Use of staircase method to validate a new design of engine pulley

A Cummins internal project demanded the development of a new crankshaft pulley. This new pulley will be mounted in the engine, but before this mounting process, a validation phase must be accomplished with the primary goal of avoiding failures during engine tests and, most of all, during vehicle operation. This article documents part of the work done to validate the new pulley, aiming to deliver a component that meets the specification demanded by the application to be released in the market. It is presented how acceptable unreliability of the pulley was assessed by means of fatigue testing. Several approaches could be used, but following internal standards for this kind of component, the procedure chosen was the staircase fatigue method. After finishing the testing phase and data treatment, the findings indicated that the new crankshaft pulley presented infinite life when subjected to the hub load found in the application with a design margin of 82%. Another validation that was carried out is related to the bolts used in the mounting. The bolted joint measurement and its audit are discussed, and the reflection about considering or not this parameter as an additional load to validate the pulley is done. The influence of the bolt load in the pulley lifetime is also explored. It was observed that the bolted joint kept its integrity throughout the test when initially tightened with an average preload of 60.4kN and it was confirmed that this force did not have a direct influence on the crankshaft pulley life, reinforcing that pulley fatigue has been tested under the influence of the applied load only. As next step, clamp load auditing is strongly advised to crosscheck if the tightening recipe of defined torque plus angle can provide adequate clamp load.