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Originally Posted by wmarshall11
Take it for what you will, but I'd suggest moving the piston further from the pivot or moving the dropped wheel closer. Increasing your piston bore would also be effective, and will provide a more dramatic (quadratic) effect than changing your radii.
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Yeah I messed up my math when calculating the torque needed to lift the robot. I will probably end up increasing the cylinder to 2" diam. which should be more than enough torque even with the piston where it is. I would go for a 1.75" pancake cylinder, but Bimba doesn't offer that and they're the only pneumatic company I've ever worked with.
Quote:
Originally Posted by Chris is me
It's super great that you've decided to teach yourself CAD and have worked on original designs just a few weeks into it. Good for you, keep doing that.
That said, this design might need some work, and building a cantilevered octacanum module like this is a somewhat complex endeavor that is probably not the best first design project for someone new to this stuff.
The "double cantilever" of supporting a drive module cantilevering off of an already cantilevered shaft can put some weird and strong bending moments on the drive shaft that it may not be ready to support. Specifically when a robot is pushing you from the side (or to a lesser extent, when you're strafing) - there's a real danger in bending the shaft this way and you'll want to be really careful.
For "west coast" drop drive modules like this, I've always been more comfortable with a design that straddles the drive tube, rather than hanging off of it. You can add some low friction plastic blocks between the module and the frame so that if the module deflects under load, the forces are transmitted to the frame rather than the axle supporting the module, and even if the axle supporting the module takes some load, it will be on both sides of the tube very close to the side walls which is a lot better of a loading condition. The Vex drop modules do this for good reason; they are a good starting point for a design.
If you are going to face mount the piston then using it only to push the module down is the correct choice. If you couple the piston to the module but rigid mount it on one end, you put a bending moment on the piston, which is bad news bears and just unnecessary. You'll have to pivot mount the piston on both ends if you want it to provide the up-lifting force to hold the mecanum off the ground, which is a lot more of a pain than just throwing a torsion spring on there.
If you're using 1/8" plates for the module, I would just get rid of the lightening altogether. The pattern you have has a web that is too thin to do anything in the middle and it gets dangerously close to those bolt holes. The lightening saves you maybe like a quarter pound per module?
Can you make the mecanum wheel a dead axle wheel? This way you can use the axle as a structural member and you get some "free" rigidity.
Overall, a good start!
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Thank you! I was originally planning on attaching the cylinder to the module, but I decided against it when I realized I would need the double hinge. When I saw that the VersaDrop doesn't have the piston attached, I thought that would be okay to do, but I forgot to add the spring return. That will definitely be something I add in the next version.
As far as the cantilevered module goes, I don't know if you saw that the module is supported by two pieces of VF Stock, not just the one shown in the render. If you take a look at the 3D model, you can see that the module is constrained on both sides. That being said, the problem of side loading is something I was keeping in mind when building it. I was under the impression that a 1/2" Hex shaft constrained on both sides would be strong enough, but I don't have any experience to prove this. If this isn't true, what do you think I could do to fix this problem?
I was wondering about the lightening. Do you think it would be better to make it a thicker plate and add pockets or leave it 1/8" plate and take out the pockets?
Since the mecanum wheel is AndyMark and the pulley is Vex, there's no easy way to attach them and make it a dead axle. Sadly, AndyMark doesn't make an 18t pulley and Vex's 4" mecanum wheel doesn't support dead-axle drive. If you have another way to do this dead-axle, I would be interested in hearing about it.