I often explain that a Finite Element Analysis (or FEA) is a great way to learn the behaviour of a design long before prototypes are made. Back of the envelope calculations are also a great tool to use but when there is complex geometry involved you can’t beat an FEA. The complexity of an analysis can be increased if required to increase confidence in a design too but sometimes that isn’t necessary and a lot can be learned from understanding the behaviour of a simplified system.
Recently we designed another variant of seat bolster for the Jolt Rider marine seat (as shown in Figure 1). As the support under the bolster was changed the question was posed: will this be strong enough? This was easily answered using an FEA and I can explain further and share the process with you (with permission from Jolt Rider).
Figure 1: JoltRider Marine Seat
Figure 2: New variant of bolster plate (shown on top) prior to analysis
I started with a simple analysis of the system. It was a cantilever system where the plate was mounted rigidly and the load distributed onto the new plate which acted as a cantilever. This setup is shown in Figure 3. The von Mises stress and displacement plots are shown below in Figures 4 and 5 respectively. You can see that the supporting plate underneath is concentrating stress on the new bolster plate and this is at or just above yield stress of the material. This shows that the plate would permanently deform under extreme loading. The inside corners of the rectangular hole in the middle and the two front counterbored holes are red where stress has concentrated. Based on the displacement plot the front edge is expected to move 5.8mm. Keeping in mind that this is a worst case example with extreme loading (and lack of other supporting components) it is clear that some design improvements are in order.
Figure 3: Setup for original seat bolster part.
Figure 4: Stress plot for the applied load on the seat bolster plate . Please note the stress data has been removed from the legend for confidentiality.
Figure 5: Displacement plot for the seat bolster plate.
The Solution
The design was easily modified to remove the central rectangular hole and the two forward counterbored holes. These were added initially for convenience and to save weight in the product. The updated component is shown in Figure 6 under load. You can see that the stress on top of the plate is much reduced thanks to removal of the stress raisers and from the extra support provided from the material where the holes were. A factor of safety plot for the updated design is shown in Figure 7 and the design is looking much better. A displacement plot in Figure 8 where you can see that the displacement of the updated design has reduced to 3.5mm from 5.8mm in the initial design.
This example shows how powerful even a simple analysis can be when designing complex geometry or for complicated loading cases. It gave us the confidence to proceed with the design without needing to add time and prototyping expenses to the project.
Figure 6: Stress plot for the same applied load on the updated design. Please note that the red used in this legend is well below yield.
Figure 7: Factor of safety plot for the same applied load on the updated design.
