I don’t have much experience with scissor lifts but was wondering what is the best way to build one. I’m wondering what is best a rack and pinion or a scissor lift with a gear and motor attached directly to the first stage? Also what do you guys think are the best parts for the slide that attaches the base of the lift to the robot? Linear slides or chassis rails?
Most people seem to find that a gear with motors driving it at the center of the first stage works best.
Teams tend to play around with different options for the slide rail depending on their chasis design.
Rack and pinion
Generically, two ways to do this, powered horizontally, or vertically.
Powered horizontally, you have the problem where the lift requires more torque to lift when compressed vs when the lift is raised up halfway. This requires you to double up the torque, just to start up the lift and then waste it when lifting the rest of the way at a snails pace because you don’t need that much power anymore. Thats why horizontally powering it sucks.
Powering it vertically eliminates this problem, however it has to be well built to avoid annoying constant lubrication maintenance and slide damage. Well built as in there has to be enough support, not just a vertical slide attached to the scissor construction. FYI my vertically powered scissor lift sucked.
Powering it from the center of the scissor(doesn’t matter first stage or what ev, you can power it at any stage)
This is in my opinion the best method, because it eliminates non linear torque requirements the horizontal powered rack and pinion has, and also is thinner than a well supported vertically powered rack and pinion.
P.S.
Its not called chassis rail its called slotted angle’s fyi.
Sorry that bothered me
Also slotted angles do not have as much travel as linear slides do, and provide both less structural support and reductions in friction/wear.
You can search some posts on here for more scissor lift threads. There’s already a lot on the forums.
With that said, the lift mechanism and placement of the force transmission point really matters.
See the documents referenced in this post. It gives different lift configurations and the effects on the force required. It also discusses lateral loads in one of the document which is another tough part when going 60 inches high.
Friction is a tricky mistress too. Never have metal rubbing metal. She will gum up a scissor more than anything else. I am not sure that has been covered too much in other posts directly other than discussions on grease.
Haha yeah I was looking through autodesk inventor at parts and probably accidentally looked at the name above the picture of the part I was talking about 
Anyway thank you very much for the help!
Having built a rotationally-powered scissor lift, I can tell you that they do have problems with the torque being high at the bottom and having less strain at the top, similar to how an n-bar needs more torque when it is parallel to the ground than when it is above or below that point. However, I still think powering the scissor at the center is the most efficient method, because it is the most direct way to transfer torque to the lift itself.
Linear slides are generally better, although you have to be careful with how you mount the scissor lift to the slide and make sure your load distribution is even, because if the weight of the scissor is twisting the slider at all, then it will have huge amounts of friction.
Good luck!
Hmm, that is a valid point I have never thought of. Perhaps it is the case, makes me wonder if then a vertical rack and pinion is truly the most linear method of powering the scissor lift.
Although there is a problem with vertical R/P, that is that when the lift is just barely fully extended, it has the same problem as horizontal R/P, just when the lift is almost fully extended, and almost as in maybe an 10-5 degrees from full extension.
Look at 400x’s scissor lift. It obviously works well, but we adapted it to where it can lift with five stages.
Also use the metal standoffs in between so your scissor lift starts partially lifted I recommend the .75 in ones.
Scissor lifts are ongoing projects and a pain in the beginning but when they work they are worth it.
I have been obsessed with scissors since my start of vex. Last year was my first year and my first design is a two stage scissor lift, powered linearly from the bottom. This year i have done a lot of research, CAD and experiment, and i can tell you few things i would focus on when building scissors.
First, balance. For new teams, one thing scissors do great at is wobbling. Therefore, don’t expect the two sides to rise at the same speed without any balancing method. Many great scissors wobble, waste time on the field, and ruin themselves. Therefore, if you are to build one, build a balanced one. Methods of preventing wobbling include adding on structure to connect two sides of the lift and using program. You can use hard structure such as 15 c channel as what 400x did, or 12 c channel as many teams do. Or rather, you can use a long shaft to directly connect motors on both sides, like what team 80n did in their gateway robot. About programming, there are a lot of options. You can use two potentiometers, one on each side, to monitor the situation. If the difference between the two exceeds a certain value, instruct one side to slow down. Also, use the second joystick analog channels to separately adjust two sides. But always remember to do backup safety code. You never know when your sensors will return incorrect values.
Second, structure. A scissor system is rather complex and a lot of options are there for structure. But whatever you do, remember to look for the most light weight and durable choice. You can simply connect c channels. Or you can do what 9090c did, cutting one c channel into two long angles and use that as structure. Remember to mind the bottom joint, better use steel there. The bottom joints take up ridiculous weight and the joint will gradually eat through aluminum.
Third, power. Four ways to power scissors in vex i have seen so far. You can move one joint on the bottom horizontally and push the lift up. This requires tremendous torque at a lower position and outputs high torque at high position. You can attach one center joint, usually the lowest one, to a verticle linear slide and power it vertical ly. This gives you equal speed at any position. You can do a rotational joint lift, which will give you relatively higher speed and consumes less space if built properly. You can also change the distance between a non-central joint and a fixed point on the base with a rack amd pinion system. Team 80n used this in gateway and it was so cool. Elastics is basically required for a scissor lift system, because there is no reason not to utilize the free power. When using elastics, i typically remove the motor and add on one rubber band at a time until the lift is able to stay at most positions without external force. I personally suggest to do a little bit testing prior to putting on any motors. Not that crazy science analysis, just remember not to use noisy, old scruffy motor on one side and brand new ones on the other.
