Software speeds joining design

Tom Shelley reports on where CAD helps design joints and where it doesn't.

Software built into CAD greatly speeds and enhances fastening and joining design, but it does not do everything, and should still in some cases be used with caution. When we put the question as to how CAD could be best used to help joining design to Steve Bedder of Autodesk, he launched into an exposition of 'Bolted Connection', a facility which now comes included in the price in all editions of Autodesk Inventor. He made the point that fasteners formed a large and important part of the software's 'Content Center', which now contains details of some 1.6 million parts.

All the mechanical fasteners are based on standards, and because there are so many possibilities, he said many companies configure the software to only offer those fasteners they would normally purchase and use. In its current version, it is not necessary to size fasteners for a particular sized hole – the user only has to take them near a circular hole edge and the software chooses the size. Furthermore, when choosing suitable matching nuts and washers, only compatible components are offered.

However, what is probably more important, is being able to make use of calculation tools to work out how many fasteners and what sizes they need to be to withstand particular loads. This is important because the traditional approach in the past has often been not to go through quite long calculations to choose correctly sized fasteners, but err on the 'safe side' and choose fasteners two or more sizes larger, adding to overall weight and cost. A calculation tab in Inventor includes three facilities: 'Bolt diameter design' 'Number of bolts design' and 'Check calculation.' These include static load and fatigue calculations based on formulae in the built-in 'Engineer's Handbook', which are derived from experience and not theoretical finite element analyses. Autodesk also includes a weldment environment, but here, the results must be treated with more caution. Again the calculations are based on formulae derived from experience, but they all assume completely sound structures.

The example that Steve Bedder chose to use to demonstrate the facility was a load carrying fillet weld, with the edge of a plate welded to the centre of another plate. This configuration is notorious in engineering design because, if weld penetration is incomplete, there remains a crack between the welds on the two sides of the end of the plate. If this happens, many years ago, TWI found that the effective fatigue limit in steel is only 3.3 tons/in2 or 45 MPa and advised that this type of joint be avoided if at all possible, since it is impossible to see from the outside whether a crack is present or not without resorting to radiography.

Bedder commented that, "There will always be a level of knowledge and experience required of engineers", and advised that if there is a possibility of a crack, this should be modelled with full finite element analysis, making use of the Algor and Plasso Tech software that comes with Inventor Professional. In the case of adhesive joints, no quick calculation facility exists, or at least not yet and so there is still a need to resort to finite element analysis and specialist tools, especially when the adhesive is to be used to bond composites, which is currently a major area of modelling research and development.

Design Pointers
• There are now excellent tools for choosing correctly sized and numbers of fasteners to withstand known loads
• There are also excellent tools for analysing weldments but these should be used with caution in situations were less than perfect welding can greatly reduce performance, particularly as regards fatigue
• Corresponding quick tools to size adhesive bonds are still to be developed

Tom Shelley

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