Bimetal

A bimetal (also bimetallic strip ) is a metal strip of two layers of different metals are connected to each material -locking or form -locking manner. Characteristic is the change in shape when the temperature changes. This manifests itself as bending. The reason is the difference in thermal coefficient of expansion of the metals used. These metals may be, for example, zinc and steel, or steel with the alloy brass.

Principle

Two metals with different coefficients of linear expansion increased with heating by different distances. By connecting the two ends of two metal strips, for example by riveting or rolls having different extension leads to a bending of the bimetallic strip.

Production

Bimetals are usually manufactured in sheet or strip form.

The bare, free of oxide layers of metal sheets are rolled each other while under pressure. In the contact zone is formed by cold welding and subsequent diffusion annealing a permanent connection.

In another embodiment, the metal ends are congruent provided with through holes and riveted together or screwed.

Application

In automotive carburetors with an automatic choke, the starter valve of the automatic with a heated spiral bimetal spring after the warm-up phase of the engine is returned to normal position.

Because of the temperature-dependent deformation of bimetal bimetallic thermometer or thermostat can be produced. To this end, an end of such a strip is attached; at a temperature variation bends the strip, the free end changes its position and moves a pointer (for example, in the thermometer or similar things ) or a contact.

Bimetallic strips are used for control purposes, such as for temperature-dependent automatic actuation of valves, switching operations and partly also in measuring instruments.

A temperature switch opens or closes a bimetallic strip - in combination with a bistable magnetic Federelemant or adhesion - as a function of temperature for one contact, which for example, a heater on or off.

Slow combustion stoves have partially a strong bimetallic strip which directly actuates a valve for the supply air.

Historic water cooling thermostats in the motor vehicle contained a bimetal thermostats are present, however, actuated by an in- flow of coolant capsule ( a wax ) is filled with a so-called wax. In flasher relay power heated by the lights an isolated wire winding around the bimetallic strip which opened and then had to cool down. Fell one of the lights was blinking slows to less than half the frequency. Today, this function is emulated by the charging of a capacitor, and semiconductor.

Find application in many bimetallic heaters, for example for the temperature control of irons and boilers, for the control of coffee machines, toasters, kettles and for triggering circuit breakers ( circuit breakers).

Instead of an elongated strip a slightly domed circular disk can be used by approximately 2 cm in diameter made ​​of thin bimetallic. Upon heating, the material of the shell inside to expand and therefore tend to evert the curvature. This device is bistable within itself. It turns suddenly and powerfully to, and only when strong cooling back, thus has high hysteresis and reliability.

If a bimetal incorporated into the flow of electricity and a light bulb placed near the filament, the lamp begins to flash after a short heat. By heating up the current flow is interrupted; upon cooling, the circuit is closed again. Accordingly, does a bimetallic relay.

In shipbuilding bimetallic also be used for joining dissimilar metals ( there usually steel or aluminum). Since the metals can not be welded conventionally, bimetallic strips are used, which were prefabricated by explosive welding. At the building site can be typically used in the shipbuilding process (GMAW / MIG ) can be applied to, for example, a page with the hull made ​​of steel, to weld the other side of the deckhouse of aluminum.

Problems through the bimetal

In many areas of bimetal causes problems or must be operated a constructive effort to avoid such.

Bimetals described above are deliberately flat components ( "stripes" ). If, however, three-dimensional components (eg L- profiles, T-sections, H-sections, components with rectangular cross section) of metals with different thermal expansion coefficients together adds (eg screwed, riveted, glued ) and the construct strongly heated, then the construct bends often such that it remains deformed even after the re - cooling.

The force occurring can be so strong that compounds are destroyed (eg screws ) between the two metals.