Tham khảo tài liệu 'designing capable and reliable products episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Case studies 205 Figure SSI models for the solenoid assembly failure modes where A cross-sectional area J polar second moment of area. An approximate relationship is commonly used to determine the torque for assembly M for a given pre-load F Shigley and Mischke 1996 . It is a standard formulation for bolts and fasteners determined from experiment and is related to the friction found in the contacting surfaces of the parts on assembly. M KFD where K torque coefficient or nut factor . Therefore combining the above equations in terms of the shear stress gives T FLKFDr r - d4 206 Designing reliable products The principal stresses at the relief section 5 and 52 are found from 5j j2 Using von Mises Theory from equalion the probabilistic requirement p to avoid yield in a ductile material but under a biaxial stress system is used to determine the reliability R as R p sy ỵ sị sị-SỵS Stress in service The shear stress T due to the assembly torque diminishes to zero with time the preload F remaining constant and so the stress on the solenoid section is only the direct stress s as given in equation see Figure b Edwards and McKee 1991 . A second reliability can then be determined by considering the requirement that the pre-load stress remains above a minimum level to avoid loosening in service from experiment Marbacher 1999 . The reliability R can then be determined from the probabilistic requirement p to avoid loosening R P L PSSp Determining the design variables Before a probabilistic model can be developed the variables involved must be determined. It is assumed that the variables all follow the Normal distribution and that they are statistically independent . not correlated in anyway. The scatter of the pre-load F using an air tool with a clutch is approximately 30 of the mean which gives the coefficient of variation cv assuming ĩơ covers this range therefore ơp For the torque coefficient K .
