Abstract: The calculational model of main drift chamber in spectrometer of Beijing electron-positron collider (BEPC Ⅱ) and the entire thread stretching procedure is built based on FEM for the research of mechanics rule and the control of accuracy the and structure. The “element activation” technic is used to avoid element rebuliding because of structure change and ensure the efficiency and accuracy of calculation. The calculation is linearized with the sequential couple method after the vis-coelasticity of the wires investigated by stress relaxation ex-periment. The method is proved effective by both theory and experiment. The result of the change of the wires’ initial stress is gained and the stretching method with pretightening force for the control of the change of the wire stress is used to ensure the accuracy of the drift element. The result used in the BEPCII modification works will exert an important role for the building of main drift chamber.

Key words: FEM, Initial stress, Main drift chamber, Viscoelasticity