Operation
The animation shows a later version (not by Harrison) which has individual hinge axes and sprung (rather than hinged) stops. Harrison developed the grasshopper escapement 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 fixes for which Harrison was not paid, so around 1722 he modified the escapement 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 folds 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, pushed by 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 momentum of the pendulum, pushes the wheel backwards slightly. This releases the second pallet, which retires gracefully to its stop, and transfers the task of impulsing the pendulum to the first pallet again. The small movement of the pallet on its hinge involves far less friction than the sliding contact 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 was only replaced following 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.
Read more about this topic: Grasshopper Escapement
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