Limitations
The tendency of the pallets to move out of the way of the wheel has some 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 may not be restrained and may accelerate rapidly and uncontrollably. To prevent this happening while the clock was being wound, Harrison invented one of his longest-lasting mechanisms, a maintaining power which is still widely used in clocks and watches. In its usual construction this consists of a ratchet wheel sandwiched between, and co-axial with, the first (and slowest-turning) driving 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. During 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 one or both pallets are long enough to sit into the gap between the teeth of the escape wheel then the wheel will run free 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.
In common with other escapements of its time, the grasshopper pushes the pendulum back and forth throughout its cycle; it is never allowed to swing freely. This disturbs the pendulum's natural motion as a harmonic oscillator. Around the same time as Harrison invented the grasshopper, George Graham introduced his deadbeat escapement which reduced this effect, and this practical and simple escapement was to become standard in precision regulator clocks.
Because of these various idiosyncrasies, the grasshopper escapement was never used widely. Harrison used it in his prototype marine chronometers, H1 - H3, and Justin and Benjamin Vulliamy made a small number of regulators using Harrison's design, but it remains today what it was in Harrison's time: a brilliant, unique curiosity.
Read more about this topic: Grasshopper Escapement
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