Bimetallic strips use differential expansion of heated dissimilar metals to convert thermal energy into mechanical motion. They are used as thermally activated switches or heat indicators in various electrical and measurement equipment. The strips can be straight or coil-shaped and are made of two dissimilar metals bonded together, such as brass and steel or copper and steel. They can be used to make or break circuits, indicate temperature readings, and act as thermal overload devices.
A bimetallic strip is a simple device that converts thermal energy into mechanical motion. It is used as a thermally activated switch or heat indicator and works on the principle of differential expansion of heated dissimilar metals. Bimetallic tape consists of two different metals that are bonded together to form a straight, flat strip or concentric coil. When the strip is heated, one of the metals heats up and expands faster than the other, causing the strip to bend. This mechanical deflection is then exploited in various ways to switch electrical circuits or move a dial for a heat value indication.
When a solid, fluid or gas is heated, its molecules begin to move away from each other leading to expansion. Until a heated material is contained to a certain point, it will expand in a balanced manner. If, for some reason, the material is contained or cannot expand along one of its surfaces, unrestrained expansion in the rest of the material will cause it to deflect or bend. Obviously some materials will heat up faster or expand more than others depending on their molecular structure. If two of these dissimilar materials come together, the one that heats up more slowly will restrict expansion along one face of the other and cause this deflection phenomenon to occur.
This principle of thermal deflection of dissimilar materials is used to provide heat-related mechanical motion in a wide variety of electrical and measurement equipment. Bi-metal strips are usually constructed of two dissimilar metals with brass and steel and copper and steel being typical combinations. The two materials are usually welded, brazed, or riveted together to produce a smooth, secure bond between the two contact faces. The bimetallic strips can be formed as straight or concentric coil spring type elements. Their physical deflection when heated is then exploited in various ways to achieve different functions.
One of the most common ways to use the deflection of bimetallic tape is to use the tape as an electrical contact to make or break circuits. Heater thermostats are a good example of this type of application and use the straight, unheated strip to maintain contact with a switch point and keep the heater circuit running. When the temperature rises to a predetermined point, the strip heats up and bends, thus breaking the circuit and turning off the heater. When the strip cools down again, it straightens and reactivates the heating circuit. This allows you to maintain a constant temperature range.
Coil-type bimetallic tape tends to unwind as it heats up and rotates the needle of a dial thermometer to indicate a temperature reading. Bimetallic strips are also used as thermal overload devices to shut off power to electrical equipment when they draw excessive current or physically become too hot. The known expansion characteristics of the different metals in the bimetallic strip allow very accurate adaptation of these devices to operate at exact temperatures. This makes the strips a reliable and cost-effective method of using heat as a control or measurement input.
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