Agitation system accelerated life testing

Abstract

Accelerated Life Testing (ALT) is an effective method of demonstrating and improving product reliability in applications where the product is expected to perform for a long period of time. ALT accelerates a given failure mode by testing at higher stress level(s) in excess of operational limits. The Agitation System is intended to mix fluids placed in the Agitation chassis to avoid separation of the fluid chemical contents. This system is one of the key sub-systems to prepare a homogeneous fluid which will adhere to the fluid quality requirements. The Agitation System reliability requirements were defined as 90% at 9000 hours. The accelerated life tests were performed using two stresses viz ambient temperature and RPM at three levels. Two reliability related failure modes were observed — Chassis Hinge Bearing failures and Connecting Rod End Bearing failures. A root cause analysis (RCA) of these failure modes was carried out. Chassis hinge bearing failure was driven by RPM and ambient temperature whereas Connecting Rod End Bearing failure was attributed to RPM. Life Data analysis was performed using ALT failure data. The individual failure modes were analyzed separately using appropriate Life-Stress and Weibull distributions. The individual failure rates were rolled up using the reliability block diagram approach to quantify the reliability demonstrated at the system level. The demonstrated system reliability was 88% at 9000 hrs. Demonstrated reliability being near to the target, the design was accepted during reliability qualification. The design failures reported during the testing have been analyzed through the RCA methods and the following design improvements are recommended to further improve the reliability. Hinge bearing failure: Optimize design to withstand higher forces exerted on the hinge by adding more supporting ribs to increase strength and also select higher grade material. Connecting Rod End Bearing failure: Redesign the crank mechanism by taking care of tolerance stack-up and putting a quality plan in place to ensure the alignment. Connector leakage issue (Packet Male connector): O-ring quality control plan to be put in place and precise quality monitored. Tube end connector leakage issue: Connector quality control plan to be in place and precise quality monitored.