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Originally Posted by RobotsThatWork
Did you add a foot gusset to the front rails you used the barrelnut on?
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Quote:
Originally Posted by KohKohPuffs
I made a custom barrel nut: take an aluminum rod, thread both ends, and drill a hole through the round face, and tap for 10-32. Screw them into place, and theoretically it should hold just fine in addition to the rivets from the belly pan 
I've never heard of a foot gusset before, but to me it sounds like one that has an L shape and can mount to the side of the boxtubing. I could do that, but probably not with those modules in the way. |
Is the axis of the barrel nut vertical? The barrel nut would have to be made fairly accurately for it to work well (length, ends square, holes for screws all in the same plane). When you start putting stress on your chassis, will the single screw holding the side tube to the barrel nut cause that part of the side tube to deform? It really isn't a tube anymore in that area. Is it possible to add an L-shaped gusset to the top surfaces where the side tubes meet the front and rear tubes? That would spread the stress out much better than the 3 screws in the barrel nut and save you having to make the barrel nut.
Since you have a large bottom pan providing rigidity in it's plane, you can probably pocket the top and bottom surfaces of the two side tubes quite extensively to reduce weight. The inside surfaces of the side tubes and the front and rear tubes can probably also be pocketed. You can probably also pocket the middle tube quite extensively too.
Quote:
Originally Posted by GeeTwo
With 1/4" plates, I would not be so much worried about permanently bending those plates as much as rubbing the axle or bearing against the inside of the tube, especially with T6. However, note that tempering usually causes little change to the elastic modulus (that is, the spring constant for a given geometry). Unless you are going for a really tight tolerance in the tube, you're not likely to get contact under likely sustained forces such as scrub and shove on the FRC field that would cause this contact. However, consider what would hit what and where they would meet if you were rammed from the side, or if you ran into a wall or another robot with a corner of your robot.
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Do you have a small gap designed in between each of the two plates and the inner surfaces of the side tubes? Can you add a piece of low friction material on the outside surfaces of the plates? Nylon screws/bolts are injection molded, I think, so their dimensions are pretty consistent. You could add some tapped holes to the drive module plates and install some nylon screws or bolts so that their heads will rub on the inside surfaces of the side tubes when the module is deflected. The screws/bolts can be cut flush with the inside surface of the module so they don't interfere with the wheels. A few threads will be more than enough to retain the screw/bolt in the module.
What retains the bearings holding the ends of the shafts with the omni-wheels on the outside surface of the side tubes?
How will your drive modules be built? What is the order of assembly? It looks like you will have to insert the hex shaft after you insert the module plates into the bottom of the side tubes with the omni-wheel and sprocket for driving the traction wheel "held in place by a third hand". Our modules from 2014 were like this and were really frustrating to work with, requiring two people working in very tight quarters to get them in or out. I cannot remember the details but 148's drive modules could be pre-assembled and formed a self contained unit. They could be swapped in about 2 minutes. I think they used a dead-axle.
You may want to delete the pocketing on your slide-drive module with the two omni-wheels. You may also want to beef up the "ears" that the two green standoffs attach to and add fillets where they join the main part of those plates. What keeps the two plates from "sliding" relative to each other when the slide-drive is pushing your robot sideways? It does not look like the main shaft extends through both plates. The two standoffs will tend to twist the ears they are screwed onto. Can this whole assembly be made from a piece of tubing with the wheel bearing holes offset so that the wheels stick out the bottom of the tube? It could be much more rigid.
It may also be helpful to extend the main shaft from the VexPro Clamping block through to the front side of the module and add a support that is attached to the bottom pan. The VexPro Clamping blocks are plastic and will deform more than metal, allowing your shaft to wobble from side to side a little.
Will the slide-drive module twist the main central shaft so that the front end moves towards one or the other side tube? This is the same issue as with the drive modules in your original design.
Be careful when installing the motor on the VexPro clamping block. One student ruined one or two of them by overtightening the screws. Since cap screws have a relatively small head, the clamping forces are very concentrated and the screw was pulled through the plastic, deforming it. The screw head ended up touching the mounting face of the motor. The hole in the plastic became a press-fit for the screw head and could no longer retain the motor properly.