Bimetal strip

BIMETAL STRIP - Temperature regulating or indicating device which works on principle that two dissimilar metals with unequal expansion rates, welded together, will bend as temperatures change. 

To consider the effect of a receptor layer, Timoshenko beam theory, which was originally developed to analyze a bimetal strip, can be used [38]. Based on the Timoshenko beam theory, an analytical model for the static deflection of a cantilever sensor coated with a solid layer was derived. In a simple cantilever coated with a solid receptor layer as shown in Fig. 4.3.10, the deflection of the cantilever is described as ... 

As solids are heated they expand at rates determined by their coefficients of expansion that vary with the different substances. The heat needed to raise the temperature of different solids varies and can be determined from its specific heat. Problems can arise where dissimilar metals are welded together that have different specific heats and different coefficients of expansion. Allowance has to be made for differential expansions where different metals are moving in contact and have different coefficients of expansion, such as in bearings. However use is made of these differences in bimetal strips for temperature measurements and in thermostats. 

Circuit breakers are electromechanical devices used for the interruption of electrical currents due to overloads or short circuits. Most circuit breakers are thermal magnetic devices. The thermal element in a circuit breaker is a bimetal strip, which is heated by the current. The deflection of the bimetal strip due to overloads causes the breaker to trip. The bimetal strip provides inverse time trip protection as the amount... 

Traditional NEMA designs use either melting alloy or bimetal strips, and have interchangeable heaters. NEMA has assigned classes in reference to the designed protection level provided by the individual overload relay. Class 10 overload relays are designed to protect artificially... 

Bimetal Overload Relays. Traditional NEMA-style bimetal overload relays share the heater mterchangeabihty of the melting-alloy type. They may also have the ability to adjust up or down the equivalent of one heater size, or about +15 percent. lEC overloads are bimetal type, but have nonreplaceable integral heaters. To compensate for this, these units can typically be adjusted +20 to 25 percent. Bimetal devices use the heat generated to cause deflection of one or more bimetal strips. Once this deflection is great enough, the trip mechanism is operated. 

Thermal overload protection is usually provided by means of a bimetal strip bending under overload conditions and breaking the starter contactor coil circuit. 

A bimetal strip is formed from two dissimilar metals joined together. When the temperature increases the metals expand at different rates and the bimetal strip bends or bows. 

A thermostat is a device for maintaining a constant temperature at some predetermined value. The operation of a thermostat is often based on the principle of differential expansion between dissimilar metals that is, a bimetal strip, which causes a contact to make or break at a chosen temperature. Figure 3.64 shows the principle of a rod-type thermostat which is often used with water heaters. An invar rod, which has minimal expansion when heated, is housed within a copper tube and the two metals are brazed together at one end. The other end of the copper tube is secured to one end of the switch... 

The low-temperature switch, which has been calibrated to open at 16 C, is a simple bimetal thermal switch. As the temperature of the switch reaches this low level, the bimetal strip which makes up one side of the switch bends, breaking the interlock circuit. The bending action takes place because the two metals used in the strip have different linear coefficients of expansion. 

Other nonelectrical thermometers are bimetal, fiHed-system, and pyrometric cone thermometers. In bimetal thermometers, two strips of metal of differing expansion characteristics are welded together face-to-face. If one end of such a strip is fixed, the strip bends in response to temperature change as the... 

Any bi-niclal combination, having large differences in their coefficients of linear expansion, such as a bimetal of brass and steel is used for sueh applications. One end of a strip is fixed and the other is left free for natural movement. When heated, brass expands more than the steel and bends towards the steel as shown, giving the desired movement to actuate a tripping lever. 

BIMETAL THERMOMETER. Thermostatic bimetal can be defined as a composite material, made up of strips of two or more metals fastened together, which, because of the different expansion rates of the components, lends lo change iLs curvature when subjected to a change in lemperaLure. 

With one end of a straight strip fixed, the other end deflects in direct proportion to the temperature change and the square of the length, and inversely as Lhe thickness, throughout the linear portion of Lhe deflection characteristic curve. Tf a strip of bimetal is wound into a helix or spiral and one end is fixed, the other end will rotate when heat is applied. The angular deflection varies directly with the temperature change and the length of the strip, and inversely with the thickness of the material, over the linear parts of the deflection characteristic curve. Bimetals show uniform deflection only over part of the deflection characteristic curve, as shown in Fig. 1. The three types of elements most commonly used in thermometers are shown in Fig. 2. 

Detailed analyses of the effects of film thickness on substrate curvature in bimaterials date back to the early twentieth century, when interest in the use of thermostatic bimetals began to expand rapidly, as described in the historical note on thermostatic bimetals in Section 2.2.3. Timoshenko (1925) and Rich (1934) derived thermoelastic solutions for curvature and stress evolution in a bimetallic strip as a function of temperature change, for arbitrary variations in the relative thickness and elastic properties of the... 

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