Investigation of a broken crank mechanism: causes and possible solutions

Reciprocating cryogenic pumps are employed in several applications where it is necessary to rise the pressure of a cryogenic liquid, such as air gases or LNG (liquid natural gas), up to hundreds of bars. One of many VT-55 high-pressure pumps produced by Vanzetti Engineering suffered a crankshaft failure, this case, although isolated, pushed the company towards the decision to verify the correct design of the crank mechanism, to understand the causes of this breakage and the operative limits of the pump. As starting point, developing a kinematic study, the position, velocity, and acceleration of the piston as a function of the crank angle are obtained. Thereafter, hypothesizing a pressure profile along a pumping cycle, a shaft static analysis was performed, estimating the forces and moments acting on it. In parallel to this, to forestall and verify the absence of resonance conditions, a torsional dynamic analysis was conducted, representing the pump assembly with a one-dimensional lumped parameter model. Analytical and experimental studies were done to represent the pump assembly as flywheels with equivalent inertia and torsional bars with torsional stiffness. To estimate the equivalent stiffness of the toothed belt, it was necessary to perform a tensile test, for which the anchoring system was designed. Once the parameters of the model have been characterized, it was possible to estimate the resonance frequencies by considering the free behaviour of the system. On the other side, by decomposing the system’s forcing via Fast Fourier Transform has been possible to obtain the various harmonics of excitation. Combining the natural frequencies and the harmonics into the so-called Campbell diagram, it was possible to identify the harmonics of excitation for a given pump speed. Returning to the static analysis results, the forces and moments previously estimated have been translated into stress values referring to each singular point of the shaft. Since the break occurred where notches are present, it was necessary to estimate the stress intensification to obtain the actual stress state. In parallel, both the Finite Element Method approach and experimental tests were conducted to validate the results obtained from the analytical model. To carry out the experimental tests, the pump assembly has been modified: the shaft and the feather key have been adjusted to allocate a series of strain gauges and their relative wiring, moreover, spacers have been designed to remove circlip rings in the neighbourhood of the sensor and to install the slip ring. By comparing the analytical model results with the ones coming from the experimental tests, is possible to state that the model well represents the shaft during the pump operation. Having identified, during the estimation of the stress concentration factors Kt the presence of a sharp edge at the base of the feather key groove, it was decided to make a rapid introduction of fillet radius with dimensions reported by UNI 6604 norm. Proof of this, in the FEM analysis the most stressed points correspond precisely to that area. Making this change, from the FEM analysis, a significant reduction in tension was felt. A second review will include the replacement of the elastic rings with spacers, as also these elements create a significant local increase in tensions.