The Case of the Awful Automation
Synopsis:
Machinery failure often involves a complex mix of accidental damage, breakdown, wear-and-tear, and design defects. Here, minor breakdown damage is separated from fatal design defects.
Background:
An automated parts handling machine was said to have crashed after a drive belt broke. It was claimed that the crash bent the machine structure and that replacement parts were unavailable due to the manufacturer’s demise. The $80k machine was thus replaced.
Analysis:
TI was requested to determine the cause and extent of damage. The machine moves on linear ball bearings running on precision ways and it was claimed that the tubular aluminum frame members and ways were bent in the crash. The tubular members were separated with ways intact and set up on TI’s Grade AA laboratory surface plate. Precision measurements were taken at uniform spaces all along each way and in both planes to determine their straightness. Then, the ways were separated from their aluminum tubes and the measurements repeated separately for the ways and their tube mounts. Next, the ways were remounted to the tubes and the measurements repeated.
Comparing measurements, it was noted that the ways and tubes had been under stress as originally measured and that the ways were curved more when free than before they were disassembled. When reassembled, the ways were less curved than before disassembly.
Similar measurements were made of the aluminum tubes and it was found that the tube alleged to be damaged in the crash carried a specific profile on the way-mounting side and a uniform curvature on the reverse side.
The inflection points on the mounting side are the result of the structural support of the solid plug installed in the ends of the tube and the rigidity of the linear bearings. Once away from the ends of the tube, the load applied to the tube by the linear bearings ironed the tube and way into a concave profile. The distortion of the aluminum tube was permanent and was the result of inability to carry the load. The curvature of the ways and structural tubes was the result of design error, not the crash that resulted from the broken drive belt. The design engineer violated every design requirement set forth by the manufacturer of the linear bearings and the ways on which they ran. TI also noted that the only parts actually damaged in the crash were the broken drive belt and one linear bearing, and that both could have been replaced for under $200. Further, structurally superior replacement tubes could have been made by any precision machine shop.
