The study of residual torque is necessary in various fields to ensure the safety and reliability of bolted joints.
The present study aims to determine, experimentally, the decrease in torque applied to a nut used in the assembly of two polymeric components (POM - Polyoxymethylene). These components are part of the fuel supply module, responsible for supplying fuel from the tank to the engine. This reduction in torque initially applied is mapped to the end-of-life of the components and is used as an approval criteria in the audit procedures of the Robert Bosch company. The first component features an overmolded metallic screw, injected into POM. The mating part is also injection molded from POM and is assembled onto the first part, secured by tightening a metal nut. Due to the plastic-to-metal interface, it’s expected that there will be a reduction in the initially applied torque required to fasten the pieces together.
The study was based on 5 steps:
- 1
Theoretical study on residual torque at the plastic-to-metal interface;
- 2
Verification of plastic component deformation due to applied torque;
- 3
Assembly of samples for practical tests;
- 4
Mapping of torque decrease in samples by time. In this case, a digital torque wrench was used;
- 5
Analysis of the data and determination of the residual torque range at the end of the product’s life.
After the study was completed, it was determined a minimum residual torque, for the end of the product’s life, in this application. This criterion enables more accurate control in field part verification, improves product quality, and optimizes failures identification.