A
bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
and
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
, or in some cases
brassBrass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin. Despite this distinction, some types of brasses are called bronzes. Brass is a...
instead of copper. The strips are joined together throughout their length either
rivetA rivet is a permanent mechanical fastener. Before it is installed it consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole. Then the tail is "upset" A rivet is a...
ing,
brazingBrazing is a metal-joining process whereby a filler metal or alloy is heated to melting temperature above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere or flux...
or
weldingWelding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature.
A
bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
and
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
, or in some cases
brassBrass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin. Despite this distinction, some types of brasses are called bronzes. Brass is a...
instead of copper. The strips are joined together throughout their length either
rivetA rivet is a permanent mechanical fastener. Before it is installed it consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole. Then the tail is "upset" A rivet is a...
ing,
brazingBrazing is a metal-joining process whereby a filler metal or alloy is heated to melting temperature above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere or flux...
or
weldingWelding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. The metal with the higher
coefficient of thermal expansionWhen the temperature of a substance changes, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bonds...
is on the outer side of the curve when the strip is heated and on the inner side when cooled.
The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices. In some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.
History
The bimetallic strip was probably invented by the eighteenth century clockmaker
John HarrisonJohn Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought and critically-needed key piece in solving the problem of accurately establishing the East-West position, or longitude, of a ship at sea, thus revolutionising and extending the possibility of...
for his third marine timekeeper (H3) to compensate for temperature-induced changes in the
balance springA balance spring, or hairspring, is a fine spiral or helical spring used in mechanical watches, marine chronometers, and other timekeeping mechanisms to control the rate of vibration of the balance wheel...
. It should not be confused with his bimetallic mechanism for correcting for thermal expansion in the
gridiron pendulumThe gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726, which didn't change in length with temperature, so that its period of swing stayed constant with changes in ambient temperature...
. His earliest examples had two individual metal strips joined by rivets but he also invented the later technique of directly fusing molten brass onto a steel substrate. A strip of this type was fitted to his last timekeeper, H4. His invention is recognized in the memorial to him in Westminster Abbey, England.
Clocks
Mechanical
clockA clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell"...
mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms. The most common method is to use a bimetallic construction for the circular rim of the
balance wheelThe 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...
. As the spring controlling the balance becomes weaker with increasing temperature, so the balance becomes smaller in diameter to keep the period of oscillation (and hence timekeeping) constant.
Thermostats
In the regulation of heating and cooling,
thermostatA thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The name is derived from the Greek words thermos "hot" and statos "a standing"...
s that operate over a wide range of temperatures the bimetal strip is mechanically fixed and attached to an electrical power source while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the
set point.
Some thermostats use a
mercury switchA mercury switch is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity, or other inertia.Mercury switches...
connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.
Depending upon the application, a higher temperature may open a contact (as in a
heaterA heater is any object that emits heat or causes another body to achieve a higher temperature. In a household or domestic setting, heaters are commonly used to generate heating...
control) or it may close a contact (as in a
refrigeratorA refrigerator is a cooling appliance comprising a thermally insulated compartment and a heat pump—chemical or mechanical means—to transfer heat from it to the external environment, cooling the contents to a temperature below ambient. Refrigerators are extensively used to store foods which spoil...
or air conditioner).
The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a
relayA relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts...
or the supply of
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
or
fuel oilFuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...
through an electrically operated valve. In some natural gas heaters the power may be provided with a
thermocoupleA thermocouple or thermocouple thermometer is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat into electric power...
that is heated by a pilot light (a small, continuously burning flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered
sparkA spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air. When a suitable voltage is supplied, a spark forms, ionizing the gas and drastically reducing its electrical resistance...
generating device.
For an illustration of a bimetal element in a simple thermostat, see the thermostat entry.
Thermometers
A direct indicating dial
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
(such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle. A bimetallic strip is also used in a
recording thermometerA recording thermometer is a type of thermometer that records temperature changes over a period of time.The recording thermometer uses a bimetallic strip that coils and uncoils as the temperature changes. One end of the strip is attached to a long, light metal lever that holds a special pen...
.
Heat engines
Simple toys have been built which demonstrate how the principle can be used to drive a
heat engineA heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle...
, (
http://www.newenergyshop.com/ See under Extraordinary Engines, the Seesaw - Candle Bimetal Engine)
Calculations
CurvatureIn mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...
of a Bimetallic Beam:
Where and are the
Young's ModulusIn solid mechanics, Young's modulus is a measure of the stiffness of an isotropic elastic material. It is also known as the Young modulus, modulus of elasticity, elastic modulus or tensile modulus...
and height of Material One and and are the Young's Modulus and height of Material Two. is the misfit strain, calculated by:
Where α
1 is the Coefficient of Thermal Expansion of Material One and α
2 is the Coefficient of Thermal Expansion of Material Two. ΔT is the current temperature minus the reference temperature (the temperature where the beam has no flexure).
External links