Grasshopper escapement
Encyclopedia
The grasshopper escapement is an unusual, low-friction escapement
for pendulum clock
s invented by British clockmaker John Harrison
around 1722. An escapement, part of every mechanical clock
, is the mechanism that causes the clock's gears to move forward by a fixed distance at regular intervals and also gives the pendulum
(or the balance wheel
) periodic pushes to keep it swinging. The grasshopper escapement was used in a few regulator clocks built during Harrison's time, and a few others over the years, but has never seen wide use. The term "grasshopper
" in this connection, apparently from the kicking action of the pallets, first appears in The Horological Journal in the late 19th century.
position was a major problem in marine navigation; Newton argued that astronomical positioning could be used, but an easier theoretical possibility was to use accurate knowledge of the time at a specific, base, location. The difference in time between local time, which was easy to measure, and the time at base gives the difference in longitude between the base and the ship, since 24 hours of time is equivalent to 360 degrees of longitude. A large prize was offered for a solution to the problem and Harrison devoted his life to devising and building highly-accurate timekeepers. Precision
and friction
were the main problems. Two advantages of the grasshopper escapement are the repeatability of its operation and its freedom from the need for lubrication
. The repeatability 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 thus completely regular in its timing. The lubricants available to Harrison were poor, messy and short-lived. This meant that conventional clocks had to be stopped frequently for cleaning and oiling. Using his clean and absolutely stable grasshopper escapement Harrison began a series of long-term investigations into the performance of clocks, leading to his invention of the gridiron pendulum
which nullified the effects of changing temperatures. The performance of his improved clocks in turn gave him an accurate, convenient standard against which to test his marine timekeepers.
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.
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 chronometer
s, 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.
, was unveiled at Corpus Christi College
, Cambridge University
, in Cambridge
, England
on September 19, 2008. Industrialist John Taylor
spent £1 million building the mechanical clock. Feeling that Harrison's escapement was not well enough known, the clock's grasshopper escapement is exposed on the top of the clock, built in the form of a demonic grasshopper
called the "Chronophage" or "time eater", which rhythmically opens and closes its jaws, representing time being devoured.
The clock, 1.5 metres in diameter, has many other notable features. It has no hands, but rather uses three concentric pairs of stacked annular disks
—one pair each for hours, minutes and seconds—slotted and lensed to allow the selective escape of light from an enclosed, continuously lit set of light emitting diodes. The arrangement of slots in each disk, along with the rotation of the foremost disk of each pair, creates a Vernier
effect, producing the illusion of lights rotating at various speeds about three concentric circumferences on the clock's face.
The pendulum speeds up, slows down, and sometimes stops, but returns to the correct time every five minutes. Taylor designed the clock to remind himself of his own mortality.
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
for pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...
s invented by British clockmaker John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...
around 1722. An escapement, part of every mechanical clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
, is the mechanism that causes the clock's gears to move forward by a fixed distance at regular intervals and also gives the pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
(or the balance wheel
Balance wheel
The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...
) periodic pushes to keep it swinging. The grasshopper escapement was used in a few regulator clocks built during Harrison's time, and a few others over the years, but has never seen wide use. The term "grasshopper
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...
" in this connection, apparently from the kicking action of the pallets, first appears in The Horological Journal in the late 19th century.
History
The grasshopper escapement was invented by John Harrison who used it in his regulator clocks, and also in the first three of his marine timekeepers, H1 - H3. Determining longitudinalLongitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
position was a major problem in marine navigation; Newton argued that astronomical positioning could be used, but an easier theoretical possibility was to use accurate knowledge of the time at a specific, base, location. The difference in time between local time, which was easy to measure, and the time at base gives the difference in longitude between the base and the ship, since 24 hours of time is equivalent to 360 degrees of longitude. A large prize was offered for a solution to the problem and Harrison devoted his life to devising and building highly-accurate timekeepers. Precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
and friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
were the main problems. Two advantages of the grasshopper escapement are the repeatability of its operation and its freedom from the need for lubrication
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...
