Study of the Causes of Boom Elements Cracking of Electric Demolition Machine with Use of Experimental and Numerical Methods

In this study authors present a method which was used to investigate the causes of boom elements cracking on Brokk90 demolition machine. This type of machine is used by ARCAD demolition company from Wroclaw, Poland. Their specialty are demolition jobs, which require use of wide variety of equipment. In particular they offer demolition with use of small remotely controlled robots. In this case operators of the Brokk90 machine discovered crack growth which appeared on the sections of the arm system, to which a variety of tools can be mounted such as hydraulic breakers, hydraulic cutters, diamond chain saws etc. Crack growth occurred much earlier than time before overhaul designed by the manufacturer. The investigation of the of the arm system was carried out by the authors of this paper. In order to minimize the time during which the machine will not be operational, it has been decided to use noninvasive methods of measuring and recording the movement of the arm system during operation. Techniques used for this study consists of using Phantom high speed camera and Commtest vb7 vibrometer. Clips recorded via the high speed imagining system were processed using state of the art processing software which determined excitation acting on the arm system while in operation. Use of vibrometer allowed to investigate natural frequencies of the assembly, and later on validate built FEA model. Based on recorded data boundary conditions for the simulations were determined and used in the simulation. During the study a fully functional finite element analysis model of the arm system was developed. Study of the model allowed researchers to thoroughly investigate stresses occurring in the elements of the arm sections. Based on the similarity between places of stress concentration on the model and places of crack formation on the arm system, high level of correlation between real life machine and simulation model was established.