Design systems and select components for bolted joints and thread fasteners

This is one of the SEED curriculum engineering design guides. SEED (Sharing Experience in Engineering Design) is an organisation of teachers of design in tertiary education.

The aim of this guide is to provide procedures to enable the user to design a bolted connection where the dominant load acts parallel to the axes of the bolts making the connection (and therefore perpendicular to the connecting faces of the parts which constitute the joint). The guide provides sufficient additional data to enable the threaded elements to be selected and specified. Typical joint configurations where the threaded fasteners are in tension/bending, simple tension or shear are shown in the following figures:

Figure 1. Types of Bolted Connection covered by this Guide

 

 

Two approaches are included. The first covers “statically loaded joints” and the second “dynamically loaded joints”. The first approach is essentially a simplified version of the second and can be adopted when the variation of the axial load at the joint in service can be regarded as negligible and the number of times that the axial load is applied to the joint throughout its working life is less than 105 applications. The guide assumes that the component parts being connected are sufficiently rigid so as to not subject the threaded fasteners to bending stresses when the joint is being tightened or when it is under load in service. The guide does not consider the design of threaded connections where the threaded fastener is subject to lateral shear – clevis joints for example. In these cases, the threaded fastener is subject to high bearing stresses on its diameter and, invariably, bending stresses accompany the lateral shear stress. These situations are considered to be outside the scope of this guide.

Section 2 looks at the limits of the guide in greater depth. The guide advises in general terms on aspects of good practice relating to the positioning of fasteners in a joint. The number and position of fasteners to be used in a given situation tends to be influenced by other features of the design work being undertaken and there is a wide variety of possibilities which are difficult to categorise in a brief guide such as this. The symbols and nomenclature used throughout the guide are defined as they occur. No derivations of equations are presented; such derivations are available for those who require them in the Bibliography at the end of the guide.

Section 3 covers the selection of bolted connections which are statically loaded. This enables the user to design safe connections for static loading using a flow chart approach. Section 4 gives the necessary background and equations for determining the load distribution and corresponding stress levels imposed on the component parts of a joint subjected to fatigue loading. It also includes a table of screw thread strength reduction factors, which should be used in conjunction with the other strength reduction factors used in designing against fatigue. A second flow chart is included, giving the reader a step-by-step procedure for the safe design of bolted joints capable of sustaining fatigue loading. Section 5 examines and discusses the relationship between applied torque and induced load and the distribution of stress in normal threaded fasteners. Section 6 deals with ISO grading of threaded connector materials. Section 7 looks at the common thread system as well as touching on past systems. Section 8 shows how to select fasteners with the best characteristics for many applications.

[Description and screenshot taken from the SEED Curriculum for Engineering Design page for this guide. (c) The Design Society used under the terms of their (CC BY-NC-SA 3.0) license.]

Link: http://www.bath.ac.uk/idmrc/themes/projects/delores/co-design-website/dpg/bol/bolhome.html
Author: S. McGuigan; V. Leedham; T. Watkin
License: http://creativecommons.org/licenses/by-nc-sa/3.0/
Rights: Copyright 2011 The Design Society - This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. See http://www.bath.ac.uk/idmrc/themes/projects/delores/co-design-website/teachers/curriculum/about.html