Bimetallic plate is a plate made of bimetal or mechanically connected pieces of two different metals . As a rule, it is used as the main part of a thermomechanical sensor.
Content
Device
A bimetallic plate is a piece of tape made of bimetal . One end of the tape, as a rule, is fixedly fixed in the device, and the other end moves depending on the plate temperature .
There are devices consisting of two plates of dissimilar metals , fixed at one end and connected (by riveting , soldering or welding ) at the other ends. When the temperature changes, the connected end of the plates moves.
Operable in a very wide range of temperatures [1] .
Application
Thermostats and safety devices
A curving bimetallic plate controls the electrical contacts that close or open the heater circuit. (In the case of protective devices - disconnecting the power supply).
They can reduce-spread contacts gradually (cheap unreliable construction - contacts sparkle and burn), and they can jump ( mechanical bifurcation ), immediately moving the contact a few millimeters (clicks from such switches are heard when irons , kettles and other similar devices are working).
They are used as protective devices: for protection against overheating (for example, in an electric kettle ) or against overcurrent ( fuses ). can be either self-repairing or requiring personnel intervention (it is assumed that the personnel will find and eliminate the cause of the malfunction, and only then return the fuse to the on state).
Pulse generators and timers
Bimetallic plate with contact and with a heater (the winding is made of high-resistance wire or the plate itself, through which current is passed).
It is used to switch the operating modes of devices after they are turned on (for example, in starters for fluorescent lamps and electric motors ). In this case, the heating of the plate continues as long as the device is turned on.
Measuring Instruments
A variety of bimetal thermometer with a heater. Depending on the method of inclusion may be a voltmeter or ammeter . During the work consumes a lot of energy, however does not contain the rubbing mechanical parts at all. Simple, vibration-resistant, little sensitive to contamination, as a rule, self-healing when wet. It is still widely used in automotive electronics.
Clock
Used for temperature compensation of the clock. They can change the diameter of the split balance rim made from a bimetallic plate, or change the effective length of the balance spring.
Thermometers
A long bimetal coiled ribbon is fastened in the center. The other (outer) end of the helix moves along the scale , marked in degrees. Such a thermometer, unlike a liquid (for example, mercury) thermometer, is completely insensitive to changes in external pressure and is mechanically more durable.
In thermographs, a bimetallic plate controls the pen of a recorder through a system of levers that draws a graph of temperature changes (used in meteorology ). For example, in a temperature controller, a bimetallic plate, heating to the maximum allowable temperature, bends and opens the circuit in a definite way. As a result, further heating does not occur.
Heat engines
Converting temperature differences to mechanical work. There are simple toys to demonstrate the operation of such engines [2] .
Microdisplacement devices
Objects (of the “preparation” type, viewed under a microscope ) with the help of bimetallic plates with heaters can be moved within small limits. The amount of movement is controlled remotely by changing the current through the heaters.
Disadvantage: the amount of movement is not constant and depends on the cooling conditions (ambient temperature, drafts, etc.)
In shipbuilding
Bimetallic (as well as trimetallic) plates are used for welding dissimilar metals in order to prevent contact (galvanic) corrosion. In shipbuilding, they are used both for joining an aluminum superstructure with a steel case, and for connecting decorative elements made of stainless steel with an aluminum structure.
Non-metallic analogues
For work in aggressive environments, properties similar to bimetals are characterized by junctions from glass or ceramics with various CTE ,
Plate Calculation
Bending ( curvature of the curve , inverse to the bend radius) of the bimetallic plate [3] :
Where:
- ;
- - Young's modulus of material 1 (here and below for material 2, the indices, respectively, 2);
- - material thickness 1;
- - coefficient of thermal expansion of the material 1;
- - the difference between the temperature at which the bend is calculated and the temperature at which there is no bend.
The curvature expression is given for the case when the Poisson ratios of the mating plates are equal to zero. The general case is considered in [5].
History
Apparently, the bimetallic plates were created in England in the 18th century by watchmaker John Harrison for thermal compensation of his “H3” marine chronometer . [4] .
Notes
- ↑ Bimetallic tape in liquid nitrogen (English)
- ↑ Bimetallic swings (photo)
- ↑ Clyne, TW. "Surface Resistance Coatings on Interfacial Debonding." Key Engineering Materials (Switzerland). Vol. 116-117, pp. 307-330. 1996 (not available link) (English) , pdf , 36KB
- ↑ Sobel, Dava. Longitude, London, Fourth Estate, 1995, ISBN 0-00-721446-4 , p. 103 (English)
[5] Glagolev V.V., Markin A.A., Pashinov S.V. Bimetallic plate in a uniform temperature field // Mechanics of composite materials and structures. - 2017. - T. 23. - № 3. - p. 331-343.