Reliability Analysis Of The Snap-action Thermostats

The snap-action thermostat is an electrical switch element to control temperature. Because ithas many advantages, such as high precison in temperature control, compact structure, small size,it is widely used in the temperature control and overheating protection, such as microwave oven,electrical tea pot and other household electrical appliances. Recently, the foreign snap-atctionthermostats’life is up to two million times, while the domestic thermostat’life is only onehundred thousand times. And also the domestic thermostat has many problems which restrictesthe competition ability improvement in the international market, such as big difference betweendifferent batches life. So, in this dissertation, T1/33F thermostat as the object, the systemictheory analysis of thermostats is presented by using FEA and the reliability analysis methods ofFMECA and FTA, weak points of the design are found out, and corresponding corrective actionsare given. All these provide technical supports to improve the domestic snap-actionthermostats’life.First, the reliability research developing history and current situation at home and abroad,reliability analysis technology and measures, the current situation and problems existed in thesnap-action thermostats was briefly introduced.Second, the operating characteristics and requirements of the snap-action thermostats wereanalysed. Due to the test and filed data of one company, FMECA of the thermostats was made inorder to get the whole failure modes; dangers matrix analysis was made in order to get the mainfailure modes(contact welding, leafspring fracture and temperature drift failure) and keycomponents (thermostatic bimetal-strip, leafspring, electrical contact) of the snap-actionthermostats. On this basis, the main failure modes as top event, the main failure reasons of thesnap-action thermostats were found by FTA.Third, through analysis of the structure, operating pinciples and operating characteristic ofthe thermostatic bimetal-strip, the relation between step temperature, step amplitude andthickness, outer boundary circle radius, arch rise, expansion coefficient and elastic modulus,poisson’s ratio was determined. After that, by ANSYS software, the relation between theparameters above and step temperatures was analysed further, and which parameters were moreimportant for step temperature was got. Finally, the failure mechanism of thermostaticbimetal-strip was analysed, and it’s concluded the temperature drift phenomenon of the snap-action thermostats was mainly due to size change caused by thermostatic bimetal-stripfatigue.Fourth, a static analysis of the leafspring of the snap-action thermostats was made,including the calculation of the leafspring flexibility and stress. Then, the stress was analysedfurther by ANSYS software. After that, the failure analysis was carried out due to its workingcircumstance, working requirement and stress analysis results. It was concluded that thephenomenon of leafspring fracture was mainly due to excessively high stress. Finally, withoutchanging the snap-action thermostats structure, the stress as goal, the contact requirement of theelectrical contact as state variables, the dimentional parameters of leafspring as design variables,leafspring optimization design was taken to make the stress achieve the requirement of fatiguestress by Ansys software.Fifth, the electrical, thermal, mechanical stress of the electrical contact, effect of contactparameters variations on contact performance was analysed. Then, the reasons of electricalcontact failure was analysed and concluded. It was concluded that the contact failure reasonsmainly included the broken constantly failure and the close out failure. Finally, through theT1/33F snap-action thermostats, broken constantly failure mechanism phenomenon was analysed,and it was concluded that the broken constantly failure was mainly due to dynamic weldingcaused by contact arc.Sixth, the type-I life test of the thermostats was made. According to the test data, usingMaximum likelihood Estimation method, it was confirmed that the life distribution of thethermostats was normal distribution. Then the life distribution parameters was estimated,reliability of the thermostats when its working life was100000was calculated, and somemethods for improving the the snap-action thermostats reliability was put forward.Seventh, gave a conclusion to research achievements and defects of entire dissertation, andpointed out the next research direction.