Thread Fastener Design and Analysis

Related Resources: Design and Engineering General

Thread Fastener Design and Analysis

Fastener and Screw / Bolt Design Formula Design Data
Strength of Materials

Thread Fastener Design and Analysis - Engineering Fundamentals

Introduction: Engineering Fundamentals of the Tightening Process The process of tightening threaded fastener assemblies, especially for critical bolted joints, involves controlling both input torque and angle of turn to achieve the desired result of proper preload of the bolted assembly. Understanding the role of friction in both the underhead and threaded contact zones is the key to defining the relationship between torque, angle, and tension.

There can be as many as 200 or more factors that affect the tension created in a bolt when tightening torque is applied (refer to paragraph 2.2). Fortunately, torque-angle signature curves can be obtained for most bolted joints.

By combining the torque-angle curves with a few simple calculations and a basic understanding of the engineering mechanics of threaded fasteners, you can obtain the practical information needed to evaluate the characteristics of individual fastener tightening processes. The torque-angle curves can also provide the necessary information to properly qualify the capability of tightening tools to properly tighten a given fastener.

Free Membership Minimum Required
Open: Thread Fastener Design and Analysis - Engineering Fundamentals

TOC

Table of Contents
1.0 Introduction: Engineering Fundamentals of the Tightening Process … 3
1.1 Energy Transfer.. 3
1.2 Modeling the Tightening Process.. 3
1.3 Where Does the Torque Go? 4
1.4 Elastic Torque-Tension Relationship 5
1.5 Stress/Strain vs. Torque/Tension …..5
1.6 Correlation of Stress-Strain and Turn …….. 6
1.7 Force-Deformation and Torque-Angle Diagrams …7
1.8 Preload-Preload Efficiency Factor ….7
1.9 Torque-Tension Correlation Coefficient ……8
1.10 Thread/Underhead Friction Measurements 8
1.11 Automated Tightening Process …… 9
2.0 Torque-Angle-Tension Control …… 9
2.1 Introduction …. 9
2.2 Torque-Angle-Signature ……10
2.3 Torque-Angle Signature Analysis …11
2.3.1 Tightening Curve … 12
2.3.2 Elastic Origin . 12
2.3.3 Loosening Curve ……12
2.3.4 Beyond Yield 13
2.3.5 Loosening Tendencies …… 13
2.3.6 Accurate Measurement …… 13
2.3.7 Torque-Angle Signature Analysis Summary …….14
2.4 Clamp Force/Strength Considerations ….. 14
2.5 Modeling the Tightening Process …14
2.6 Torque-Tension Coefficient: Nut Factor, K …….. 16
2.7 Experimental Determination of Friction Coefficients ….. 17
2.8 Thread/Underhead Friction Coefficient Measurements 17
2.9 M-Alpha Diagram …. 18
2.10 FM-Alpha Diagram .. 19
2.11 Estimating the Tension-Angle Coefficient 20
2.11.1 Ultrasonic Stretch .. 20
2.11.2 Strain-Gaged Bolt . 20
2.11.3 Force Washer …….. 21
2.11.4 Model-Calculation: Estimate of Angle-Tension Coefficient … 21
2.11.5 Laboratory Measurement of Friction Coefficients …….. 22
2.11.6 Material Property-Yield ….. 23
2.12 Torque-Angle Tension Control Summary .. 23
3.0 Torque-Tension Audits …… 25
3.1 Introduction .. 25
3.2 Hand Torque Audit-Tool Torque Capability …….. 26
3.3 Tool Torque Capability/ISO 5393 .. 26
3.4 Release Angle-Tension Audit ……. 26
3.5 Loosening-Embedment or Loss of Preload ……. 29
3.6 Measurements Verify Fastener Torque and Tension … 29
4.0 Other Strategies ….. 33
4.1 Torque-Turn-To-Yield …….. 33
4.2 Prevailing Torque Locknut Signature Analysis … 33
4.3 Hand Torque Tightness Quality Audits …. 33
4.3.1 Redefining The Audit ……. 34
4.3.2 Hand Torque Audit Qualification … 36
5.0 Using Torque Angle Records to Determine Joint Stiffness …. 36
6.0 Material Yield Point 38
7.0 Glossary of Important Terms ….. 38
8.0 References . 39