# Bimetallic strip

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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 steel and copper, or in some cases steel and brass. The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Steel is an alloy of iron and carbon, and sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons.

Copper is a chemical element with the symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Brass is an alloy of copper and zinc, in proportions which can be varied to achieve varying mechanical and electrical properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.

## Contents

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 earliest surviving bimetallic strip was made by the eighteenth-century clockmaker John Harrison who is generally credited with its invention. He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring. [1] It should not be confused with the bimetallic mechanism for correcting for thermal expansion in his gridiron pendulum. 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, H5. Harrison's invention is recognized in the memorial to him in Westminster Abbey, England.

A clockmaker is an artisan who makes and/or repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique shops, and places devoted strictly to repairing clocks and watches. Clockmakers must be able to read blueprints and instructions for numerous types of clocks and time pieces that vary from antique clocks to modern time pieces in order to fix and make clocks or watches. The trade requires fine motor coordination as clockmakers must frequently work on devices with small gears and fine machinery.

John Harrison was a self-educated English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of calculating longitude while at sea.

A marine chronometer is a timepiece 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 accurately measuring the time of a known fixed location, for example Greenwich Mean Time (GMT) and the time at the current location. When first developed in the 18th century, it was a major technical achievement, as accurate knowledge of the time over a long sea voyage is necessary for navigation, lacking electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and testing that revolutionized naval navigation and enabling the Age of Discovery and Colonialism to accelerate.

## Applications

### Clocks

Mechanical clock mechanisms are sensitive to temperature changes as each part has tiny tolerance and it leads to errors in time keeping. A bimetallic strip is used to compensate this phenomenon in the mechanism of some timepieces. The most common method is to use a bimetallic construction for the circular rim of the balance wheel. What it does is move a weight in a radial way looking at the circular plane down by the balance wheel, varying then, the momentum of inertia of the balance wheel. As the spring controlling the balance becomes weaker with the increasing temperature, the balance becomes smaller in diameter to decrease the momentum of inertia and keep the period of oscillation (and hence timekeeping) constant.

A balance wheel, or balance, 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 torsion spring, the balance spring or hairspring. It is driven by the escapement, which transforms the rotating motion of the watch gear train into impulses delivered to the balance wheel. Each swing of the wheel allows the gear train to advance a set amount, moving the hands forward. The balance wheel and hairspring together form a harmonic oscillator, which due to resonance oscillates preferentially at a certain rate, its resonant frequency or 'beat', and resists oscillating at other rates. The combination of the mass of the balance wheel and the elasticity of the spring keep the time between each oscillation or ‘tick’ very constant, accounting for its nearly universal use as the timekeeper in mechanical watches to the present. From its invention in the 14th century until tuning fork and quartz movements became available in the 1960s, virtually every portable timekeeping device used some form of balance wheel.

Nowadays this system is not used anymore since the appearance of low temperature coefficient alloys like nivarox, parachrom and many others depending on each brand.

Nivarox is a Swiss company formed by a merger in 1984 between Nivarox SA and Fabriques d'Assortiments Réunis (FAR). It is currently owned by the Swatch Group. Nivarox is also the trade name of the metallic alloy from which its products are fabricated. Its notable property is that its coefficient of elasticity is remarkably constant with temperature. Nivarox is most famous for producing hairsprings that are attached to the balance wheel inside a mechanical watch movement, as well as mainsprings which provide the motive power for the watch.

### Thermostats

In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetallic 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.

A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.

Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.

A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs but they all share the common design strength of non-eroding switch contacts.

Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).

The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple 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 spark generating device.

### Thermometers

A direct indicating dial thermometer really common in daily use devices (such as a patio thermometer or a meat thermometer) uses a bimetallic strip wrapped into a coil in its most used design. The coil changes the linear movement of the metal expansion into a circular movement thanks to the helicoidal shape it draws. One end of the coil is fixed to the housing of the device as a fix point and the other drives an indicating needle inside a circular indicator. A bimetallic strip is also used in a recording thermometer. Breguet's thermometer consists of a tri-metallic helix in order to have a more accurate result.

