Stress Analysis and the Cause of the Fracture of a Clevis at the End of a Hydraulic Piston Rod
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TECHNICAL ARTICLE—PEER-REVIEWED
Stress Analysis and the Cause of the Fracture of a Clevis at the End of a Hydraulic Piston Rod Tim A. Jur Æ Ronald L. Windham
Submitted: 8 July 2009 / Published online: 19 September 2009 Ó ASM International 2009
Abstract A hydraulic piston assembly fractured while in service. The fracture involved a clevis assembled to the end of the cylinder piston rod. In fact, there were two fractures: one where a threaded end on the clevis was assembled to the piston rod and a second involving a fracture through the wall of the clevis. Analysis by plant personnel suggested that the stresses involved were small and that fracture should not have been expected. As a more detailed stress analysis of the clevis ring demonstrated, the stresses associated with the clevis, treated as a closed ring, were more than sufficient to have resulted in the fracture through the clevis wall. The other fracture, the fracture at the point of assembly to the piston rod, was proved to be secondary. Stress analysis as well as fracture analysis proved the point and allowed appropriate recommendations to be made to avoid a reoccurrence. Keywords Hydraulic piston Fatigue fracture Clevis Stress analysis Closed ring
operation was such that the two pistons operated on either side of an ejection beam. Raising the beam ejected the product from the machine. With the beam in the at-rest position, the cylinders would be near full extended. The ejection operation involved driving the beam under load by retracting the cylinder piston rods into the cylinder bodies. Figure 1 shows the rest and ejection positions of the apparatus and the cylinders in side view and in schematic form. The engagement of the end of any one of the two cylinder piston rods with the ejector beam was by way of a clevis. A sketch of the clevis as a separate part threaded into the end of the piston rod is shown in Fig. 2. Figure 3 shows the basic dimensions of the clevis itself. It was the clevis on one of the cylinder piston rods that fractured in service (Fig. 4). The fracture shut down the machine for an extended period of time, with a consequent economic loss associated with the interruption of business. It was important to analyze the fracture, assess the cause, and recommend action to avoid a reoccurrence.
Background Information The piece of equipment involved in this fracture analysis was a hydraulic piston. The piston was one of two that were installed on either side of a machine and used as part of an apparatus to eject finished product from the machine. The concept, in general, is shown in Fig. 1. The ejection
T. A. Jur (&) R. L. Windham Engineering Design & Testing Corp., Columbia District Office, Columbia, SC, USA e-mail: [email protected]
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Examination of the Part and Interpretation of the Fracture Mechanism The fractured end of the clevis is shown in Fig. 4. The basic dimensions are shown in Fig. 3. Of particular interest to the analysis was the presence of two fracture lines, identified by Arrows 1 and 2 in Fig. 4. Arrow 1 p
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