Above is a platform for an 18,000 pound motor and clutch assembly, the shaft is vertical. The clutch will be a new modification to the existing system. The addition of the clutch, by it’s mass addition and because it raises the c.g. of the motor, would lower the resonate frequency of the existing structure to within the operating range of the motor.

The objective of the engineering undertaken was to increase the stiffness of the platform to raise the resonate frequency out of the operating range of the motor.

The parallelism of the motor mounting surface to the frame supports is important. These surfaces were machined parallel after stress relief on the original weldment. The addition of stiffeners with minimal welding distortion and residual stress was a design objective. Of benefit is the fact that the original frame was designed for stiffness and the service stresses are very low.

Presented are the resulting design, analysis for weld requirements, distortion control procedure for welding, and a weld qualification test procedure where preheat was to be avoided.

The final configuration achieved the stiffness objective with minimal welding and distortion less than 0.0005 inch.

Finite element analysis (FEA) was used to evaluate the stiffness improvements for a variety of configurations evaluated. FEA was also used to evaluate stresses in the existing structure, stresses in the modifications and, in conjunction with FEWeld, determine the weld throat requirements for the modifications.

Shown above are the finite element analysis stress results for the final configuration. The top and bottom of the platform are shown.

The stress plots shown are informative. They show that the stress in the base material is well below the design allowables. They also indicate that welds can likely be specified to meet the design allowables for weld strength. The do not, however, indicate what the weld requirements are. Because minimizing distortion on this modification is critical, overdesign of the welds is to be avoided. For a redundant structure such as this, reasonably accurate determination of weld loads using classical analysis would be time consuming. Additionally, the finite element analysis was performed to determine the stiffness effects of modification variations and the results were already available. FEWeld, a tool developed by Weaver Engineering, takes that last step, quickly calculating weld throat requirements for all specified weld joints and all load cases of a solved FEA shell element model.