My dr4b is built well and it is similar to the previous ones I have built, however this time we r using a chain bar intake and When the lift is down the chain bar sits perfectly in line with the mobile goal and cones, however as the lift goes up, the chain bar gearbox shifts forward slightly knocking into the cones. This limits my team from scoring any cones past 5.
I attatched a picture of a robot from early on in the season just as an example of what I mean.
Credits: Team 202
Based on your description I’m not sure what type of robot you have and where the issue is. It would be helpful to have a picture of the real robot. Did you mean:
A. The robot has a DB4B cone lift with virtual 4 bar (chainbar) acting as a elbow on the end of the DB4B, with a claw at the end of the elbow. When you said “intake” you were referring to how the elbow and claw together pickup a cone, as circled in the picture. And your issue is that when you try and place a cone on the stationary goal (as pictured), the elbow lurches forward as it goes down.
OR did you mean:
B. The robot has a DB4B cone lift with a claw on the end, and also has a chainbar mobile goal lift (not pictured) . In this case the intake is the mogo lift. And the issue is that as you stack cones on the mogo the stack is not raising in the same direction as the DB4B cone lift. So you either have a leaning tower of cones, or your DB4B is not raising straight up.
Are your bars on the bottom four bar and the top four bar the same lengths?
2.When the bottom bars are parallel to the ground are your top bars parallel to your bottom bars?
If not I would recommend doing these things
Make sure that the top and bottom 4 bars are geared together directly, with bars attached to each gear. Then make sure that the gears are meshed in the right place. Usually, the top bar needs to be a few teeth in front of the bottom in order to get a vertical lift. It requires a lot of patience to get the gears right, but they are absolutely required to a good lift. You might need to put the motors on the middle section as well, we had problems when they weren’t there. If your bars aren’t the same length, it wont work properly.
So I have found, after several cad models, the only thing that effects the longitudinal drift in a perfect slop-free environment is the bar length. The attatched pic is the opposite of what you are describing, the top drifts back in this pic. However the concept is still true. If I had to guess without any pics to confirm my theory, your top half is shorter than the bottom half, by somewhere around 10-15 holes. In order for the lift to move completely linearly, the top and bottom 4bars need to be the exact same length.
yes, thats how rd4b’s work. The two 4 bars cancel out each others lateral movement, which results in 0 movement. However if one 4 bar is moving less than the other, you’d get this problem as they all dont cancel out.
Well, like you said, if you have a shorter bar, you’ll get less movement. For example if you have an infinitely long 4 bar, you’d get infinite transnational movement. However if you have a 7.5" bar you’d get not infinite, which is less. Essentially you have to imagine it as an equation. If both bars are equal you’d end up with the equation where x represents the horizontal movement 1x-1x=0 which makes sense seeing as you canceled out the horizontal motion to 0. However if you have a shorter 4 bar you’d have 1x-1/2x=1/2x. Which as you can see no longer cancels out resulting in the motion you’ve been seeing, in reality you can cancel out this motion 2 ways, 1) by having your bars be the same length, or 2) having a gear ratio that results in the top bar moving quicker than the bottom one, I’m not sure what this ratio would be, but I’m sure it exists.
Ah, OK thats what you meant. There was a thread about a dr-chainbar, using a 30t sprocket and a 15t cut down tank sprocket to get dr4b movement, but without the center gearbox. Same kind of math as what you got here.
I am also having a similar problem, but not exactly the same. My DR4B has the exact same bar lengths on the top and bottom, but when I pull the lift from the back side the lift is pulled backwards by about 4 holes, which is making it collide with my tower mount in my case. However when I pull the lift from the front bars it goes up perfectly linear.
I suspect this is because the bottom four bars are moving faster than the top four bars but I am not certain. Is there any solution to this? Right now I am wasting a lot of space by having to move my four bar 4 squares forward since it collided with my mount due to the lift moving slightly backwards.
Something that might be an issue for you is that you have the motors based on the towers connected to your drive base, or some variation of that. Whats happening is you have too much slop, and a way to get rid of that is to attach rubber bands to the top part of your lift, meaning the top part is pulled upwards resisted by the weight of the bottom portion plus motors.
Yeah my motors are on my tower mount. I have half cut c channels on the top so I don’t know if I should rubber band those because I am afraid of bending them.
Should I have all the rubber bands on the top half or should I just have more rubber bands on the top then the bottom
Part of the issue is that that the pieces are bending a little bit or there is some slack somewhere. Pull the lift from the back and it moves to the back slightly, pull from the front and it moves to the front slightly. In your case it appears that when you pull from the front it is not moving, but in reality it probably is. Your lift is moving 2 holes in either direction, so overall it is moving 2 hole back when lifted. @Silicon provided the solution for this issue:
Hey guys, thank you for the responses. I mean situation A. Sorry I cannot post any pictures as my teacher does not want us to post pictures on Vex Forums. I have worked on the lift today and I think that the issue was that when the bottom set of c channels were parallel to the ground, the top set of c channels were not parallel. So I adjusted the gears on the gearbox to make sure they were parallel.
