Analysing bolt designs on PC
Original article date: April 1999
The critical weakness in many products is the joints. Bolted joints in particular can be a source of concern for the engineer. In spite of the term ‘nuts and bolts’ being used to describe the simplest aspects of a particular topic, bolted joints are far from simple. They are not easily analysed and frequently engineers have to make major assumptions to assess whether the joint is capable of sustaining the forces applied to it. Failure of a single bolted joint can have disastrous consequences for the reliability and safety of the product.
A great deal of sophistication can be applied to the analysis of structures. Computer-based techniques, such as finite element analysis can determine the stresses in almost any part of a structure. However, developing an accurate model of a bolted joint can take a lot of time and effort. Difficulties include the bolt’s preload compressing the clamped parts together and preload scatter due to the method of tightening being used.
The importance of a bolt’s clamp force in ensuring that a reliable joint is achieved is often underestimated. The clamp force prevents gaps from occurring at the joint faces. It provides the loading that generates the frictional force between the joint faces to prevent slippage and the load to ensure that any gasket within the joint seals effectively. Most engineers recognise that a bolt’s clamp force is subject to some variability. Research completed in Europe has shown that this variability is largely due to the tightening method used. For example, a greater variation in a bolt’s clamp force is to be anticipated when using an impact wrench than when using a torque wrench to tighten the joint.
Now, an updated version of the computer program Boltcalc has been launched by Bolt Science (formerly Westcom) to aid engineers to design more reliable bolted joints. It includes these effects in the program, so a joint can be analysed in minutes.Downloadable demo version
The Boltcalc program completes an analysis of a bolted joint by considering the forces acting upon it. It will determine whether or not a bolted joint will be able to sustain the forces acting upon it. The program will allow for variations in the bolt clamp force (or preload) due to the tightening method and embedding due to plastic deformation in the thread and under the nut. Based upon published research information, the program will determine the fatigue endurance limit of the thread (rolled or machine cut), the amount of embedding anticipated in the joint and the tightening torque for given friction conditions.
The program determines the bolt size needed in a joint and the factors of safety, relative to what clamp force is required, for the bolt being overloaded, for bolt fatigue and also for excessive bearing pressure. The empirical data used by the program, such as embedding losses and bolt fatigue endurance strength, may not be readily available elsewhere. The program has an in-built database of standard thread data for metric coarse, metric fine, UNF and UNC threaded fasteners. This database makes it possible to enter thread and joint information without having to look up standard information.
By defining to the program the details of a joint, the program will estimate the bolt size required and then perform a detailed analysis to determine whether or not the bolt or joint will fail by any of a number of modes. The program will allow for:
- Clamp force loss from embedding due to plastic deformation in the threads and under the bolt thread
- Any residual clamp force that may be required for gaskets
- Fatigue of the bolt
- Insufficient clamp force to resist the applied forces
- The bolt being overloaded due to the application of applied forces
If the joint is being designed, it can estimate the size of bolt required for the application. Once the bolt size has been estimated, or details of an existing joint have been entered, detailed calculations can be performed. The calculations will determine whether or not the bolt will fail by the joint not being clamped together sufficiently, direct overload of the bolt or by fatigue failure.
Boltcalc is designed for use under the latest versions of the Microsoft Windows operating system (W95, W98 and NT). A number of new features have been added, including the capability to use Imperial units.
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Bolted joints – Four failure modes Failure to provide sufficient clamp force The main factor as to whether a bolted joint will sustain the applied forces is whether the bolts sustain sufficient clamp force. For any application, there is a minimum clamping force required to prevent joint failure. The clamping force is required to prevent joint movement being caused as a result of axial and/or shear forces. Both axial and shear forces, acting individually or simultaneously, can be taken into account, together with any residual clamp force that may be required to maintain a functional requirement such as gasket sealing. In many instances, when a gap in the joint or slippage occurs, then failure will occur by the bolt loosening or fatigue. When fatigue failure occurs, the cause is frequently insufficient preload rather than poor fatigue strength. Bolt overloaded by the applied force If a very high axial force is applied to a joint, there is the possibility that the bolt will sustain additional loading that will cause its yield strength to be exceeded. If this does occur, then either the bolt will fail due to direct tensile failure, or, when the load is removed, will sustain a plastic deformation that will result in preload loss that could cause the bolt to loosen. The program checks for this possible failure mode and determines a safety factor. For the majority of joints, failure by direct overloading is unlikely because the bolt usually sustains only a small proportion of any force applied to the joint. Fatigue failure of the bolt All materials have a tendency to fail under repeated loading at a stress level considerably less than the static strength of the material. This characteristic of materials is known as fatigue and it is a common cause of failure in many products, including bolts. The program will calculate the alternating stress in the thread of the bolt, based upon the forces entered and the characteristics of the joint. The program will calculate the fatigue strength of the bolt material, based upon the user’s selection as to whether the bolt thread was rolled or machine cut. The program uses lower bound, empirically derived values for the fatigue endurance strength. If the user has specific information on the fatigue endurance strength of the bolt being analysed, then this can be used in preference to the program value. Excessive bearing pressure If the bearing pressure under the nut force exceeds the compressive yield strength of the joint material, plastic deformation will occur. The preload loss from embedding determined by the program assumes that the compressive yield strength is not exceeded. If it is, then such preload loss can increase uncontrollably. The program checks that the bearing pressure is within acceptable limits and if it is not, recommends a number of design alternatives. |
- Bol t Science
- 0961 300952
- Bill Eccles
April 1999