Tutorial on the Basics of Bolted Joints

Joint Diagrams with External Forces Applied

Joint Diagram 3

When an external tensile force is applied to the joint it has the effect of reducing some of the clamp force caused by the bolt's preload and applying an additional force to the bolt itself. This is illustrated in the joint diagram shown above. The external force acts through the joint material and then subsequently into the bolt. At first sight it may seem a bit strange to place the applied force in the position shown in the diagram. However, it should be realised that the load on the bolt cannot be added without decreasing the clamp force acting on the joint. As can be observed from a study of the diagram, the actual amount of increase in the bolt force is dependent upon the relative stiffness of the bolt to the joint.

Joint Diagram 4

As an illustration of the importance of the relative stiffness of the bolt to the joint, presented above is a joint diagram for a 'hard' joint (a low stiffness bolt with a high stiffness joint). In this case, because of the steep stiffness slope of the joint, the bolt will only sustain a small proportion of the applied force.

Bolt with reduced shank
With a 'soft' joint (a high stiffness bolt with a low stiffness joint), because the stiffness slope of the bolt is greater than that of the joint, the bolt would sustain the majority of the applied force. Study of these diagrams provides understanding of why high performance bolts have shanks that have been reduced to a diameter below that of the outside diameter of a thread. By reducing the shank diameter in this manner the stiffness of the fastener is reduced so that it will not sustain as much of any applied force that it would otherwise do. If the shank diameter is not reduced to a diameter below that of the stress diameter (see stress area in the glossary) then the strength of the fastener will not normally be impaired. Go to the Previous Page
Go to the Next Page

Click Here for Further details about Bolt Science Training Courses