How designs the strongest hook? - Polymaker competition
Of course this is a marketing campaign by Polymaker, though I haven’t been paid for this video and will even participate on my own. Our task is to design the strongest hook with some design restrictions, print three of them in Polymakers PolyMax PLA, and send them to Polymaker until September 25th 2020, who will be then testing them against each other to find a winner. The requirement to print the parts in precisely their material has a strange aftertaste for a competition. Still, I can understand it in a way so that everyone uses the same material and doesn’t have an advantage by using some fancy carbon fiber reinforced filament or whatever. Well, and it’s good for sales… If you don’t have any Polymax PLA or don’t want to buy some or maybe even don’t have a 3D printer, stay until the end of the video where I’ll tell you about a way you could still enter the competition.
The rules are as follows:
You need to print in Polymax PLA using a 0.4mm nozzle and the print must be one piece. The hook can’t be heavier than 50g. It needs to allow 2 steelhooks to be attached at both sides for the test conduction. So make sure that they can be inserted. The hook must be open and can’t contain any closed holes. This is definitely the most important design requirement but more on that in a bit. You can’t do any post-processing on the hooks like coatings or annealing and you can’t reinforce it in any way. So 100% printed in Polymakers Polymax PLA.
Interface Hook
Keep in mind that the hooks you see in ths video are just for illustratiuonal purposes. They aren’t printed dense, nor are they perfectly optimized. That’s up to you! The best and strongest way would probably be just something like a chain link or some kind of a dog bone shape, but the rules don’t allow that. Thus, we need to be a bit creative. If you have ever been rock climbing and have been taking a close look at the carabiner on which your life depended on, you might have seen three ratings on it, which are also interesting for our problem.
Climbing Carabiner with Strength Ratings
There is the normal “gate closed” rating, which is the intended loading scenario with a strength of usually around 20kN. In this case, the load goes through both sides, depending on the shape, more or less equally. The next one is transverse loading, which is the loading perpendicular to the intended direction. This axis is significantly weaker because the lever arm is higher, causing higher stresses. Last comes the “open gate” load rating. This is the load-bearing capability if the gate is not closed, which is, in real life, highly dangerous. In this case, the load only goes through one of the sides and will cause a significant bending action during loading, causing this rating to be even lower than half of the one with the closed gate.
So this is interesting now and is also the challenge we need to work on because we’re not allowed to print a closed loop for optimal load-bearing capability. You could now either get fancy with the shape and optimize a C or S shape with all the tools and ideas you know like topology optimization or any other method that you have available or bent the rules.
If we take a closer look at the carabiner, it’s also not a closed loop, because you need to quickly attach a rope by using the gate. The climbing carabiner uses two geometrical features that after a bit of deformation make a closed loop out of an open loop. And printing something with a gap that closes after some deformation should be totally within the rules. Honestly, I think that there is no way, that a C or S-shaped design, even optimized to its maximum, will beat even a fairly simple carabiner design. But this doesn’t make the contest pointless because designing one of these closing mechanisms can be challenging and even the shape of the hook itself can be tweaked quite a bit. I’m really interested in how creative you guys will become! Possible weak points you should look for is the failure of the locking mechanism, so try to transmit the load as smoothly as you can here. Next is the lever arm from the load introduction lugs to the sides, which needs to be as small as possible, and finally, make sure that your attachment lug doesn’t shear out because it’s too thin.
Reduce the Lever Arm
Make sure your Lug doesn’t Shear
Print settings are honestly not that critical. Obviously, you should print the hook flat. Instead of a lot of infill, increase the number of perimeters, because in a hook, the printed lines will be in the direction of the internal forces, so the material is ideally loaded. Make sure that you don’t under-extrude, because even though the print lines are oriented in the proper direction, bending will cause transversal and shear stresses, separating the perimeters and weakening the part. If you’re over the 50g limit, which should be hard to reach, maybe make use of mesh modifiers, to adjust the density in regions that need more strength, in comparison to less loaded ones. Also, make sure to watch 3DMakerNoobs video on the contest he posted a couple of days back, where he also goes over some tests and findings and shares some modeling techniques.
More Perimeters is better than more Infill
Polymaker will accept entries until September 25th with shipping locations in the US, EU, China, Japan and Australia! I’m not sure how many will participate but pay attention because they will only accept the first 128 entries.
Strength results of some “illustrative” Failure Load comparison tests
So have fun designing the strongest hook, bending the rules even more if you have good ideas and I guess I’ll see you on the test stand! Please leave your thoughts about this contest and your design ideas in the comments for discussion.
Join the PolyMaker contest
https://polymaker.com/3d-printed-hook-tournament
3D Maker Noobs video
Download the ANSYS models
Non-Linear stress/failure Simulation
All models were created in ANSYS 2020 R2. The model size was limited to a maximum of 32k nodes/elements that they work in the free ANSYS Student Edition.