Posted: 2018-02-24

Energy drink car holder

Keywords: FEA, 3d printing, SLS, PLA, 3d design

Modern cars are fitted with all sorts of gizmos that are aimed at increasing safety and, comfort. That includes a cup / 330ml can holder. Unfortunately few cars have holders designed for slim 250ml cans.

Goal

Design and manufacture a can holder; requirements:

  • matches a 250ml energy drink can
  • ergonomic placement within the cabin
  • easy mounting/dismounting from the car; no drilling required
  • low cost

Design

Based on the can dimensions I designed a tapered basket that would fit around the can. The basket is attached to the vent grill via a slender piece pressure fitted on one end to the vent and screwed on to the grill.

FEA analysis

To be fair, running a FEA on the can holder model is a bit of an overkill. However since I'm new to PLA/3d printing, and lack the gut instinct when it comes to dimensioning parts, conducting this training exercise now, should pay off in the future.

To conduct a finite element analysis we need the following information:

  • geometry of the analyzed piece
  • material properties
  • boundary conditions
  • loads acting on model

For more information regarding theoretical foundations of FEA as well as some practical tips regarding structural analysis / implementation, please check out Metoda elementów skończonych w mechanice konstrukcji - Gustaw Rakowski, Zbigniew Kacprzyk.

Back to our simulation, we could of course model the vent grill, energy drink holder and a can filled ~95% full with liquid, establish contact between all the model parts and then load it with a recording of the accelerations experienced while driving, but this 100% realism approach has few problems. Preparing such a model is uneconomical; computing it would take ages; and worst - such a simulation would give us only approximate answers regarding the serviceability limit state SLS of the holder. We are interested in the minimum dimensions/cross-sections that won't fail under normal load; and we want those answers in a reasonable time with as little additional work as possible. Based on our initial shape, two possible points of failure were identified.

Material specs for the simulations were taken from this sources:

Extensive hanger flexure

The hanger may be represented as a cantilever with one end being fixed in the vent grill and a mass load on the other end. We will check how much positive/negative deflection can the hanger stand before it snaps.

Figure 1. Negative hanger flexure - equivalent strain

Figure 2. Positive hanger flexure - equivalent strain

The strain values for hanger flexure in both directions seem within material spec, moreover the physical constrains of the vent and mass of the can won't permit such high values of deformation during normal usage.

Basket-hanger connection shear

Due to manufacturing limitations the can holder had to be split into pieces. As a result of that decision the basket connects with the hanger via two M3 flat-head screws. We will check under what value of load stress values at the connection exceed the bearing capacity of PLA. A full can weights 2.5N, since we have no data regarding vertical accelerations while driving, we'll load the basket with 10 times the nominal use case load and see if the plastic around our mechanical connection will start shearing.

Figure 3. Connection shearing - Von Mises stress

The Von Mises stress in this use case are within spec, so this connection should hold.

Testing

Since the FEA suggest that the holder shouldn't break under normal conditions, it's time to test our design in real life. After more than 1000km under normal load, no signs of damage/fatigue can be seen. For more information about FEA model verification and validation check out this presentation by Prof. Lesław Kwaśniewski.

Figure 4. Working 250ml can holder

Summary

The holder fulfills it's purpose. The 3d model can be downloaded from my thingiverse.

DB