Browse Publications Technical Papers 2009-26-0082
2009-01-21

Analysis of Breaking and Loosening of Bolts Due to Change in Rust Preventive Coating through Online Breaking Tests (OBT) and Endurance Loosening Tests (ELT) 2009-26-0082

The growing concern of elimination of harmful substances, from automobiles, which affects human health and environment adversely, has mandated certain regulations like ELV, RoHS, and REACH world wide. These regulations limit the usage of substances and chemicals which are hazardous in nature. Hexavalent chromium, a recognized carcinogen [1], which is used over zinc plated bolts for rust prevention, is also banned under ELV directive (2000/53/EC).
One of the alternatives to hexavalent chromium is trivalent chromium. Both these passivations have different chemical properties hence have a varying effect on the coefficient of friction of the bolts. The breaking/ loosening of bolts has co-relation with the coefficient of friction of the bolt.
While switching over from hexavalent to trivalent type of passivation, theoretical checking for torque values was done and the torque range was found within current operating limits. But while implementing, many problems of bolt breaking/loosening were reported from vehicle assembly line and during test evaluations. To counter these issues, Online Breaking Tests (OBT) and Endurance Loosening Tests (ELT) were performed. Investigations were carried out and iterative trials of OBT and ELT were done along with the suppliers to study, analyze and find solutions to these problems. A compilation of the OBT and ELT analyses and the solutions which were found are discussed in this paper.

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