other maker. The escapement is the part that communicates with the movement and lets it run down in equal steps while the movement gives up energy via the escapement to keep the balance wheel oscillating. A plan view is shown in Figure 1 and a perspective view in Figure 2, and you may find it helpful to refer back and forth between the two diagrams while reading the explanation. In Figure 2, modified from a figure in the official overhaul manual, the horn of the detent obscures some details of the discharge jewel and so in the inset, the horn has been displaced so this can be seen.

Figure 1: Plan view of escapement (After Rawlings)

Figure 1: Plan view of escapement (After Rawlings)

The escape wheel, driven by the movement, rotates in clockwise steps. The impulse and discharge rollers and their attached jewels are mounted on the balance staff, below the balance wheel and they are shown rotating anticlockwise. In both diagrams the discharge jewel is pressing on the end of the passing spring which in turn is pressing on the horn of the detent. Upon further anti-clockwise rotation, the detent will move its attached locking jewel out of engagement with tooth A of the escape wheel, leaving it free to rotate. Meanwhile, the discharge jewel slips free of the passing spring, allowing the locking stone to fall into the path of tooth B, ready to lock the escape wheel again, and the rollers continue their anti-clockwise rotation. Tooth C catches up with the impulse jewel, delivering some energy to the balance wheel to keep it going, and then tooth B arrives at the locking jewel and the rotation of the escape wheel is arrested.

The balance wheel and the rollers eventually reach the end of their anti-clockwise rotation and return, clockwise, but this time the discharge jewel simply lifts the delicate passing spring off the horn of the detent and continues past to complete the clockwise rotation before returning to recommence the cycle. Thus, when listening to a chronometer going, one hears a loud tick as an escape wheel tooth is locked against the locking jewel and a much softer tick as the passing spring drops back on to the horn of the detent. Unlocking, impulse and locking occur only once per full oscillation back and forth of the balance wheel and for the rest of the time the it is free of interference or detached.

Figure 2: Perspective diagram of escapement

Figure 2: Perspective diagram of escapement

The structure of the detent is most easily seen in Figure 2 and a photograph of an isolated detent is shown in Figure 3.

Figure 3: Model 21 detent.

Figure 3: Model 21 detent.

A foot is attached to a support block with a screw and washer. Two guide pins engage in a groove in the support block and allow only longitudinal movement when the depth adjustment screw is turned. This adjustment determines how deeply the passing spring engages with the discharge jewel, which in turn determines for how long the locking jewel is lifted out of the path of the escape wheel teeth. If it is out of engagement for too long, locking on the next tooth may fail and the chronometer is said to trip. It will then go twice as fast as it should. The support block allows the detent to be removed from the chronometer and replaced without disturbing its adjustment in relation to the rest of the escapement.

The detent spring in the Hamilton Model 21 is in two parts, unlike most chronometers which have a simple leaf spring, typically about 0.04 mm thick by 2 mm wide. Having two parts cannot be to allow a thicker spring to be used, as the stiffness of a spring varies directly as its breadth and as the cube of its thickness. Possibly it was felt that the increased effective breadth resisted torsional forces better. In any case, the spring must be stiff enough to resist buckling as the escape wheel tooth locks on the locking jewel while being weak enough so as not to interfere significantly with the balance wheel when unlocking.

The slender and flexible passing spring is mounted on a Z-shaped bracket. However, it must not be so flexible that it cannot cause unlocking, nor should it be so strong that on resuming its seat on the horn of the detent the percussion causes the locking jewel to release a tooth. The lock adjusting screw determines the position of the stop button which in turn determines how deeply the locking jewel engages with the escape wheel teeth. It must be deep enough to lock despite any shake in the escape wheel bearings. Depth of the passing spring and lock, and the angular position of the discharge jewel in relation to the impulse jewel are interdependent; if any one is disturbed, the others may need to be adjusted to regain an efficient action of the escapement. The official Manual for Overhaul, Repair and Handling of Hamilton Ship Chronometer explains in detail how to carry out adjustments for the Model 21 and Appendix 1 of my The Mariner’s Chronometer does so for chronometers in general. Without a clear understanding of the action of the escapement and the effects of individual adjustments, one is best advised to leave well alone, and if the instrument has fallen out of adjustment through wear or accident, expert help and wide experience may well be required to put it right.




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