Tham khảo tài liệu 'failure analysis case studies ii episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 163 The applied torque and thrust during the reaming of the hole on which failure occurred was as follows torque varied between 358 599 and 466 170Nm. the original thrust was thought to be constant at approximately 4454 kN however on further checking it was found that the actual thrust was 5033 kN . 13 above maximum. By design the drive torque is transmitted through the connection via splines and the drive head bolts are intended to carry only the applied thrust. The bolts are 32 mm in. in diameter by 89 mm in. long SAE Grade 8 hexagonal head cap screws having a torque specification of 1140Nm 840 ft lb . Prior to installation all the bolts are coated with an anti-seize compound. 4. SITE VISIT A site visit was made in order to carry out an inspection of the raise boring machine which had been brought to the surface and had been dismantled in the company workshops. During brcaking-out it was noticed that the torque of the drive head cap screw or centre bolt was well below the normal figure. The fractured bolt sections in the locating holes had been extracted and clearly identified in clockwise sequence from 1 to 32 position 1 being at the 6 o clock position for reference purposes . It was not possible to extract the sections of bolts 21 25 and 28 due to seizure in the holes. The fractured bolt sections were subsequently matched to their corresponding bolt head sections by fracture surface comparison. The original orientation of each bolt in the locating holes had been marked on the bolt heads. It was clear from the position of fracture of the bolt sections still situated in the body of the machine and the positions of fracture of the other bolts that failure had occurred at or near the joint between the cover and the body. The underside surface of the cover including the area containing the locating holes showed general rusting from the ingress of water. 5. EXAMINATION OF THE FRACTURED BOLTS The fractured bolts were visually examined on-site and then .
