Session: CT-01-02 Design and Analysis of Bolted Flange Joints-2

Paper Number: 152289

152289 - Investigation of Friction Coefficient Change on Bolted Joints Under Transverse Vibration Conditions 

Abstract: 

Bolted joints are loosened spontaneously when the slippage has occurred between clamped parts by applying continuous transverse vibration. To solve this loosening issue in industrial applications, it is common to use anti loosening nuts to prevent spontaneous loosening by generating additional friction force between thread engagements as prevailing torque. However, sometimes loosening issues happen under certain vibration conditions even though these anti loosening nuts were installed.

It is the theory to design bolt joints to prevent spontaneous loosening that the displacement of slippage between clamped parts is designed less than certain critical limit named the critical relative slippage (Scr) that is not slipped bearing surfaces of bolt heads and nuts from surface of clamped parts. However, sometimes there is a difference between theoretical values and actual results in applications. It is assumed as one of the reasons that friction coefficient of bolt joints has changed during certain vibration conditions, but this spontaneous loosening mechanism of bolt joints under transverse vibration conditions is not obvious completely.

The purpose of this investigation is to verify the mechanism of spontaneous loosening of bolt joints under continuous transverse vibration conditions by evaluating friction coefficient changes on tightening and untightening processes. The first test method is by measuring bolt preloads with applying certain constant tightening torque under various transverse vibration conditions using Junker vibration machine to calculate each friction coefficient on tightening process. As a result of the first test, calculated friction coefficient of bolt joints by applying tightening torque have decreased significantly and measuring achieved preload have increased along with the increase of the acceleration of clamped parts slippage, and this tendency was applicable for another size of bolt joints. The conclusion of this test on tightening process was that the friction coefficient decrease during transverse vibration conditions has calculated by the transverse acceleration between clamped parts. The second test method is by measuring spontaneous loosening torque under various vibration conditions to calculate friction coefficients during spontaneous loosening process and to verify how friction coefficients change affects spontaneous loosening on bolted joints. In the result of the second test, values of measured spontaneous loosening torque were significantly smaller than values of untightening torque under non-vibration conditions. And friction ring lock nuts which have typical anti-loosening function also behaved same tendency in certain transverse vibration conditions.

As conclusion of this investigation, it could say the friction coefficient decrease during transverse vibration conditions is assumed to be one of the key factors of spontaneous loosening of bolted joints. And to prevent the spontaneous loosening on bolted joints, it is important to consider in bolted joints design that the friction coefficient of bolt joints will decrease along with the increase of acceleration between clamped parts slippages.

Presenting Author: Masato Takenaka Nord Lock Japan Co., Ltd.

Presenting Author Biography: Masato Takenaka is APAC Engineering manager in Nord Lock Group. His work is to design bolts and nuts for solving failure issues on various application, especially Power Gen and Chemical industries.
He is member of The Japan Research Institute for Screw Threads and Fastener (JFRI) and got award in 2022.
In his spare time, he likes to do jogging and challenge marathon together with his wife.

Authors:

Masato Takenaka Nord Lock Japan Co., Ltd.
Koji Sato Nord Lock Japan Co., Ltd.
Fabbro Lee Nord Lock Korea Co., Ltd
Wan Yang Nord Lock Japan Co., Ltd