Stamping Press Failure Analysis
Envista was retained to investigate the cause of failure and determine whether any outside party was responsible for damages after a 3,000-ton stamping press used in the production of automobile parts failed, resulting in prolonged downtime and extensive repair costs.
Prior to the loss, the subject stamping press had been taken offline for a partial rebuild. This rebuild had been performed by a local company under direct supervision of technicians from the manufacturer. The technicians were responsible for providing drawings and specifications to be used during the rebuild.
As part of the rebuild process, the hydraulic overload protection system, the system meant to protect the press from accidental overload, was disconnected, rebuilt, and reinstalled. When the master relief valves were reinstalled, the incorrect pressure setting was used based on incorrect manufacturer drawings. Instead of the master relief valve releasing the correct load on the press, at about 3,000-tons, the valve allowed the press to generate over 4,000-tons of force based on this setting. Although, the incorrect relief valve setting alone would not result in the press being overloaded.
The hydraulic system relief is also controlled by a human-machine interface (HMI), which sets the air pressure of the system – the primary means of pressure relief in an overload scenario. While this air pressure was never meant to exceed 60 pounds per square inch (PSI) based on the manufacturer’s design, the repair technicians had bypassed this setting and raised the air pressure to 80 PSI in order to generate a higher tonnage during calibrations. This is another indication that the technicians working on the rebuild process did not fully understand the press protection system. Had the pressure relief valves been set to the appropriate specification, they would have discharged before ever allowing the air pressure to reach 80 PSI. Under these two conditions, the overload protection system for the press had been essentially eliminated.
On the date of the loss, final calibrations were being performed by the technicians on the tonnage monitoring system. Given that the tonnage monitoring system displays the tonnage of each press stroke, the only means for the technicians to read the tonnage of each test stroke using a measurement instrument placed beneath the ram. Typically this is performed with load cells, however, due to a lack of availability, the technicians had instead used a different tool. While attempting to calibrate the tonnage using this unfamiliar method, the technicians were heavily reliant on the overload protection system to ensure press damage did not occur. Assuming that the overload protection system was in place, the technicians intermediately increased the press tonnage until eventually striking the press in excess of 4,000-tons, well past the designed capacity. When the stroke occurred, linkage in the crown of the press overloaded and fractured.
Envista was able to determine that the failure and damage of the press were the result of these three critical errors made during the repair process that resulted in over $1 MM worth of immediate repair costs as well as several weeks of additional downtime.
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