My team's trying to get a head start for Toss Up and we're considering, among numerous other options, a scissor lift. We gave the scissor concept a few days' work during gateway and failed miserably. Since then, we've done a fair amount of research, but some questions still remain:
1.) How would you suggest powering a scissor lift: using rack gears and linear slides or using direct driven gears that walk around a central fixed gear?
2.) Just how compact can a scissor lift get before it jams and will no longer raise?
3.) What is the best part to make the actuall scissor beams out of? I’ve seen a lot of L and C channel, but confirmation would be wonderful.
Any help you can offer will be greatly appreciated!
A scissor lift is perfect for Toss-Up! I would recommend using either 1X2X1 channel or 2X2 angle. Keeping in mind the second moment of area the 1X2 slotted angle that many teams use would be too prone to twisting under rigorous game play. With proper design which I will leave up to you the lift can be very stable even all the way raised, we have very little lateral movement of ours even at 42" after finishing the reinforcements. I would also recommend a direct drive, but that’s a matter of preference.
A 3 stage scissor lift made with a 25 hole material will get you under the 12" bar and still allow you to hang without the construction of a complicated hanging mechanism. Also keep your weight distribution in mind, you will need to be able to raise the base fairly promptly to hang. With some careful consideration during your design process you can also maintain a relatively constant center of gravity with scissor lift.
Although I don’t have too much experience with Scissor lifts, our C team had an amazing scissor lift this year. They had the “walk-around a central gear” mechanism like you described. They used 25 hole 1x2x1 c-channels.
We also plan on a scissor lift this year! We we’re thinking about 35 hole C-channels instead of just 25 hole for more height, but again, we have little experience.
It’s certainly slower than a 4 or 6 bar lift, but we really don’t think it’s an issue with this game. There are going to be very few robots that can descore well from what we can tell. The robots that can descore such as ours will most likely only be able to get the top 2 out leaving the bottom 3 balls in so once you have stashed the balls if you and your alliance can get the first 3 balls in each 24" goal then you are good there. We can high hang in about 4 seconds using a 21:1 ratio which we believe is fast enough, and raise to the 224" goal in about 3 seconds… Moving around the field and gathering the balls is where the speed will be important in our opinion.
There will be some brilliant designs that perform better than scissor lifts, but I think a large number of the good performing robots that can perform all of the game objectives will be utilizing scissor lifts. But there will be those designs that we never thought of that blow everyone else away.
Great work sbdrobotics and your team! That kind of versatility is exactly what my fellow designer and I were hoping to hear of. I really can’t wait to get building now!
So you want to go the Scissor route huh?
Think twice
Please take my advice, Scissors work well when they work but there is alot of “Behind the Scenes” at work. What I mean by this is that although scissors look awesome and are good at lifting and collapsing it takes NUMEROUS Hours of tweaking.
We started building our first scissor at the beginning of the Sack Attack Season, and rebuilt it at least… 7 or 8 Times!!!
You go through lots of Rubber Bands, Rebuilds, GREASE, Motor Gears, and Last but not least Patientce. You just have to kind of go with the flow because eventually you’ll get the hang of it.
I would not reccomend this because it will take up the majority of your time and then you would be rushing to finish a intake and/or manipulator and even a base.
If you really want to know how to power, and build and stuff I can be helpful in the Process.
Here was my team that did not make it to worlds, but made it to semi’s at Nationals. The scissor cost us Worlds
I would just like to point out that it doesn’t have to be slower than a 4 or 6 bar lift. The speed really depends on the gear ratio and the number of stages in the lift. During the Sack Attack season the robots that we brought to competitions had lifts with gear ratios ranging from 7:1 to 21:1. The resulting speed from ~14" to ~45" went from less than 1 second to around 3 seconds. While in the end the worlds robot used 21:1 to be able to hold more than 15 sacks, the point is that a scissor can be just as fast, if not faster than a 4/6-bar.
I would also partly agree with this, but what lift doesn’t require hours and hours of tweaking, especially when you build it for the first time? True, it may take more time than a 6-bar for most teams, but that is because many teams have built 6-bars before. If you’re willing to put in the numerous hours to learn to build something new, you’ll receive a great benefit. I know I plan to use all that we have learned this past season in many future designs.
I agree with Tiger, we tried several different lifts last year and each posed its own challenges and took a lot of tweaking. We have a lot of time invested in our current lift and have made a lot of adjustments and revisions.
While not competition tested our lift raises fairly quickly and evenly, but again we are not a public school based team that cannot start building until September either. We have plenty of time to abandon our ideas and start over if needed, but so far we are very happy with what we have.
I say if yo want the versatility of a scissor lift go for it, but be prepared to spend some time getting it right. I think the time it takes to work the bugs out is worthwhile and rewarding as long as you don’t wait until the week before your first match to build it.
Although this one is fast, We managed to get a even faster speed with a ratio of 24:36 and it was extremely fast. It lifted well as well. I would advise this ratio for optimum speed and enough torque to lift 20+ lbs
This type of learning by doing is what my team really excells at too. We have an incredibly efficient design/build process that allowed us to be happy with our robot (which had a downright weird gearing system) in just over a month. From there, we spent every hour of our TEC meetings driving until we found a problem, tweaking, and repeating. Our bot did pretty well too: got us second in robot skills at the US nationals while hardly differing from the original design.
I definitely think that we're up for the challenge....I'm kind of excited really. Nothing makes my day more than spending six hours of it building and testing!
edit: Like I said, 21:1 is the largest gear ratio we used, might be about enough torque to hang. (2 motors) If you don’t plan on hanging with the lift, you could do 7:1 just fine. With the right amount of elastics, 7:1 can be crazy fast.
Vamfun has written a few blog posts about scissor lifts. I suggets you read his stuff.
He referenced two really good papers in showing what loads are at different lift points. Look at these papers and based upon how flat you are versus your connection points, the force available for the lifting if pretty bad for fully flat scissors, but really good up top. Those first few inches of lift are painfully slow but it gets much faster as it rises in many commercial scissor lift configurations.
Like others have said, scissors can take a heck of a time to build but once you get it working, you will probably be happy. Just make sure you have 2-3 months to build and tune it. Side to side loads can be tricky as can motor skips if you are not using linear slides.
And lastly, with all those connection points, friction is your main enemy besides the weight added to address side to side loads.
He referenced two really good papers in showing what loads are at different lift points. Look at these papers and based upon how flat you are versus your connection points, the force available for the lifting if pretty bad for fully flat scissors, but really good up top. Those first few inches of lift are painfully slow but it gets much faster as it rises in many commercial scissor lift configurations.
Like others have said, scissors can take a heck of a time to build but once you get it working, you will probably be happy. Just make sure you have 2-3 months to build and tune it. Side to side loads can be tricky as can motor skips if you are not using linear slides.
And lastly, with all those connection points, friction is your main enemy besides the weight added to address side to side loads.
We we’re thinking about 35 hole C-channels instead of just 25 hole for more height, but again, we have little experience.
