Marine Chronometer

Just what is a marine chronometer?

Put simply, a marine chronometer is an extremely accurate clock which is designed to be very stable and reliable in it's operation, even while operating on the deck of a rolling ship at sea. Chronometers were used in sailing ships from the 1700's until about the 1970's in order to determine the longitude of a ship's position. Another way to think about it as an antique version of GPS for ships. Before the invention of the chronometer, there were many sailing and navigational mishaps which could have been easily prevented with the positioning precision that is provided by such a device as the marine chronometer.

How does all this relate to Engineering at the Thayer School at Dartmouth?

This year for the class ENGS 146 students were given the same challenge as the British public back in 1714 but with a modern twist: Build as accurate a clock as possible for use on the rough seas, using a desktop 3D printer and online CAD software working in small teams. Each student was shipped an inexpensive desktop 3D printer in kit form and given a few smaller assignments in order to learn the intricacies and nuances of 3D printing. Then students worked together in small groups to design, print, test, and optimize their marine chronometer. The class culminates in a live competition, where students compete head to head and data from their clocks is uploaded and analyzed in real time to determine which chronometer is the most accurate. Students tested components as they designed, refining and optimizing their sub-assemblies as they progressed through the assignment, and some in-process images can be see to the left.

Who invented the first marine chronometer?

Perhaps the most famous marine chronometer is known simply as H1. It was invented by an English carpenter named John Harrison in 1730. He constructed it largely from brass, and built it for a competition the British Government of the time was hosting, which had a 20,000 British pound note reward (equivalent to about 4 million pounds in today's !). Like any good engineer, John went on to make many revisions to his initial design, and other inventors and clock makers would contribute to the eventual solution which for the first time in history provided captains with a reliable method for determining their longitude.

How did we switch it up from last year's marine chronometer challenge?

This year, the instructors decided to add a fun twist to the marine chronometer project. Students will mount their clocks onto the top plate of a foam-suspended platform assembly (as diagramed on the left). The four spring-damper-foam “shock absorbers” will permit the top plate of the platform to rock and wobble relative to the base plate. The clocks must be able to reject mechanical disturbances which arise from build bed movement.

Components of a Chronometer

The Mainspring:

The mainspring is the primary power source of the chronometer. The mainspring can also take the form of a falling weight or pendulum, it just needs to provide a torque input to the clock in order to power the workings of the clock. The length that the mainspring or power source can run largely determines the run time of the clock, so it's design was a primary focus of student groups from the start of the project. What most students found was that plastic mainsprings are less than ideal when compared to their metallic counterparts. Students found that plastic mainsprings had a tendency to deform and loose much of their strength after only 5-10 uses, so most groups elected to make them easily accessible and replaceable within their chronometer assembly.

The Escapement:

The escapement regulates the motion of the clock, and it is responsible for the universally recognized tick-tock noise of most mechanical clocks. The escapement pictured on the right is called a lever escapement, as the lever arm connected to the pallet forks (the two pronged teeth) engage with the teeth of the escape wheel. The pallet forks and lever arm transmit motion from the hair spring and balance wheel (on the far right in the image) which "locks" and "unlocks" the escape wheel periodically. This regulated "locking" and "unlocking" allows power from the main spring to be transmitted through the gear train to the various parts of the clock, in a highly consistent manner. While all parts of a chronometer are important, the regulation function of the escapement mechanism makes it a very crucial part of a well built clock.

The Balance Wheel:

The balance wheel in conjunction with the hairspring provides a regularly oscillating mass which is used to "trigger" the escapement mechanism. The balance wheel would typically be made of a relatively hefty piece of brass or other weighty metal, and would have precision jewel bearings in order to inhibit its motion as little as possible. When making a clock, friction and miss-alignment of parts are two of the biggest challenges to overcome, and students were not helped by the inherent somewhat loose tolerances which are associated with 3D printing. At the beginning of the course students were shipped a basic assortment of fasteners and bearings, and many groups wisely chose to use the ball bearings in order to reduce friction wherever possible. Many groups also chose to use left over nuts and bolts from their hardware kit to add mass to the balance wheel, dramatically improving it's performance in their chronometers.

The Hairspring:

The hairspring aids in the regulation of the oscillation of the balance wheel. In a sense, the balance wheel and hairspring are always fighting each other, in that the balance wheel wants to continue it's rotation, but the hairspring prevents it from making a full rotation and reverses the direction of rotation of the balance wheel. When the rotation of the balance wheel goes too far the other way, the hairspring steps in again to reverse the motion, and the whole cycle repeats. As previously stated, this oscillation motion in conjunction with the lever arm, pallet forks, and escapement gear are what all serve as the sub-system which regulates the motion of the clock. If any one of these parts were missing from the clock, it would not function as intended or in a predictable and precise manner. To the right you can see the motion that just a balance wheel and hair spring make on their own when operated independently from the other mechanisms of the clock.

The Gear Train:

The gear train in a chronometer serves to distribute and transmit power to the various parts of the clock, as well as reducing or increasing the speed of the clock as appropriate for the various time increments: hours, minutes, and seconds. The ratio of the size of two gears interacting in a gear train will determine how the speed and torque is affected. For example, two gears which interact with each other and have a 2x size relation (where one gear is 2x the size of the other) are said to have a 2:1 ratio. By using the ratios between gears in the gear train, students were able to optimize their chronometers to "tick" with as close to a 1 second interval as possible, which was the stated design goal of the project and competition. While a somewhat unconventional design to be found in a clock, the planetary gear system shown to the right was one group's approach to transmitting power through their clock.

The Housing or Base:

All the parts of a chronometer need to be held together somehow, and that is the primary job of the housing or base of a clock. Most fancy clocks are luxury or decorative items, and so often a secondary purpose of the housing or base is to show off the mechanisms and internals of the clock. In a chronometer, which will be used in a harsh, corrosive environment, obviously the exhibition of internal workings is less than ideal, so most chronometer housings would be less showy then their terrestrial based clock counter-parts. However this is ENGS 146, where we are all about showing off the wicked cool internals and mechanisms that we designed, so most student groups opted for large cutouts and keeping most of the mechanisms exposed, as can be seen in the example to the right. Rigidity and stability in the housing and base were also key design aspects that students paid attention to while they were designing and building their chronometers.

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