Dynamic Crank Web Strain Measurements for Reciprocating Compressors (Research)
The current industry approach to document crankshaft alignment is to measure static web deflections. These measurements are made at regular crank angles and historical limits have been defined, which relate observed deflection values to estimated crank stress levels associated with once per revolution bending stresses. Recently, more refined finite element models have been developed to define the crankshaft stress fields when subjected to the measured web deflection values. The effects of dynamic loads have largely been ignored. These loads, resulting from inertia and gas forces, will add to the stress field generated by the static misalignment. Furthermore, the location of the rotating shaft in the bearing will affect the estimate of stress field based on the static web deflection measurements. In an effort to advance the state of the art in addressing these issues, SwRI has developed a device, which will record the dynamic strain at the web root. This Strain Data Capture Module (SDCM) records the dynamic strain levels at predefined intervals and stores the information for subsequent retrieval and download to a PC. In an effort to refine the device and evaluate the implications of the data on the understanding of the crank stress levels, the GMRC and PRCI have jointly funded a program to acquire multi-web dynamic strain data on an industry representative machine and compare the estimate strain (and stress) levels with that predicted using the web deflection approach. This paper presents the initial results of this effort.