Failure investigation and redesign of piston- and pump shafts

For the application of glue during the production of cars, a double-acting pump driven by a pneumatic piston is used. The shafts of the pump and the pneumatic piston are fixed to each other with a C-shaped split bush, which fits over a groove in both the pump and the piston shaft.A number of piston- and pump shafts failed prematurely at the location of the coupling. Following the first fractures, all shafts in use were subjected to a magnetic particle inspection and cracks were found in a large number of them.From the fracture investigation it was shown that the shafts failed due to the propagation of a fatigue crack, starting from a bottom radius of the groove for the coupling bush, caused by cyclic loading during operation. No major initiating defects could be found. The corner radius of the groove was found to match the drawing of the manufacturer. From the investigation of the material it was found that two different materials (42CrMo4 and C45E) were used for the production of the shafts, which matches the materials specified by the manufacturer. The loads acting on the shafts during operation were checked using strain gauges, and found to be inside the design range of ±45 kN.Using a stress method it was shown that C45E steel was inadequate for this application and that even with 42CrMo4 steel, more care had to be taken in the surface finish of the corner radius. Two new shafts were produced from 42CrMo4 steel, with a polished (instead of turned) bottom radius. These shafts were subjected to a fatigue test of 2 million load-cycles with load varying between +45 and −45 kN (constant amplitude test). Both shafts were able to withstand this load without failure. The shaft design has been changed and no further shaft fractures have occurred.