Research On Screw Joint For Precise Assembly Of Miniature Parts

Screw joint used as the most widely connection method directly affect the assembly quality and reliability of products. With the development of miniaturization and precision of electromechanical products, a higher control precision is presented for the miniature screw joint. To improve the fatigue life, reliability and quality of screw joint, the key point is to control the precision of pre-tightening force. Nowadays, the commonly used method of tightening screw thread is torque control method, which causes decentralization of axial pre-tightening force, and decreases the reliability of the joint. Other methods are not suitable for the miniature screw thread precise assembly which need target pre-tightening force determined, since the control force is too large. The torque/time method presented in this paper can effectively improve the control precision of pre-tightening force without adding other parts, like angle sensor.Aimed at the precise screw thread assembly for miniature parts, the research for improving the precision of pre-tightening force was studied, including:â‘ Initial screw assembly status was investigated, and the allowable wrap of location and angle between bolt and nut was confirmed.â‘¡Based on the function of pre-tightening force and torque in the elastic region of screw, a new tightening method, which is followed the slope changes of torque to time and the system stiffness of the screw thread, was proposed. Offering different tightening torque to every screw thread recording to the method can effectively reduce the uncertainty influence of the friction and improve the conformity of the pre-tightening force. The assembly process of tightening M1.4 screw thread with dishing flexible washer was simulated by finite element analysis and dynamic simulation.â‘¢An automatic screw thread assembly device was developed. The device consists of two functional modules, including tightening spindle module and linear stage module. Tightening spindle module as the core of the whole assembly system, mainly consists of power, torque sensor and the holding unit.â‘£In screw thread assembly experiments, torque control and torque/time control method was used separately, and the data acquired from the two methods was contrasted. The dispersion of pre-tightening force was detected by the force sensor.Experiments shows that torque/time control method can improve the control precision of the pre-tightening force and the assembly quality of products.