How to rubberband


#1

How do you rubber band a DR4B? I know that you should make a triangle and I know that @antichamber made an online rubber band calculator tool but the tool is in 35 hole C channels when I’m using 25 holes 1 x 1 cut c channel. So do you just randomly pick places? Or is there a way to use the online rubber band calculator for this too?

PS. How do you use the calculator? I didn’t want to “revive” old threads because people seem to not like that.


#2

@RougeScaless
In our In the Zone robot, we used 28 hole 2 x 1 c-channel for an RD4B and our hole distances can be found in the attached picture. I don’t think that it particularly matters for the spacing to be an exact ratio, because we just picked reasonable distances that ensured structural rigidity. As for the calculator, I am not sure how to use it for this specific scenario, because, as you pointed out, it automatically assumes that your c-channel is 35 holes long. If you really want that kind of perfection, you can start off by using the online tool for 35 holes and using proportions to scale the distances down.

For more advice, I recommend this thread, https://vexforum.com/t/how-to-rubber-band-a-dr4b/42553/1

Screen Shot 2018-07-02 at 11.29.26 AM.jpg


#3

Thanks!


#4

@skhan21 I don’t know why but when I did my rubber bands, it did not assist the lift and just made the lift worse. Any ideas why that happened?
Screen Shot 0030-07-04 at 12.47.16 PM.png


#5

The purpose of rubber banding is to pull the 2 sections of 4-bar closer together as the lift raises.
The first picture is your lift with some lines on it. The orange rubber band is doing nothing, as the lift raises closer together, the rubber band stays the same length. The green rubber band is doing nothing, it is attached to the bar of the lift. The red rubber band is actually pulling your lift down: When the lift raises, it’s length increases.
I recommend you move that third point where the blue arrow points to. This way, your rubber bands will be helping rather than hurting your lift.
The second picture is 675C’s lift. Their rubber banding is good. If the blue arrow doesn’t work, just try something like that.
Screenshot 2018-07-04 at 7.18.26 AM.png
Screenshot 2018-07-04 at 7.20.41 AM.png


#6

Thanks for the diagram and the explanation!


#7

np. Did you fix it?


#8

It’s currently 20:00 in Bangkok. Will do tomorrow morning and certainly will report.


#9

Oh right!!! your signature (duh…)


#10

Sorry I couldn’t answer earlier. @Baguette123 has a great answer though so definitely follow what he says.


#11

@Baguette123 , @skhan21
I did the 675C version of rubber banding and it worked! Here is a video of it!
link text

Thank you both so much for your advice.


#12

Awesome! Glad it worked!


#13

Usually, you make a triangle, set two points as far away as possible, then experiment with the third point.


#14

@RougeScaless Congratulations! Glad it worked!


#15

I’m surprised that people are still able to find that, seeing as it’s so old now. It wasn’t the best program and was not very intuitive.

Glad you were able to make it work! As a side note, would you be interested in me re-making the rubber band calculator?


#16

I haven’t seen you post here in a while, welcome back!
The calculator is really useful, but It would be really awesome if you could re-make it. It’s completely up to you though.


#17

I don’t remember. What is the calculator based on trying to keep uniform? I’ve seen/heard a bunch of people talking incorrectly about the proper uniformity, saying things like you want to keep the force of the rubber band as constant as possible. I seem to recall your calculator taking a more proper approach, but I only have vague memories of the calculator.


#18

The rubber bands must approach an equilateral triangle as your lift fully extends or else the triangular rubber banding system is no different from a two-point system as the upwards force is not linear :slight_smile:


#19

Ok, I’ve remade the rubber band calculator! I’ll make a tutorial video soon, but I’ll make a simple explanation for now. Here’s what the different colored curves mean:

  • RED: Actual force curve of a doubled up VEX #64 rubber band, X-axis = distance, Y-axis = force
  • ORANGE: Distance between banding points, X-axis = angle, Y-axis = distance between points
  • GREEN: Distance between banding points scaled by band curve, X-axis = angle, Y-axis = scaled distance between points
  • PURPLE: Output force onto lift, X-axis = angle, Y-axis = force

The purple curve is most important curve, and this is the line you will focus on. If you want the same amount of force for the entire specified motion range on the lift, you want this to be completely horizontal between the pink lines. If you want less force at the very bottom of the lift (this allows you to have more lifting force overall while not having to force the lift down continuously while it’s at the bottom)
https://scratch.mit.edu/projects/235244490/#player