Further arbors (axles), wheels and pinions (gears) are added, in order to complete the movement train.
A device commonly referred to as 'Harrison's maintaining gear' is installed on the arbor driving the minutes hand (called the centre arbor). The concealed maintaining spring (not shown), maintaining ratchet wheel and two maintaining pawls shadow the rotation of the train (and produce that clicking sound you've been hearing on occasions, as the pawls fall of the ratchet teeth). Whenever torque from the driving weight is isolated from the train, most typically during winding of the clock, the maintaining ratchet engages with one of the pawls as the maintaining spring attempts to unwind. The spring output, thus constrained in one direction, feeds torque up through the train to the remontoire in order to prevent unwinding of the remontoire spring and provide for a single rewind, should it be demanded. By this means, the escape wheel continues to receive torque, maintaining operation of the escapement and preventing loss of timekeeping. When a grasshopper escapement is fitted, maintaining gear is vital ; any loss of torque to the escape wheel would disengage the grasshopper and could lead to damaging, high speed 'free-wheeling' of the saw-toothed escape wheel upon restoration of drive from the weight (such as when winding ceases).
Below the centre arbor lies the 'great arbor', which carries the barrel, winding ratchet and two opposing 'clicks' (spring-loaded pawls). The line attached to the driving weight is coiled around the barrel when the clock is wound. There is a groove cut into the surface of the barrel, which guides the line onto (and off) the barrel in an orderly fashion, ensuring no overlap or binding and, thereby, a reliable, constant torque to the great arbor. In addition, the barrel groove positions the driving weight clear of the pendulum as the line unwinds. The winding ratchet permits free winding of the clock in a clockwise direction (viewed from the front of the clock) and transmits anticlockwise torque from the descending weight during normal operation.
The great arbor is not supported on the brass and lignum vitae anti-friction wheels and hubs described in an earlier video ; instead Harrison developed a variation, in the form of dry running, caged rolling element bearings, better suited to supporting high radial loads with negligible friction and no requirement (in this application) for lubrication. We are all familiar with ball bearings (by which I mean complete bearings, not just the balls). It is likely that John Harrison invented the first caged version, now used throughout the world by the billions (trillions?) per year in just about every machine you care to mention (including the hard drive, cooling fans etc. fitted to the computer you're using to view this video). Who knows what financial reward Harrison might have amassed, had he patented the caged rolling element bearing concept during his lifetime.
Towards the end of the video, the first view of the complete movement train indirectly demonstrates the maintaining gear in action. I wind the remontoire spring and then release the train ; the upper train rotates using maintaining power alone.
The second view of the lower part of the complete movement train clearly shows how unbelievably free-running Harrison's machines are : a pencil (near the bottom of the screen) is deliberately used in the manner shown, in order to prove that very limited impulse is being applied to the great wheel. The advantage, compared to typical clockwork, is significant : low friction and no requirement for lubrication can only permit negligible variations in friction, thereby contributing towards consistent remontoire rewind duration and superb timekeeping.
Further information is available at http://soptera.blogspot.com and
http://www.hsn161.com/HSN/Heskin.php
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John Harrison RAS Regulator 'Replica' - Video 12 of 13by glathoppa995 views
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