How to Build a Double Reverse 4-bar | VEX Override

[00:00:00]How do you double reverse four bars work? These are a monster of an arm that I usually stay away from at all costs.

[00:00:06]However, I just built one. So, let's go into how it works.

[00:00:09]Now, the reason I don't like these is because they take a lot of pieces.

[00:00:13]They're big, heavy, complex to build, and just not super intuitive. Um however, they are really really good at one thing. And that one thing is moving vertically up and down. And the reason they're good at that is because from the name, double reverse four bar. What does that mean? That means that you have two four bars on top of each other. This is a four bar. This is a four bar. A four bar is just like a normal arm. Um and it keeps your keeps your claw on the end perfectly vertical. So, for example, right here, we have a four bar.

[00:00:40]This kind of you can see this claw on the end stays vertical as the four bar goes up and down.

[00:00:45]So, take it over here. We got a four bar here. Kind of this piece right here stays vertical up and down. However, it kind of you can kind of see this piece moves side to side. So, that's where the second four bar comes in cuz you want your thing to go straight up and down. You don't want it to go side to side at all. So, if you take another four bar, you put it backwards on top of the first one, then it kind of cancels out that side to side motion, and you end up with just perfectly vertical motion. And the reason that this is so good is cuz we're making vertical stacks. Now, if you're looking at this and you're like, that looks really cool and I want to build that, but I'm not really sure how it works. You send me some more pictures.

[00:01:17]Go ahead and click that link in the description below. I'll send you some pictures and details on it. Now, couple things to think about when building this. You can notice that wherever I put it, it just kind of stays there. And that's really important. That's because I have these bands here. And I have them in a triangle. You can do it just in a line just like if you have just two of them.

[00:01:32]As long as you make the rubber bands not fall off. Mine were falling off a lot, so I actually zip tie them to the metal so that way they don't fall off the end of the screws here.

[00:01:40]Um but yeah, just got equal bands on each side and just band it until you're trying to counter out gravity here. So, gravity pulls it down, the bands pull it up. Um you can put bands on the this one, too. So, like half the bands here, half the bands here. That is also totally valid way to do it. Um but yeah, and then I've got the motors back here, right here on here. You don't have to do it that way. You can also put the motors like inside here and here, um or on outside there and there. That totally works, too. Uh the reason I didn't do that is cuz that introduces a lot more slop. So, basically you got the like slop from the gears. Cuz slop is just like See how the gears wiggle a little bit even though the motors aren't really spinning? That's slop. You want to minimize that as much as possible. Um you get a lot of that in your drivetrain when like just there's so many gears going on, you get some slop. Um so, you you want to minimize that on a big arm where like a little slop here translates to more more more more It means a lot of slop at the end. So, you want to minimize that as much as possible.

[00:02:31]So, pointing them down here means the gears slop here, gears slop here, all the way along the arm. That means a lot of slop up here. So, that's why I put them back here. Um the downside is you are lifting the motors the entire time.

[00:02:40]The upsides mean you get more precise precision control from your motors, so you don't need any uh rotation sensors on uh your arm, which is a big win. And then also, uh you don't really have much slop because they're going directly to the gears here that you can just power it like that. Um these are both green motors, which is really cool. Um but uh yeah, I tried red ones and I was like, "Wow, that's really slow." And then just the green ones kind of worked.

[00:03:03]And they work because I have good banding on here. Um if you have horrible banding, you're going to need some a lot more power than you should uh on your arm.

[00:03:11]Now, um this arm works really well. And then we've got our uh rotating claw to uh grab them from our intake. So, it kind of rotates down like that to grab it from our intake and then our claw here.

[00:03:23]But, how do you actually know where you are on the arm is when you turn on the program, it's really important to be able to zero it. So, when I run the program, it's going to put the arm all the way down until it hits this hard stop here, which Make sure you have a hard stop. Um and then it'll set that point at zero. So, here we go.

[00:03:40]There we go. And now it can go up and down just like that.

[00:03:47]So, yeah, that's a That's super important.

[00:03:51]Make sure you have that zeroing uh sequence there. Um another thing that's a little tricky is wiring. We got all these wires up here. Um and it's really important to make sure when this rotates, you got a little bit of a slack here to make your wires work. So, we got every time there's a pivot point, we got a little slack here, got a little slack uh right right here, and we got a little right here. This is probably a little more than you need. Um but yeah, making sure that there's not too much that gets tangled in other robots, but also making sure there's not too little that you'll like pull your wires tight and cut holes in your tubing and stuff like that. Um if you really need to, you can like rubber band them against something so they can like like um be stretched off of that something, but be banded to it once uh once there's some more slack in the the lines. But yeah, overall, this has worked really well for me. Um just This is how turrets work, too. You just have a little bit of slop or a little slack in in your cables here. Um for for like small turrets and stuff, that's how they work, too. So, making sure you have that down uh for your four-bar or double-reverse four-bar. Super super important.

[00:04:47]Um yeah, that's pretty much all there is for a double-reverse four-bar, but yeah, complex arm, probably one of the most complex arms out there. That's why I don't really like it. Um like we got this four-bar here, and gear on the top one, gear on the bottom one on this one, and then kind of goes up there. Uh sometimes you if you want extra height, you can add another set of two gears in here. Make sure you don't just add one gear though, otherwise it'll spin backwards, and that's a mess. Um so, you can like add extra height like move this whole thing up and put two more gears in here for extra height. Yeah, that's kind of all there is to double-reverse four-bar.

[00:05:18]And let me know if you have any questions in the comments below, and I look forward to seeing you in the next video.