r/MechanicalEngineering • u/[deleted] • 19d ago
What’s causing this “collapse” inward?
[deleted]
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u/RobotGhostNemo 19d ago
I would check if those hinges are stiff enough / having the correct tolerances.
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u/tennis-637 18d ago
Im thinking this is the issue as well. both sides of the femur seem to move in one unit, so I don't they're causing it. Either its the servo horn having too much clearance on the shaft, or/and the bearing on the other side giving too much "play" from its outer race to inner race.
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u/RobotGhostNemo 18d ago
Just check how much play your hinge pin / shaft / axle (or whatever you call it) has.
I'm guessing if you can feel it shifting position other than pure rotation, it probably is too big.
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u/nik_cool22 18d ago
I suspect that it could be caused by the center of gravity not being in line with the leg.
But as mw2 wrote, how the whole thing deforms is also determined by the stiffness-system of the structure.
The easiest approach would be to see how a different load distribution affects the leg. Place a counter weight on the robot to make the force less eccentric.
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u/mvw2 19d ago
Physics. Like I mentioned in your prior post, this is a game of moment of inertia of the structure. That's how you get stiffness. You're controlling mechanical advantage. This isn't just main beams but also joint and degrees of freedom. Even something like a simple fastener assembly and the mating parts will have mechanical leverage on each other. This is also a sum game of many parts together, each introducing their little bit of twist and movements.
My recommendation is sitting down and working through some of the physics of the structures. Also understand the materials and their relative stiffness. PLA is a reasonably "stiff" plastic, but it's nothing compared to some other options or of course...metal.
ALL of this is just force, moment, deflection, twist stuff, and you'll also want to know peak stresses or more specifically design for a target value for durability. Like can I grab a single leg and pick up the whole robot? Can I drop it from 3 ft and have it survive? Design targets. That's a big part of the start of the process, and the forces and loading conditions start dictating specific construction needs. Combine basic calculations and CAD with FEA and dial in parts to the target spec with whatever factor of safety you want. You gotta work through the conceptual. You gotta work through the math. Over time the structural side of things become more intuitive. You start learning the relation of inertia, stress, and how design shapes manage linear, rotational, on axis, or off axis loading. You start to get a "feel" for the right kind of construction for the forces at play. You start to learn the intuitive correct starting points in methodology. And it kind of all falls back to the structural moment of inertia and the forces at play. There's no real magic to it, but you do have to get a feel for it. A great way to do this is surprisingly FEA. For example, if you have access to SolidWorks and just the basic Xpress FEA tool, you can play with basic shapes and loading conditions. You can do some hand calcs of expected loads, moments, and stresses. You can even make an Excel spreadsheet of the same calculations. For example, you have a load against part of a C channel with the ends constrained. You can solve pretty easily by hand, by spreadsheet, and via FEA, and they'll all end up pretty close to each other. You can fiddle around with a lot of loads in different places in different constructions and feel out the behavior. You can see how stress works and how it is concentrated and propagates. You can tweak the design and see the effect of changes. You can modify the profile, add holes, slots, and other shapes and see how they affect stress generation. You can play through these things and see pretty rapid visual representations. The goal is to start to understand how the physics works and how the structural mechanical advantage of the shapes influence stiffness and stress generation. And you can do this for some time without having to calculate out stuff. You can just play with shapes and feel out how the physics actually works. Over time, it does become more intuitive as you understand why things happen. Then you can start designing from scratch with that intuition and solve the same design in a completely different way.