SIT-University of Edinburgh joint project develops quantitative assessment method for loosened bolts

New technique detects loosening at bolt tip based on correlation between frequency and axial force

A research group from Shibaura Institute of Technology has developed a method to quantitatively and cheaply assess the decrease in joint stiffness of bolt/nut assemblies.

The technique involves measuring frequency modes in the ultrasonic frequency range – sound inaudible to people – of the bolt’s threaded portion that protrudes from the nut to identify a correlation between the vibration modes and axial force, or fastening force.

This simple method is not dependent on the tester’s technique or skill, thus allowing tests to be conducted cheaply. It can be introduced in machines that check loosened bolts.

Professor Naoki Hosoya of SIT’s Department of Engineering Science Mechanics, and Dr. Francesco Giorgio-Serchi of University of Edinburgh’s School of Engineering headed the research team.

◆Background

Bolt/nut assemblies are commonly used in machines and structures ranging from airplanes and trains to roller coasters and chairs. As these machines and structures move, the axial force of the bolt/nut assemblies declines, resulting in a phenomenon known as “loosening.” In fact, loosened bolts have caused airplane, train and roller coaster accidents. However, there is currently no flawless method to detect loosened bolts.

During the conventional hammering test, an inspector listens to and evaluates changes in the hammering sound. However, the accuracy of this test depends on the skill of the inspector. The new technique developed in this joint study is capable of detecting even slight loosening that conventional methods cannot identify, through the use of sound in the ultrasonic frequency range. This new method can be used in general-purpose vibration testing systems, so it will be cheap to introduce. Using the method in inspections and for other purposes could prevent accident-causing bolt loosening.


◆Characteristics of the new and conventional assessment methods　
The latest method can detect changes in the natural frequency of the bolt tip with regards to axial force.

New method	Conventional methods
	Torque test	Hammering test
Does not depend on an inspector’s technique and skill, and in the future allows quantitative evaluation　by contactless and Automated measurements.	Difficult to measure axial force because of the changing frictional property on the seating surface	Difficult to quantitatively evaluate degree of loosening because accuracy depends on an inspector’s skills.

◆Research results
The team hammered bolt/nut assemblies to make them vibrate. When an assembly is tightly fastened, the bending mode natural frequency at the bolt tip is high (vibrating finely and quickly). When it loosens, this frequency declines (vibrating slowly and largely). The results showed a clear relationship between the natural frequency at the bolt tip and the axial force, and that the frequency declines as the axial force declines.

The research team and a private company are conducting joint research with the aim of establishing a bolt/nut assembly inspection system using this method. It is expected this method will have practical benefits in developing countries because of the low cost required.

◆Article information

Naoki Hosoya, Takanori Niikura (Master’s Program in Mechanical Engineering), Shinji Hashimura, Itsuro Kajiwara, Francesco Giorgio-Serchi, Axial force assessment of the bolt/nut assemblies based on the bending mode shape frequency of the protruding thread part using ultrasonic modal analysis, Measurement 162(2020)107914.
https://doi.org/10.1016/j.measurement.2020.107914