Unintended Load Paths Leading To Fatigue

Xmas Greetings from Origen

This month’s Technical Tip considers an interesting failure arising effectively from a misunderstood load path. The failure in question involved a 1000 tonne 600mm diameter hydraulic cylinder after approximately 3 months in service.

Owing to the large size of the cylinder, the piston had been threaded onto the rod and was screwed down against a shoulder on rod (as opposed to the original forged piston and rod). A threaded locking ring was used to prevent the piston from coming loose. This locking ring screwed over the end of the rod that protruded into a recess in the top of the piston and was secured by means of six grade 10.9 M16 cap screws running through the locking ring into the piston. These cap screws effectively pulled the piston toward the locking ring and induced significant contact loads between the threads of the piston, locking ring, and cylinder rod to provide the requisite locking action.

Removal of the end cap of the cylinder highlighted, extensive damage to the surfaces of the cap, the piston, locking ring, and the bore of the piston, and that the head of one of the M16 cap screws used to secure the locking ring to the piston had broken off. The damage to the cylinder was consistent with having been caused by the head of the failed cap screw and securing Nordlock washer becoming displaced and moving around the region between the piston and the cap where they were deformed whenever the piston was retracted fully. In due course remnants of the displaced components became lodged between the piston and the bore causing extensive damage to the cylinder bore and piston. Removal of the cap screws and locking ring facilitated detailed inspection of the components and highlighted that three of the six cap screws securing the locking ring, had fatigued one below the head and the other two in the threads, at the interface between the locking ring and piston head. Witness markings on the load-bearing surface of the head of the cap screws and the locking ring, showed the cap screws had carried significant load prior to failure.

Analysis of the material from the cap screws showed it had a tempered martensite microstructure which was consistent with the composition of the material, and both the composition and the hardness were within specified limits suggesting failure was not due to material issues.

During disassembly, it was noted that the locking ring could be rotated by hand from its as assembled position through approximately 120ᵒ indicating there was a gap of some 1.3mm between the locking ring and the piston. This implied that although the locking action was indeed achieved, the piston was pulled toward locking ring away from the land on the rod and against the lower face of the threads on the rod by the locking screw, while the load ring was tightened down by the head of the cap screw onto the upper faces of the piston rod threads. Depending on the magnitude of the torque that was applied to the piston this would have resulted in a relatively large proportion of the cyclic piston load being carried by the bolts. Owing to the gap between the members being secured and the resulting low member stiffness a large proportion of this load was carried by the bolts. With the applied cyclic loading, of typically 400 tons, the consequent stresses were high and development of fatigue cracking as observed was inevitable.

This Tech Tip highlights the importance of understanding the true loading on components and of getting the stress analysis and modeling right. If this understanding is not correct the consequences can be expensive!