Akshay Harlalka
Curious Builder
Analysis of Snapfit Buckle of Travel Bag
So, this week, as I was preparing to pack my bags for travelling to MIT, I used the snapfit buckle to secure the clothes in the bag. I began to think about snapfits more and wondered how the engineers ensure that snapfits would not fail due to fatigue during the expected service life of the bag. So, I decided to do a basic analysis and small test to check if my analytical model is correct. Following that, I decided to calculate the expected life of the snapfit.
Snapfit Buckle in the Travel Bag
Following are detailed analytical calculations that went behind the scene. The figure below are shows the key dimensions and features of the snapfit buckle.
Dimensions and Key Design Features of the buckle (Attn: Check features like hardstop for the snapfit beam to prevent excessive bending and the slot in the center which slides into a guidance feature in the female part of the buckle. )
Calculations to predict the required push-in force of snapfit buckle and the corresponding stress at the root of the cantilever
Model Validation
To check if this model is correct, I conducted a small test to measure the push-in force of the snap fit. I used a digital spring scale to measure the force I am applying.
The prediction for the push-in force basically dependent on the friction force. Friction coefficient (dynamic) for polypropylene on polypropylene is 0.3. The normal reaction force on each beam is 5 N. Therefore, the predicted friction force is 0.3(5+5) = 3N.
Model Validation Procedure
The spring scale recorded a force of 0.3 kg which was approximately 2.94 N. This suggests that the model can be applied and is a good approximation of what is happening in the real world.
Now to calculate the expected life of the snap fit, we will need the S-N curve of polypropylene. After a lot of research, the best approximation I could find was the S-N curve of 40% talc filled Polypropylene. I know that this would be an approximation on the higher side (predicted life will be more than the actual life of the component), so I would have to be careful before making any final judgements. However, I also know that curve (normal to the flow direction) is not be too far from the 100% polypropylene as it correlates reasonably with the stress value for 10^7 cycles which is 11 MPa which is close to 14 MPa as seen in the graph.
Based on our calculation, the stress at the root of the cantilever was 13.5 MPa. So, the expected number of cycles before it fails will be less than 10^7. Depending on the slope of the line (assuming it is same for 100% PP and shifted downwards to match the value of 11MPa at 10^7 ), it seems that the number of cycles would be closer to 10^5 cycles. This seems reasonable design target for a travel bag which is ideally supposed to be used for many years.
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