. The repeatability of its operation is inherent in its design. One pallet
Pallet fork
The pallet fork or pallet lever is an integral component of the lever escapement of a typical mechanical watch. Its purpose is to release the escape wheel one tooth at a time, at each swing of the balance wheel, and also give the balance wheel small pushes to keep it going.The lever is shaped like...
is released only by the engagement of the other; the impulse given to the pendulum is thus completely regular in its timing. The lubricants available to Harrison were poor, messy and short-lived. This meant that conventional clocks had to be stopped frequently for cleaning and oiling. Using his clean and absolutely stable grasshopper escapement Harrison began a series of long-term investigations into the performance of clocks, leading to his invention of the gridiron pendulum
Gridiron pendulum
The gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726. It didn't change its effective length with temperature, so its period of swing stayed constant with changes in ambient temperature...
which nullified the effects of changing temperatures. The performance of his improved clocks in turn gave him an accurate, convenient standard against which to test his marine timekeepers.
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
Anchor escapement
In horology, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward...
which he built for a turret clock to go in the stable block at Brocklesby
Brocklesby
Brocklesby is a village and civil parish in the West Lindsey district of Lincolnshire, England. The village is 1 mile south of Habrough, 4 miles southwest of Immingham, close to the border of both North Lincolnshire and North East Lincolnshire, and near Humberside International Airport...
Park in Lincolnshire
Lincolnshire
Lincolnshire is a county in the east of England. It borders Norfolk to the south east, Cambridgeshire to the south, Rutland to the south west, Leicestershire and Nottinghamshire to the west, South Yorkshire to the north west, and the East Riding of Yorkshire to the north. It also borders...
. 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
Hinge
A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
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.
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 powerMaintaining power
In horology, a maintaining power is a mechanism for keeping a clock or watch going while it is being wound.-Huygens:The weight drive used by Christiaan Huygens in his early clocks acts as 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
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....
. 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 chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
s, 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.
John Taylor's Corpus Clock
A unique public clock built as a tribute to John Harrison's grasshopper escapement, the Corpus ClockCorpus Clock
The Corpus Clock is a large sculptural clock at street level on the outside of the Taylor Library at Corpus Christi College, Cambridge University, at the junction of Bene't Street and Trumpington Street, looking out over King's Parade. It was conceived and funded by John C...
, was unveiled at Corpus Christi College
Corpus Christi College, Cambridge
Corpus Christi College is a constituent college of the University of Cambridge. It is notable as the only college founded by Cambridge townspeople: it was established in 1352 by the Guilds of Corpus Christi and the Blessed Virgin Mary...
, Cambridge University
University of Cambridge
The University of Cambridge is a public research university located in Cambridge, United Kingdom. It is the second-oldest university in both the United Kingdom and the English-speaking world , and the seventh-oldest globally...
, in Cambridge
Cambridge
The city of Cambridge is a university town and the administrative centre of the county of Cambridgeshire, England. It lies in East Anglia about north of London. Cambridge is at the heart of the high-technology centre known as Silicon Fen – a play on Silicon Valley and the fens surrounding the...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
on September 19, 2008. Industrialist John Taylor
John C. Taylor (inventor)
John Crawshaw Taylor OBE is a British inventor, entrepreneur, horologist and philanthropist best known for his extensive research into electric kettles.- Career :...
spent £1 million building the mechanical clock. Feeling that Harrison's escapement was not well enough known, the clock's grasshopper escapement is exposed on the top of the clock, built in the form of a demonic grasshopper
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...
called the "Chronophage" or "time eater", which rhythmically opens and closes its jaws, representing time being devoured.
The clock, 1.5 metres in diameter, has many other notable features. It has no hands, but rather uses three concentric pairs of stacked annular disks
Annulus (mathematics)
In mathematics, an annulus is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. Or, it is the area between two concentric circles...
—one pair each for hours, minutes and seconds—slotted and lensed to allow the selective escape of light from an enclosed, continuously lit set of light emitting diodes. The arrangement of slots in each disk, along with the rotation of the foremost disk of each pair, creates a Vernier
Vernier scale
A vernier scale is an additional scale which allows a distance or angle measurement to be read more precisely than directly reading a uniformly-divided straight or circular measurement scale...
effect, producing the illusion of lights rotating at various speeds about three concentric circumferences on the clock's face.
The pendulum speeds up, slows down, and sometimes stops, but returns to the correct time every five minutes. Taylor designed the clock to remind himself of his own mortality.
External links
- Other Animated Escapement Mechanisms
- Benjamin Vulliamy's regulator clock — Movement of 1780 regulator by Vulliamy, one of the few that adopted the grasshopper escapement