So I have heard about how people are able to make ways on their lifts so that there is constant force helping the lift up. so my question is how does this work and how can I apply this to my robot and how effective is it really?
Also I have seen this thread https://vexforum.com/t/uniformly-tensioned-rubber-band-system-analysis-guide/28402/1 and I don’t really understand it. anyway I am just trying to learn something new.
-thanks
The thread you linked in your post answers all your questions. It’s a very detailed guide and it looks like a lot of time was taken to write it up, you should make use of it. You might want to try to reread it and understand it.
I’m fairly sure the system described in the thread you linked doesn’t give you a torque from the rubber bands that is *strictly *uniform (whether or not it would be possible to make one using that design that would be strictly uniform would depend on the stress-strain function of the rubber band). The design is supposed to give *approximately *uniform torque.
The way the rubber band is arranged around the joint means it has less mechanical advantage where the rubber band gives more force, and more mechanical advantage where the rubber band has less force. The thread explains pretty well how that part works - torque is equal to force times distance from the pivot.
thanks this help out a ton 
Agreed. One type of assist helps the lift when the lift is low, while the other helps more when the lift is high due to angle change. The key advantage is that adding the second type of rubber bands assist gives you great assist when the lift is raised while barely inhibiting the lift from staying down.
Check out my post on this thread:
https://vexforum.com/t/rubberbands-on-elevator-lifts/27439/1
That’s the only way I’ve seen to (theoretically) achieve truly constant force out of your rubber bands, regardless of the stress-strain characteristics of the polymer itself.
It’s worth noting that most rubber bands do have a fairly constant force once stretched, due to a variety of factors, lower cross-sectional area and of course the science behind the polymers themselves.
One thing that the thread mentioned previously in this post does not address (at least that I saw), is that your desired lift force-theta curve should actually be a sine curve, because when the lift is horizontal, gravity causes a larger torque at the pivot than when the lift is vertical (when there’s 0 torque due to gravity). This of course applies to arm lifts. Scissor lifts or reverse double-four bars can ignore this.
Here’s an oldie but goodie thread about rubber bands.
https://vexforum.com/t/rubber-band-properties/23885/1
Seems that if you keep the deflection length relatively constant it will exert the same force. But the question posed above is do you need the same force? Based upon the angle and that pesky gravity thing you can adjust it to result in the same effective force on the arm. But that’d be a pretty fancy linkage path to generate the correct compensation for both the angle and the deflection of the rubber band to generate the required force.
Still, give them some props since it’s pretty sweet to keep the pulling force the whole way you need it! Don’t get the classic rubber band looseness putting a great strain on the motors.
The above thread links to this graph which shows stretch vs forces…
http://design.caltech.edu/Archive/2001/handouts/Rubber_Bands.pdf
Easy solution to the gravity thing:
[ATTACH]9020[/ATTACH]
Have whatever constant-force spring device you use attach to an extension of the arm sticking past the pivot point, mounted vertically below it (it’s not exact as the vertical-ness of the rubber band changes, but it’s pretty darn close).
I’m not quite sure how this mechanism works. If you could explain a little more that would be great.
The only reason I didn’t mention this fact in my thread is that this deviance (in my opinion) is negligible. Most lifts start at -45 degrees approximately and go up to around 50-60 degrees. If a lift started at -90 degrees (lift bars perfectly vertical downwards) and rose to 90 degrees (lift bars vertical upwards) that would be a different story. So a deviance of less than maybe half a pound wouldn’t make much of a difference.
I don’t believe this is true. Scissor lifts and double-four bars require more force to start them up near the bottom than at the top. Think of it this way, when a double four bar is completely extended, it requires no force to hold up. The same goes for scissor lifts. I can understand that maybe you thought that vertical motion requires constant force, but the velocity of the scissor and RD4B lift is variable, making the force required to raise it variable. Elevator lifts, on the other hand, require constant force (save for multi-stage elevator lifts in which subsequent stages stop).
All these research papers and graphs of rubber bands are great and provide interesting information, but they aren’t that all practical or helpful for vex. That’s why I made the guide on how to make approximately constant force rubber band systems, which requires low number of parts and no complex designs. Completely uniform force is simply unneeded in vex
The principle is that you transfer the energy of stretched rubber bands into torque by constantly unspooling more tight rubber bands and winding up the excess on the collector spool (the blue one, which rotates slower). The same can be done in reverse. We ran into some issues when spooling 12 feet of rubber bands, but with only a few, I’d imagine that it would work quite well.
Well at 45 degrees, torque due to gravity is 30% less than when horizontal. So it depends on what you call significant, but I see your point.
It depends on how you actuate them. In my head, double four bars and scissor lifts should be actuated vertically, but in practice those aren’t always the easiest ways to power them.
The general curve is very helpful to know when designing these systems. I might test some VEX #32 rubber bands this semester in my school’s matsci lab for fun.
For me, it’s a fun design challenge to see how far VEX can be pushed. Did you do linkage simulation, or just iterate until it worked how you wanted it to? Just curious. I’d do linkage simulation if I had the time…
Ahh, interesting concept. But is it truly theoretically constant force? For every 2 inches of rubber band you release, you spool up lets say only 1 inch, so as the rubber band is being spooled up/ released, the rubber band becomes more and more slack.
I should have done this simple calculation before I posted. Sin(45) = .7 while sin(90) = 1. Now that I look at it, that is quite significant.
Usually in vex, scissors are powered by horizontal linear slides or rotationally through one of the “X”‘s while RD4B’s are powered rotationally through the tower, rather than by vertical 5’ linear slides 
I just thought it out logically. When the rubber bands are fully stretched, that’s when you want the acting angle to be low, and as the rubber bands become more slack, the acting angle should become greater. I just found the corresponding mounting point that enabled that function.
There’s no feedback system built in as I drew it, which would have to be devised before this could be reliable for long rubber band lengths. When you’re only using ~1 foot of unstretched rubber bands, I think this would work significantly better, as the error would compound less, reducing the need for constant feedback into the spooling speed.
You only need to power the first stage of the scissor lift, or a short segment of the arm. I’ve seen it done, but you are correct in that VEX lifts are predominantly built to be powered non-linearly.