I would personally recommand a rotational lift, rubber band and a lot of lubrication. With these things in mind, you should be able to start smoothly. Remember that every successful design is the combination of tremendous amout of research, design, experiment and program. It takes extra dedication to build a perfect scissor, but a perfect scissor has advantages other systems do not possess, including attracting judges…
Good luck building yours!
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Could you clarify what you mean by problems in vertical slides here?
Most of the designs I’ve seen so far involving vertical slides put far too much strain on the poor plastic inserts 
From personal experience, I would highly recommend physically linking all the motors on your lift, because even a small difference in the bottom stage of the lift is multiplied 4-5 times near the top of the lift, and it really doesn’t take that much to have your lift flop sideways, especially if one side becomes caught on something. After a certain point, no amount of programming could really save a tipping lift.
The problem that Stanley is talking about is the fact that, because a vertical-slide-powered scissor is essentially a horizontal-slide-powered scissor turned sideways, it has the same torque problems, but reversed. A horizontally powered lift requires more torque when it is nearly or fully compacted; a vertically powered lift requires more torque when it is nearly or fully extended.
In addition to that, any rack and pinion system will have tremendous friction, especially on a scissor lift that reaches to 60 inches. Plus, the weight of a scissor lift that tall is likely to shear teeth off of the rack gears unless the system is very well supported.
The vertical slide has the same minimum required force for any height of the scissor lift. If you think about the amount of torque on the center joints required to lift a certain load, a vertical slide exerts the most torque at the bottom and the least torque near the top.
You’re right in that since rubber bands become very loose when the scissor lift is all the way up, the motors on a vertically powered scissor lift can have more trouble than a centrally powered or horizontally powered scissor lift, which have a lot of excess torque near the top of its range of motion.
You definitely can use vertical slides this year- you just have to be smart about how you build it.
I don’t understand quite what you mean here. First you say that the vertical slide has the same required torque no matter the angle. Then you say there is more torque at the bottom than the top? 
I meant force for the first bit, and torque (as observed on the scissor joints) for the second bit. Sorry if I wasn’t clear
I am sorry to interrupt, but i will have to disagree on this one. A vertical rack amd pinion scissor lift theoredically requires the same torque at the top and the bottom. When you change the height of a joint directly with rack and pinion, the total height of the system is linearly proportional to the distance the motor has traveled.
The situation is different in a horizontal rack and pinion lift. To calculate the height with the distance the bottom motor has traveled, you will use Pythagorean theorem. The function between the distance the motor has traveled and the height is a radical, concaving up function, which proves that the lift rises faster at lower position and slower at high position. This is why more torque is required for a horizontal rack and pinion scissor lift at a low position.
Again, sorry to interrupt the discussion, but i don’t want people to get mislead. Please tell me if i am wrong. Just my personal opinion.
You don’t have to let this happen. Try to reduce the shape change in rubber bands by mounting them in a certain way and use more rubber bands than usual, so that the rubber bands are very tight even at the top and the force they provide doesn’t drastically change. Pretty sure you know how to do that.
No, because if you have a one-X scissor lift, horizontally powered, and you turn it on its side; you have a vertical scissor. Only the direction you power it is inverted, so the points of high strain will also be inverted.
Picture a one-X scissor with really bad CG (center of gravity), that tipped over, and became a wall-bot. Now you have a vertically powered scissor lift.
However, as a scissor lift nears its max height, the scissor itself moves more horizontally than it does vertically, so the vertical motion of the rack and pinion is being turned into horizontal motion by the lift. Since horizontal motion isn’t very useful as far as lifting is concerned, this energy can be considered wasted.
Those two instances are not the same thing. The direction the scissor lift will try to lift things will also be inverted, or else you’d have five useless stages acting as a single vertically powered stage.
CCA’s right, the height of the scissor lift is directly proportional to the distance travelled by a vertically powered rack and pinion.
As a scissor lift reaches maximum height, the linear slide also has to move less to move the scissor. These two factors cancel each other out. In fact, vertically powered scissor lifts are the only design out of the three most common (horizontal, Chinese, vertical) that don’t “waste” energy at the top.
GUYS
when i talked about an increase in the need for torque on a vertical scissor lift, i meant when the scissor is almost at the top
WHY???
because, when teh scissor is almost at max height, the two linear plastic slides attached to the rail on the bottom of the lift are so close together, that when the vertical rack and pinion is pushing up, its pretty much pulling straight on the scissor without enough leverage (because the X i almost fully compressed in raised lift mode) to move the slides together at the bottom of the lift
I hope you understand what i mean
It can be very misleading because I meant when the lift is just, just barely at max height due to the scissor being so close to reaching max vertical compression. Although the logic of putting a horizantily powered scissor sideways is correct.
If you stand on a vertical pole, it wont fall over.
If you stand on a slanted pole in the ground, it will start to bend downwards under your body weight.
Now picture it with your body weight as the way the scissor is powered, and the poles angle the amount the scissor is compressed and you can see what I mean
Again may be hard to understand because of the lack of diagrams.