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Marine Chronometers

( Originally Published 1918 )

MARINE chronometers require very careful handling. Take the movement, together with the inner brass box, out of its gymballing. To take the movement out of the box, unscrew the bezel and turn the box upside down, receiving the movement in the hand.

Before the balance is removed insert an oiler to block the fourth wheel, in case the detent is accidentally touched. Remove the balance from the cock and put it under a glass cover for safety. Let the mainspring down, observing to what point it was "set up." The ratchet is generally marked to go on the right square, and the teeth are marked where the click point comes. Free the maintaining detent from the ratchet so that all power is off. Then take out the detent and its banking arm. Remove the hands, dial, motion work, and the small pinion on the fusee bottom pivot (the latter with brass-nosed pliers; it is pushed on only). Take off the cup around the fusee winding square, and take out the barrel and bar. The plates may then be taken apart. Great care must be taken of the scape wheel. It may be put under another glass cover with the detent.

All pivots should be examined for roughness or signs of wear. Cut or rough pivots should be re-polished, and every pivot must be perfect. When re-polished, those running in brass holes should have the holes bushed. Those that run in jewels must have new holes fitted by a chronometer jeweller. If a scape or balance pivot is so cut that it will be too thin when polished, shorten it, re-turn and polish, and send the frame and arbor to the jeweller to have the jewelling sunk further in the plate or cock to suit the new pivot.

Polish the frame and cocks with rouge and oil on the finger-tips by rubbing all over, not forgetting the edges. Clean off with benzine on a watch brush, and brush clean and dry as a watch. Take the fusee apart, clean and oil its internal parts. Wipe out the mainspring and re-oil with best French clock oil.

In putting together, first screw on the brass edge to the pillar plate. It can then be well handled without fingering the polished parts. Oil the fusee and bottom centre pivots with the clock oil, the rest with chronometer oil. Brush the balance clean, clean its ruby pallets with a dry peg point, and put in place. Clean the detent by laying it flat on tissue paper and applying a peg point to remove dirt from the locking jewel face (see that this is clean and bright) and the gold spring point. Put the detent and its banking arm in last of all, after the chain has been put on, the spring set up, and the balance put in. Put it in its brass box, and proceed to put on the motion work, dial, and hands. Wind it up, seeing that the chain runs true upon the fusee, and put on the " up and down" hand at " up."

See that oil is put to the maintaining detent pivots, the point where the spring touches it, the stop joint, and the stop spring. Put no oil on the pallets, the scape-wheel teeth, or the detent.

Put in its gymballing again and oil the pivots ; see that when hanging free it is quite level. There are adjusting screws to level it in both directions.

In putting on the seconds hand, see that it falls truly on to the seconds mark, and stop the chronometer before putting it on, or damage to the scape-wheel teeth may result.

Rust on any steel work must be scraped out with a sharp graver. On the balance spring it is fatal, and a new spring must be applied.

Adjusting and Rating.—For a chronometer to have a good steady rate, it must be compensated truly and the spring must be isochronous. The long and short arcs may be tried by letting the mainspring down a turn and setting it up again, testing several days in succession. The balance must be in perfect poise, for although the " position rates " are absent, a want of poise throws a side strain on the pivots and introduces error in the rate.

For loosening the screws that hold the balance weights, the spring must be removed and a screwdriver with a blade like Fig. 191 used. This should be arched enough to turn well over the top balance staff pivot, and long enough in the arch to span over the opposite weight. It is always well to make a slight mark on the rim before moving a weight, as once the screw is loosened the position is apt to be lost sight of. If a chronometer is carried or sent upon a journey, it is well to wedge the balance firm with two thin cork wedges inserted under the rim, one on either side.

Adjusting the Mainspring.—To ensure that the second day's rate of a two-day chronometer is the same as the first, or that the successive days of an eight-day chronometer are equal, an adjusting rod must be used to test the force of the spring when wound up and when run down.

A = fusee square, B = key, C = adjusting rod, D = balance weight. It consists of a steel rod 15 in. long with a fixed weight of 2 lb. on one end. It is put through a key handle as shown, and shifted until it balances the pull of the spring. If tried thus, any difference in pull between "up" and "down" can be noted by a scale marked on the rod. If too strong when wound up, the spring must be set up a little more. If strongest when nearly run down, the spring must be set up less.

New Pivots.—A new pivot is best put in a balance staff or a scape pinion, as shown in Fig. 193. The staff or pinion is cut off where marked at C, and drilled as at B, with as large a hole as possible. A plug is turned in a split chuck from tempered steel, as at A, and driven home. A pivot put in thus will be as good as new and defy detection. The cone shoulder can be made as large as before. To drill up, the staff may be held by one end in a split chuck, and the other may be steadied by coming through a hole in the back cone plate runner.

Chronometers that have run a year or two generally have a hard sediment caked on the face of the locking stone and on the impulse pallet. This is probably dirt from the scape teeth or fine particles of brass driven upon the polished stones by constant little blows, until they stick hard. When cleaning, these jewel surfaces must be carefully scraped clean, if necessary, with a graver point, and left perfectly smooth.

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