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The photo underneath shows a dead beat escapement set up for the purpose of adjustment. Note that the pallets are held in the arms by little clamps. This was a method developed by Benjamin Vulliamy. It's a clever system, becaus if one pallet becomes worn, it may be turned around and the other end used. Also, the pallet adjustment is made a lot simpler. This method never really found much favour in England, but it was used by a number of manufacturers on the Continent. Vienna Regulators in particular were fitted with them.
In the illustrations above, you will see two examples of a clock escapement.
The top one is the DEADBEAT type, and you will see how the escape wheel stops 'dead' after every beat.
The bottom one is the RECOIL ANCHOR, but here the wheel 'recoils,' or moves back slightly before proceeding to the next tooth.
The clockmaker/repairer will work mainly on three types of escapement. The Anchor, the Deadbeat and the Verge. 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 Verge type consists of a crown, or contrate wheel, one with its teeth running around the top and not the sides, which of course is more usual. The pallets, the bits that go in and out of the teeth, are just two small flat plates set at an angle for the teeth to give them maximum impulse. 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.
Then along came the 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. Now, though, minute hands were a necessity because of the vastly more accurate timekeeping given by this new device. 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.
Secondly, this was occasioned partly by the fact that a much heavier pendulum 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 goes around, it recoils slightly? It doesn't simply stop dead with each beat, but goes backwards a little way? Hence the correct name for this type of escapement. The Recoil Anchor.
But we were quietly swinging along to even greater things. In about 1675, a Mr. Richard Towneley invented the Deadbeat escapement. The great English clockmaker, George Graham, is often, and erroneously, given credit for this, while in fact he was the one who developed and used it.
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 deadbeat impulse is given by the pallets, with flat faces on the end, angled, running over the teeth and as they drop off, they form a circle with the arc of the escape wheel, thereby ensuring that there's no recoil.
There's still supplementary arc, though - but that's for another time!
The picture above shows John Harrison's famous Grasshopper Escapement in action.
We've had a brief look at John Harrison's remarkable life, but we've yet to examine one of the great man's finest inventions, The Grasshopper Escapement.
Like so many other innovations, Harrison stumbled upon this purely by accident. He made a stable clock at Brocklesbury Park, Lincolnshire, regulated by an anchor escapement. You may remember our discussion of this mechanism in an earlier article. The Anchor Escapement, or more properly the Royal Anchor, was invented by either Robert Hooke or William Clement in about 1657.
Harrison used this escapement on the Brocklesbury Park clock, but it kept giving trouble. Lubricants in those days were nothing like the quality that we enjoy now, and of course the Anchor must have oil to run properly. But it was a Catch 22. You had to apply lubricant, but over a short period, this would form itself into an unpleasant grease and stop the clock.
What he did was to hinge both pallet arms and have them pointing in the same direction against the rotation of the escape wheel. He realized that, by hinging the arms, one pallet could only be released by the other dipping into the escape wheel. Both pallets are slightly heavier in the tail so the natural tendency is for them to move away from the escape wheel.
Unfortunately, and partly because of this action, the escapement does have its limitations. If the gear train 'hangs up,' or is deprived of power, then because neither pallet is in contact with the escape wheel, the whole train will run free once the power is restored. This can cause serious damage, depending on the heaviness of the weights, or the power of the mainspring. Teeth may become bent or broken, most especially in the escape wheel. Wheel arbors can be bent and of course the pivots can suffer an even worse fate.
Another problem can occur if the pallet arm pivots become dirty. They may jam, then, in any position and cause the clock to stop. Harrison recognized this and came up with his idea of maintaining power. We've looked into this before, but a further quick explanation won't hurt.
A large diameter ratchet toothed wheel is fitted onto the great wheel arbor, of a diameter slightly smaller than the root, or dedendum, of the great wheel teeth. The great wheel, which should be at least three sixteenths of an inch thick, is cut away on the inside, allowing for the placing of a relatively powerful leaf spring. This acts against a pin fixed to the maintaining wheel, so that when the clock's wound, power is always applied, or 'maintained,' upon the gear train.
As the clock goes, a click, or ratchet, falls into each tooth of the maintaining wheel. When the power is released from the train, this click comes into its own, pushing hard against the wheel, so keeping power on the going train. It may be seen, of course, that if the going train is denied power for any other reason than during winding, the maintaining wheel will act in exactly the same way.
It can be readily understood that the great benefit of the maintaining wheel is that the clock can never lose time during winds. Normally, when a clock is wound, the tendency is to reverse the gear train. Remember that on a longcase clock in particular, the great wheel meshes directly into the centre wheel pinion. Up above we have the third wheel, and above that, the escape wheel.
Now, because the great wheel is rotating anti-clockwise - remember the centre wheel, which carries the hands, must rotate clockwise - then the escape wheel, too, will rotate clockwise. However, the clock must be wound anti-clockwise, so taking the power off the going train and in the course of a year, the clock will lose a considerable amount of time, unless a maintaining wheel mechanism is mounted.
Even so, the poor old grasshopper never really found favour, even though it doesn't require any oil at all which, in those days especially, was an enormous benefit. Nevertheless, without the grasshopper, we may never have found the idea of maintaining power, which is a standard feature of all the best regulators and marine chronometers made today.
It's a most fascinating design, though, brought to life by the mind of a genius.
A most interesting picture of a Verge and Foliot escapement, used for hundreds of years before the advent of the Anchor escapement.
