Grasshopper escapement
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The Grasshopper escapement is a remarkable, low-friction escapement - a control device for the step-by-step release of a clock's driving power.
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[edit] History
The escapement was invented by John Harrison for use in his clocks and he also used it in his first three marine timekeepers. It has seldom been used by anyone else except for decorative purposes. Two advantages of the grasshopper escapement are its regularity of operation and its freedom from the need for lubrication. The regularity of its operation is inherent in its design, one pallet is released only by the engagement of the other; the impulse given to the pendulum is uniform in both its amount and its timing. The lubricants available to Harrison were poor, messy and short-lived. This meant that clocks had to be stopped frequently for cleaning and oiling. Using his clean and absolutely stable grasshopper escapement Harrison was able to begin a series of long-term investigations into the performance of clocks. It allowed him to determine the effects of temperature on his clocks which in turn led him to invent the gridiron pendulum. The performance of his clocks in turn gave him an accurate, convenient standard against which to test his marine timekeepers. The term 'grasshopper' in this connection first appears in "The Horological Journal" in the late 19th century.
[edit] Operation
Harrison developed it from a conventional anchor escapement which he built for a turret clock to go in the stable block at Brocklesby Park in Lincolnshire. This proved to be unreliable, needing repeated attention for which Harrison was not paid. So he modified the escapement (around 1722) by putting a hinge in the middle of each arm of the anchor. The hinged pallets both pointed the same way, opposing the rotation of the escape wheel. As the escape wheel pushes the pallet, the hinge moves away from the escape wheel. The pallet pivots about its contact point with the wheel as it pushes the anchor. At the same time, the other pallet is approaching the wheel. When it contacts the wheel, it pushes it backwards slightly and contact between the wheel and the first pallet is broken. Both the pallets are slightly tail-heavy so that they naturally tend to move away from the wheel. The first pallet therefore moves out of the path of the escape wheel and the job of impulsing the pendulum passes to the second pallet.
The first pallet comes to rest against a stop which holds it in the correct position so that when the pendulum is reaching the end of its travel in pushing the second pallet, the first pallet swings down into the path of the wheel again. It makes contact with the wheel and, driven by the pendulum, pushes the wheel backwards slightly. This releases the second pallet, which retires gracefully to its stop, and having transferred the job of impulsing the pendulum to the first pallet again. The small movement of the pallet about its hinge involves far less friction than in a conventional escapement, it does not need lubrication and there is so little wear that Harrison was able to make his pallets from wood. One of the original pallets at Brocklesby Park is still working and the other had to be replaced after an accident in 1880. Harrison later modified the layout of the escapement by having one pallet pull rather than push, putting a little hook at the end of the pivoted arm to contact the teeth of the escape wheel. He also brought both hinge axes together on a common pin.
The stops that the pallets rest against are extremely ingenious. When the pallet is pushing the escape wheel backwards it is also being driven hard against its stop. To prevent wear, or damage, the stops are designed to give way. Each stop is hinged about the same axis as its pallet. The pallets are tail-heavy but the stops are nose-heavy tending to fall towards the wheel. The stops are sufficiently nose-heavy that the combination of pallet plus stop also tends to fall towards the wheel but this is prevented by a fixed pin on the anchor. This means that : the pin holds the stop which holds the pallet in just the right place to engage cleanly with the escape wheel. When the pallet meets the wheel, it pushes the wheel backwards and as it does it lifts the stop off its pin. When the wheel then pushes the pallet, the stop comes back down onto its pin and loses contact with its pallet. Each stop is also lifted off its pin once in each cycle by the momentum of the arriving pallet. The animation shows a later version (not by Harrison) which has individual hinge axes and sprung (rather than hinged) stops.
[edit] Limitations
The natural tendency of the pallets to move out of the way of the wheel has a couple of serious consequences. The first is that any time that the drive to the escape wheel is interrupted the pallets lose contact and when the drive is restored, the escape wheel is not restrained and accelerates rapidly and uncontrollably. To prevent this, Harrison invented his longest-lasting mechanism, a maintaining power which is still widely used in clocks and watches. In simple terms, this consists of a ratchet wheel that fits between the first (and slowest-turning) gear of the movement and the barrel that the weight (or spring) is attached to. When the clock is wound, the barrel goes backwards and a ratchet on the maintaining wheel slips over teeth cut on the barrel. The first gear is still driven forward however because there is a spring between the maintaining wheel and the first gear which pushes against it. As it does so it tries to push the maintaining wheel backwards. This is prevented from happening by a ratchet fixed to the frame of the clock which engages with teeth cut round the edge of the maintaining wheel. Once the clock is fully wound, pressure on the key is released and the barrel drives the maintaining wheel and the first gear in the normal way. It also rewinds the maintaining spring ready for the next time the clock is wound. In normal operation the ratchet that stops the maintaining wheel from going backwards simply slips over the teeth of the maintaining wheel.
The second consequence of the pallets' tendency to move out of the way of the wheel is that when the clock runs down and stops both pallets return to their stops. Unless the ends of the pallets are long enough to sit into the gap between the teeth of the escape wheel then the wheel will run away as soon as the clock is wound. The same problem can arise if the hinges for the stops get dirty and stick in their raised position.
