Friction Coefficient Influence on the Fastening Torques of a Bolted Joint

Vinicius Vinhaes¹; Dr. William Manjud Maluf Filho¹

¹ Centro Universitario da FEI

doi:10.20906/CPS/COB-2015-1285

Resumo

One of the most common way a component or a system of components fail is due to the wrong clamp force applied by a nominal torque that results on a bolted joint getting loose or breaking during operation. Still, one of the several methods available to join and connect parts within a component is by using threaded elements. The objective of a joining bolt is to join different surfaces and avoid them to separate. While united, loads like torque and forces appear in the joined parts. The significant moments that are experimented during the joining process are named as pretension torque, grip torque and spin torque. The distribution and magnitude of these moments are affected by several factors. To understand this variance of clamp forces achieved by the same applied torque, mathematical models based on literature were applied first to understand and model the friction coefficient influence on common torques applied on threaded bolts. To verify this approach an experiment using a device capable of measuring the actual friction coefficient during the torque application was used to compare the results. During the machine cycle a motor was responsible for fastening the bolted joint while a load cell measured the clamp force until it reached the desired value and the real friction coefficient was one of the results available. The experiment was repeated several times to validate the method and the results gathered was within the expectation showing that the torque required to achieve the same clamp force can vary a lot due to the friction coefficient variance depending on material, surface finish, presence of dirt and debris in between the threads. Therefore it was verified that using only torque based on moment application for bolted joints can be inaccurate on achieving the desired clamp force and the recommended method to avoid this is by fastening with a torque angle.

Palavras-chave: Fastenning torques; Friction coefficient ; Variance