Tiny carabiner (load tested)

Details

I had mounted my GoPro to my helmet to shoot outdoor action movies. I love the original GoPro cameras for their great stabilization features, but they do come at a price. So who wants to see a costly camera banging on the ground, or loose it completely, in case the helmet mount should come loose. 

What I needed was a small and lightweight carabiner to secure the GoPro with a thin robe to one of the eyelets on my backpack, just in case. The eyelets have a material strength of 4mm, so one design requirement was that the cut in the carabiner needs to allow a 4mm object to pass through when it is bent open. Another design requirement was that the robe should have it's own section in the carabiner, so it cannot detach from the carabiner. 

Off course I was curious how much I could load my designs before they break. I started with the straight design, which obviously has it's week point diagonally opposite to the slot. This is where it broke at around 100N (10kg). So I enforced this part of the design and reached more than 140N (14kg), then my doubled robe tore apart. For me that was good enough, as I don't see a point in making the carabiner even stronger than double the robe breaking force. 

The reason for the three different enforced designs is mostly usability. The one with the 17mm long inner oval is just barely able to open wide enough for the 4mm eyelet. But it works and if a tiny size counts for you, that would be your choice. The 18mm and the 20mm type simply have more material length to bend and are therefore a bit easier to use. I personally find the 20mm version the most practical one. It is the easiest to attach and detach and it is still by far stiff enough to securely hold on the eyelet.

I have not yet load tested the 18 and the 20mm types, but I doubt that they are significantly weaker than the 17mm version.

Printing Instructions

Print the parts on their side (as uploaded). I strongly recommend to use PETG, as the material needs to bend quite a bit and PLA does not do that. I used 100% infill and I recommend that, as there is no point to weaken the parts to save a fraction of a gram of material. There is no need for supports or a brim.

I have printed all parts on a Prusa Mini with no troubles at all. I am fairly sure that budget printers would also be able to do the job. The opening slot is designed to be 0.5mm wide, which should give enough margin for imperfections of the print.

Disclaimer

I seriously hope that no-one is stupid enough to use a 3D printed carabiner for applications that cause a risk to health or life of people or significant loss of property. Use this design completely at your own risk!