John Harrison's H4 Remontoire

 H4 is the large chronometer watch that John Harrison produced in 1759 after many years of toil that met the Board of Longitude requirements for accuracy and won him a £20,000 prize. It was the first timekeeper that could be used at sea that was sufficiently accurate enough to determine longitude by comparing local time with Greenwich time which the watch carried.

It possessed a number of innovations that lifted its accuracy above the common watches of the day. One of these was a spring remontoire .

 One important requirement for precision at that time was that the balance wheel should swing consistently and evenly. When the mainspring is wound up the torque available through the gear train is high and the balance wheel may swing through a large arc and take a longer time to oscillate. Conversely  when the mainspring is run down the balance wheel will have a small arc and beat faster. A fusee on the mainspring can even out some of these differing forces but is not perfect.

Theoretically the frequency of oscillation should be independent of the arc, but balance springs of the period were not perfect. A decade or two later more was known about balance springs and the remontoire was discontinued, at least for chronometers.

A remontoire completely detaches the balance from the mainspring by having a small spring that is rewound at regular intervals by the mainspring and powers the balance wheel nearly directly.  In H4 this rewinding occurs at 7.5 second intervals. Thus it is wound one eight of a turn, allowed to run down one eight of a turn and is then rewound again. The balance wheel sees an effectively constant force and will perform better. By  detaching the balance in this way frictional forces arising from the train are also eliminated.

There are a number of other innovations to improve accuracy but we are not interested in those here.

Below is the  layout of my demonstration model. It follows the general arrangement of H4 but has many differences to suit wooden construction. 

 

Perhaps the main difference is that the escapement is a lever escapement rather than a verge and balance in the original. I find that my wooden lever escapement is reliable and was available rather than develop a verge escapement. Harrison used a contrate wheel as the remontoire wheel to mesh with his crown escape wheel but I have used a plain one.

Normally the centre wheel is powered by the fusee and via the third wheel, fourth wheel and the remontoire spring powers the balance. Wood is not very forgiving of the forces involved so I have driven this model from further up the train with a weight rather than a spring. The tooth count is also very different but effectively does the same thing.

The action is as follows. The fourth wheel is connected to the remontoire wheel above it by a spiral spring.(here 3d printed).  As the fourth wheel turns it winds up the spring which transmits power to the remontoire wheel and hence to the balance, The remontoire wheel rotates continually but the fourth wheel intermittently.

Also connected on the same arbor as the remontoire wheel is an eight tooth lantern pinion. 

The fifth wheel, meshing with the fourth wheel, is held stationary by a detent on its rim interacting with a cunningly shaped arm terminating in a claw to pick up the lantern pinion. As the lantern pinion rotates under the action of the remontoire spring the claw is pushed by a tooth and releases the detent on the fifth wheel allowing the fourth wheel to be rotated by the mainspring thus winding up the remontoire spring. A cam on the fifth wheel lifts the claw out of engagement of the tooth and as the fifth wheel rotates one turn it locks on the detent and the claw waits for the next tooth to come along.

Thus the remontoire wheel turns one eighth of a revolution and the fourth wheel turns one eighth of a turn to replenish the spring. This happens every seven and a half seconds in Harrisons watch and about 10 seconds in mine. The fourth wheel via the third wheel moves the minutes hand on by seven and a half seconds at a time or 8 to the minute. A seconds hand is driven from the remontoire wheel and moves continuously.

It is complicated to describe its action and reference to the video below might help.

The red wheel seen in the video is part of the weight drive system and not really part of the nechanism.

Remember this is just a demonstration of the action of the remontoire and not a complete clock.