Swing with Me
Copyright (c) 2009 Mike Bond
You remember the escapement. The way the power of the clock's released in equal time through the use of things called pallets dipping in and out of a saw toothed wheel?
The clockmaker/repairer will work mainly on three types of escapement. The Verge, Recoil Anchor and Deadbeat. There are others, of course.
The grasshopper, the double three-legged gravity escapement, (used originally in Big Ben, invented by Edmund Dension, later Lord Grimthorpe) and used now in many tower clocks. But for the purposes of this article we'll stick to those escapements first mentioned.
The escape wheel of the Verge escapement is made as a crown, or contrate wheel. The teeth are cut at 90 degrees to the face. An arbor runs over the wheel, on which are set the pallets. One on one side of the wheel, the other 180 degrees to it.
The pendulums on these clocks are very light, and once the clock's underway, the pendulum is made to swing in a huge arc, anything up to 100 degrees. The great Dutch scientist and clockmaker, Christiaan Huygens, realized that such an excessive movement of the pendulum would fail to make the clock's escapement isochronous. There's that awful word again!
The swing was so great, that variations due to power differentials as the mainspring unwound were inevitable.
Following the Verge came the Recoil Anchor escapement. Robert Hooke said that Robert Hooke invented it! This was challenged strongly by a gentleman named William Clement, who did in fact make the first longcase, or grandfather clock. Whoever invented it, there's no dispute that the first clock to use the Anchor escapement was made by Joseph Knibb in about 1670, for Wadham College, Oxford.
Up until then, there was really little point in having minute hands, such could be the time variations, hour to hour. Timekeeping improved very dramatically because of this new device. Because of this, the minute hand was added to clocks.
There are two basic reasons for this. Firstly, the arc, or swing, of the pendulum was lessened dramatically from the huge 80 - 100 degrees of the old Verge, down to 4 - 6 degrees. Another great benefit was that instead of the lightweight pendulum of the Verge, a much heavier one could be used. One of the most vital aspects of timekeeping is that the pendulum controls the clock and not the other way round. But then they went one step further.
Not only was a minute hand included, but by extending the arbor upon which the escape wheel was set so that it came right through the front plate of the clock, a seconds hand also could be added. Now, while this did show the seconds, provided the gearing was correct and allowed the escape wheel to revolve once a minute, or even twice a minute, when the clock would beat half-seconds, it wasn't totally satisfactory.
Have you ever looked at a clock with a seconds hand and noticed that as this little hand goes around, it recoils slightly? The geometry of this particular escapement causes the escape wheel to reverse very slightly with each swing of the pendulum? Hence the correct name for this type of escapement. The Recoil Anchor. But improvements were even then on the way.
In about 1675, a Mr. Richard Towneley invented the Deadbeat escapement. The great English clockmaker, George Graham, is often, and erroneously, credited with this, but in fact he was the first one to actually develop and use the system.
The main difference between the two types is that with the Anchor, as the pallets dip in and out of the escape wheel teeth, they give impulse as their sides run down the teeth faces.
The difference between the Anchor and the Deadbeat is that there's no recoil on the latter. The pallets pass into the wheel, receive their impulse from the tips of the pallets and not the side faces.
There's still supplementary arc, though - but that's for next time
(ArticlesBase SC #854848)
Article Tags:
deadbeat escapement
,recoil anchor escapement
,verge
,pallets
,escape wheel
,pendulum
,arc
,three legged gravity
,minute hand
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