### Heat engine

Heat engines are not the most efficient ones, and with the use of bimetallic strips the efficiency of the heat engines is even lower as there is no chamber to contain the heat. Moreover, the bimetallic strips cannot produce strength in its moves, the reason why is that in order to achieve reasonables bendings (movements) both metallic strips have to be thin to make the difference between the expansion noticeable. So the uses for metallic strips in heat engines are mostly in simple toys that have been built to demonstrate how the principle can be used to drive a heat engine.[ citation needed ]

### Electrical devices

Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. A coil of wire is used to heat a bimetal strip, which bends and operates a linkage that unlatches a spring-operated contact. This interrupts the circuit and can be reset when the bimetal strip has cooled down.

Bimetal strips are also used in time-delay relays, lamp flashers, and fluorescent lamp starters. In some devices, the current running directly through the bimetal strip is sufficient to heat it and operate contacts directly.

## Calculations

Curvature of a bimetallic beam:

${\displaystyle \kappa ={\frac {6E_{1}E_{2}(h_{1}+h_{2})h_{1}h_{2}\epsilon }{E_{1}^{2}h_{1}^{4}+4E_{1}E_{2}h_{1}^{3}h_{2}+6E_{1}E_{2}h_{1}^{2}h_{2}^{2}+4E_{1}E_{2}h_{2}^{3}h_{1}+E_{2}^{2}h_{2}^{4}}}}$
where ${\displaystyle E_{1}}$ and ${\displaystyle h_{1}}$ are the Young's modulus and height (thickness) of material one and ${\displaystyle E_{2}}$ and ${\displaystyle h_{2}}$ are the Young's modulus and height (thickness) of material two. ${\displaystyle \epsilon }$ is the misfit strain, calculated by:
${\displaystyle \epsilon =(\alpha _{1}-\alpha _{2})\Delta T\,}$
where α1 is the coefficient of thermal expansion of material one and α2 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). [2] [3]

## Related Research Articles

A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. Thermocouples are a widely used type of temperature sensor.

A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.

A thermometer is a device that measures temperature or a temperature gradient. A thermometer has two important elements: (1) a temperature sensor in which some change occurs with a change in temperature; and (2) some means of converting this change into a numerical value. Thermometers are widely used in technology and industry to monitor processes, in meteorology, in medicine, and in scientific research.

The electrical resistance of an object is a measure of its opposition to the flow of electric current. The inverse quantity is electrical conductance, and is the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the notion of mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).

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An infinite switch, simmerstat, energy regulator or infinite controller is a type of switch that allows variable power output of a heating element of an electric stove. It is called "infinite" because its average output is infinitely variable rather than being limited to a few switched levels. It uses a bi-metallic strip conductive connection across terminals that disconnects with increased temperature. As current passes through the bimetal connection, it will heat and deform, breaking the connection and turning off the power. After a short time, the bimetal will cool and reconnect. Therefore, infinite switches vary the average power delivered to a device by oscillating quickly between on and off states. They may be used for situations that are not sensitive to such changes, such as the resistive heating elements in electric stoves and kilns.

A measuring instrument is a device for measuring a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.

Overheating is a phenomenon of rising of temperature in an electric circuit. Overheating causes potential damage to the circuit components, and can cause fire, explosion, or injury. Damage caused by overheating is commonly irreversible; i.e. the only way to repair is to replace some components.

## Notes

1. Sobel, Dava (1995). Longitude . London: Fourth Estate. p. 103. ISBN   0-00-721446-4. One of the inventions Harrison introduced in H-3... is called... a bi-metallic strip.
2. Clyne, TW. "Residual stresses in surface coatings and their effects on interfacial debonding." Key Engineering Materials (Switzerland). Vol. 116-117, pp. 307-330. 1996
3. Timoshenko, J. Opt. Soc. Am. 11, 233 (1925)