We purchased some aluminum screws from Robosource.net. We figured out that they don’t take much to break. They do have a long “elastic” period, as shown in stress strain curves. Tightening feels way different than steel screws.
Steel screws obviously survive much more stress than aluminum screws do (with aluminum screws being unable to survive even slight tightening). We are still unsure which applications max tightening for aluminum screws will work (e.g. whether aluminum screws for the base would loosen or not, etc.).
We wanted to know others’ experiences with aluminum screws. Do you use aluminum screws? Which parts of the robot do you use them? Is there any reasoning behind where you use them? etc.
I’ve personally used aluminum screws on the base and they do not come loose that easily. However, in the case that you are worried, you can mix and use metal screws on the base and use aluminum screws where weight reduction is a necessity. For example, you can use the aluminum screws on intakes and mounting channels on channels.
In general, I like to use steel screws for any joints that move and points that support a significant amount of force, for example if you have your lift connected to the base of the robot at one point. The majority of structural connections (as well as stuff like bearings and sensors) are fine with aluminum screws. Also, if they are tightened well, I have not seen any of them come loose by themselves.
One important thing to note is that the forces the screw experiences when being tightened and when on a robot are very different, and the screw reacts differently. Yes, it is correct that you can easily break an aluminum screw while tightening it, and this is because they are not very strong to torsional force. However, once a screw is tightened (but not over-tightened), the force it will experience on the robot will mostly be tension, and it is actually very difficult (probably even impossible in a normal use) to break an aluminum screw from tension. If your screw is in a position where it does experience torsion or shear forces, this would also be something that is better suited by a steel screw.
In terms of how much to tighten aluminum screws, it’s kind of hard to explain, and takes a bit of getting used to. Unlike steel screws, you will not hit a “hard stop” when it is tight, due to what is explained above, instead the force will just increase until eventually it snaps. However, after the screw is done freely spinning, you should turn it a little bit more, and you can feel the resistance increase quite a bit.
Sorry for the long post, I’ve done lots of testing and playing around with these screws, so if you have any questions I’m happy to help.
I use aluminum screws with low profile steel locknuts on everything that I don’t use nylon on. I tighten them as far as they can go and I’ve never had one break, bend, or strip ever. I don’t use the ones from Robosource though. I use the ones on McMaster because they have a better selection of sizes, but I don’t imagine there would be a ton of difference between the two. Just take an extra moment to make sure your tools are in good shape and mated properly with the screw. I’ve had 2 aluminum screws support the whole weight of a hanging robot and I’ve used 4 aluminum screws to support the force of lifting of other much heavier robots. As has been said many times by many users on this forum, your build quality and attention to detail are the most critical thing, whether it be for robot performance or part failure, it usually comes down to how much care you take with your build.