Tham khảo tài liệu 'designing capable and reliable products episode 2 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Case studies 245 Figure General arrangement of the con-rod and pin of an overload situation is also covered. Overloads typically occur if a pressed part is badly orientated within the press dies or if a foreign body is caught in the die set literally described as a spanner in the works . Analysis of the bolts is not covered. Design strategy In production the press will be operating at millions of cycles per annum. Therefore it should be designed against fatigue failure and the con-rod and pin must be engineered in the light of the distribution of the associated endurance strength in shear. For the purposes of the analysis it was assumed that the applied stress would have an extreme value corresponding to the application of the 280 kN load. Also in carrying out the analysis approximate statistical models were needed for the material s endurance strength based on the available data. As mentioned earlier the system was to be designed with a weak link. To satisfy this requirement the pin was designed such that it would fail in an overload situation. The material selected for the pin was 070M20 normalized mild steel. The pin was to be manufactured by machining from bar and was assumed to have non-critical dimensional variation in terms of the stress distribution and therefore the overload stress could be represented by a unique value. The pin size would be determined based on the 3 standard deviation limit of the material s endurance strength in shear. This infers that the probability of failure of the con-rod system due to fatigue would be very low around 1350 ppm assuming a Normal distribution for the endurance strength in shear. This relates to a reliability R which is adequate for the 246 Designing reliable products Figure Stress-strength interference for the pin non-safety critical nature of the failure mode. The situation is represented in the stress-strength interference diagram given in Figure . Determination of the pin diameter and con-rod .
