Assembly Design and Analysis
Continuing on with part 4 of our blog series dedicated to showing the differences between Autodesk Inventor Professional 2011 and SolidWorks Premium 2011 for digital prototyping workflows, we examine the ability to select and analyze the correct clevis pin for the assembly shown below.
This test focuses on the design of a clevis shear pin and its related holes in this hydraulic clamping assembly. The pin needs to be optimized so that if a failure occurs, the pin fails and not the other components. We’ll seek a factor of safety of 2 for the clevis pin, and 4 for the rest of the assembly. The Clevis Pin shear pin must withstand 250 N force with a factor of safety of 2. The bending force on the pin is important. If it exceeds maximum allowance the pin cannot be removed. The pin should be sized to meet the forces at the designated safety factor and fit within the support structure. The pin should further be selected from a standard library of components and created with the size required. The design calculations should be stored for documentation.
Autodesk provided a Parasolid model of the above assembly and a video of Inventor performing the design and analysis to select the proper pin to fit within the clevis opening.
The key differences you will see demonstrated below
Both Autodesk Inventor and SolidWorks can solve this problem; however there are important differences in the steps required to complete the exercise and the confidence in the engineering optimization.
Inventor executed all of the steps within one command sequence. SolidWorks uses separate commands for each of the three key steps. Both Inventor and SolidWorks seem to have an equally robust clevis pin library, and automatic sizing capability.
The engineering calculation is the differentiator in the example. For this problem Inventor designed a solution using its engineering calculations library for clevis pins. The only solution offered by SolidWorks was to use FEA, which in this case, proved to be difficult to verify and is a questionable strategy.
Inventor users have the ability to leverage standard engineering calculations shown below which are included in design accelerator tools. In this test we examine the clevis pin generator. The same concept applies to bolts, bolts, frames, shafts, gears, bearings, belts, chains, keyways, cams, splines, o-rings, and springs. Inventor does not require the user to know or learn the engineering equations; the software does it for you. In the case of using FEA, the engineer must be concerned with the accuracy (error) inherent to FEA methods, necessitating a higher skill set, and certainly more time. Inventor’s automation of standard engineering calculations provided a better solution and reduced design time.
What’s Important in Assembly Design and Analysis
- Select the most appropriate purchasable clevis pin that meets the specifications
- Weigh design decisions that affect cost, product reliability, and weight
- Evaluate the performance of the final design
Inventor executes all of the steps (make the hole, insert the pin, perform the engineering calculation) required within one command (feature). SolidWorks uses separate commands for each of the three key steps. Both Inventor and SolidWorks seem to have an equally robust clevis pin library, and automatic sizing capability.
The engineering calculation is the large differentiator in the example. Both Inventor and SolidWorks offer integrated finite-element analysis (FEA). However, using FEA methods for this kind of problem is a questionable strategy. Autodesk prompted the user to determine the correct pin size by using its engineering calculations. SolidWorks does not provide this kind of functionality. SolidWorks’ concept was to have the designer select the pin size and then perform an FEA analysis in an iterative fashion to arrive at the correct sizing. SolidWorks was able to use its own no-charge Simulation Xpress to perform the FEA analysis. SolidWorks Simulation Xpress is a first-pass basic stress analysis tool that comes with every SolidWorks Standard and Professional software package, offering limited FEA functionality.
Standard engineering calculations (analytical methods) exist for this situation, making FEA methods unnecessary/overkill. The FEA boundary conditions necessary for this case present a convergence issue (singularity) for most solvers. This increases the expertise required to verify the accuracy of this FEA study with any level of certainty. Some FEA programs will simply never reach a reliable result for this case. This is typically referred to as a divergent case. SolidWorks Simulation Xpress did not allow us to individually manipulate the mesh to test for convergence.
See how Inventor solved the problem in this video.
See how our engineer solved the problem with SolidWorks.
Here are the calculations Inventor uses to solve the clevis pin selection.
The next blog in this series will examine exporting a mechanical design to a BIM system. Stay tuned or sign up to be notified of my blog updates.
Ray, would you recommend using FEA to analyze the factor of safety for the other components in the assmebly?
Perhaps you could compare the FEA offerings from Autodesk and SolidWorks.
Good question John. I’d be a little surprised to see FEA methods applied to such a simple assembly, but that doesn’t mean a design engineer wouldn’t do any performance calculations at all. As Ray’s example shows, the design calculators included with Inventor were able to help properly size the clevis pin. Ray also says the same concept applies to bolts, bolts, frames, shafts, gears, bearings, belts, chains, keyways, cams, splines, o-rings, and springs in Inventor. So, the factor of safety for all of the other pins and fasteners in this assembly can be analyzed using the Inventor design calculators. The piston itself would be very interesting to analyze with FEA, but probably not necessary since that’s a purchased component and you can get the specs from the supplier. That leaves us with the grey mounting plate and blue clamp jaw shown in the picture. If the grey mounting plate is welded, Inventor also has calculators for weld joints…so no FEA needed there. If you were concerned with stress concentrations in the blue clamping jaw, that would be a good use of FEA in this simple assembly.
Don’t get me wrong. I think FEA methods are very important to mechanical engineering. However, so are free body diagrams, hand calculations, and good old experience.
Given the grey mounting plate does not appear to be welded, FEA would be my tool of choice to analyze it. Design calculators are of limited value to me.
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