Re: Scissor Lifter?
 

Nothing wrong with scissor lifts!:)

Re: Scissor Lifter?
 

Why and how? This seems to be a very contradictory veiwpoint here, and I'd like to hear your thoughts. What is your experience?

Re: Scissor Lifter?
 

We built one in 2003 stack attack and it stacked bins very nicely. The scissors lift only had to go 2 bins high max. ;)

Re: Scissor Lifter?
 

We built one in our rookie year (1996), we never even thought to replicate that experience since. I use a scissor type man lift here at work on a regular basis. In order for it to work well, a rigid precision frame and bearings in the joints are essential. The problem with most is the non-precision with which they are built. This leads to instability while moving and therefore intermittent loads that exceed the ability of the drive to overcome.
We have a do not use list too...
No casters/shopping cart wheels for obvious reasons.
No omni wheels on ramps.
Anything else that does not fit the game.

Re: Scissor Lifter?
 

Two more things that make scissor lifts hard:

1) One or both of the bottom sections has to slide across the bottom surface as it extends upward. Getting that to work smoothly with low friction and without wobbling around is tricky.

2) Powering a scissor lift without hydraulics is hard if you're lifting a decent amount of weight. When it's fully retracted, you have to supply a lot of force to get it started, because the initial angle is unfavorable. Certainly it can be done with motors, but I think it's a lot trickier than powering an elevator or an arm.

Re: Scissor Lifter?
 

Let me just repeat every one here: NO SCISSOR LIFTS unless you want to know what hell feels like. The only time one should even consider a scissor lift is when you have to lift for small distances, with only one layer. Anything more will be waaaaay unstable.

Re: Scissor Lifter?
 

Thank you! We'll keep these in mind if we prototype one.

Re: Scissor Lifter?
 

In our initial prototyping, we drew a number of designs on a big whiteboard for lifting the totes, including a scissor design. That image was quickly circled with red marker multiple times, and then in bold and exclaimed letters indicated as "Do Not Use!!!" I cannot tell anything else about our design (because we are still finalizing stuff) other than we will most certainly not be using a scissor lift because its incredible complexity and inconsistency. There are many more successful designs available, if you are stumped I suggest you start by looking at the various RI3D elevators and arms, they might serve you better.

Re: Scissor Lifter?
 

If you look at my avatar, we built one during my high school team's rookie year (1999, team 311). Took us 4-5 weeks and 2-3x the allotted weight to get it working properly. I tend to steer students away from the scissor lift, though I don't like to outright eliminate it as a possibility.

The 1999 application was before pneumatics and I think using pneumatics would make it to develop the scissor lift, but I still don't think it's worth the weight, size and complexity when other options are available.


One note; team 311 went to the rubber match as the alliance captain in the finals on Einstein in 2002 with casters.

Re: Scissor Lifter?
 

Our team had great success with a scissor lift in 2012. We actually won a regional with it. Here is a video of it. http://youtu.be/jFwkC_s2BAQ?t=53s

Re: Scissor Lifter?
 

While our team probably won't be making one, I'd bet that it's still possible to create an effective one for specific purposes.
How ironic that the best VEX robots this year all use scissor lifts, where the game requires precision and going really high.

Re: Scissor Lifter?
 

A scissor lift to raise totes is probably an overly complex and likely too slow in operation compared to the alternatives.

Ideally you should want to raise the totes in an about a second or two at most.

Re: Scissor Lifter?
 

I remember doing a scissor lift and once you get all the kinks worked out its fine, but the amount of moving parts makes it more likely that things will go wrong the taller you get with it. There is a niche that scissor lifts fill and if this niche needs to be filled go for it.

Re: Scissor Lifter?
 

I am quite frustrated by the "we'll never do X because we tried it once and it didn't work" attitude. I find it very narrow-minded and a bad precedent for our students. Technology, collective experience, and requirements all change, and we should be willing to reconsider past decisions and be able to admit to ourselves that we could have done something better.

95's name-sake robot, Grace Hopper, used a scissor lift with great success. The compact storage profile of the lift allowed the robot to be completely mobile on the field and still reliably, and quickly, score in the high goals. Also, Grace's scissor lift was fabricated with a lathe, drill press, and belt sander. Precision machining equipment is not required.



Here is an old match video to show what the field was like: https://www.youtube.com/watch?v=YCcBXMrR8DY

Scissor lifts have a very specific application: they allow a mechanism to reach very far while being stored in a very small space. With the lack of height-restricting obstacles and lack of size constraints this year I don't think a scissor lift is appropriate. But I would consider it in the future.