I am looking for new concepts on scissor lifts, i’ve looked through the gallery and forums but keep adding up to the same design.
I am searching for scissor lift ideas in which it could possibly withstand weight and move in a fast manner.
At 2011 worlds the chinese team for middle school had a scissor lift in which i learned about but didnt have alot of information on how it works and pictures that correspond to it. Any information on that?
I have also developed a design similar to corpralchee’s design but it didnt have the correct speed or torque to move in a quick yet strong pace.
Any new ideas Im all ears.
Please need info on new or better ideas?
I do understand what you are saying
However i thought it would have been best to invent concepts and ideas other than look up information on the chinese team, although it would be a good use to have info on that type of scissor lift.
We used a scissor in FRC a while back and one of the important lessons I learned from using them is that when you try to drive them by pulling the two bottom point of contact together it is very hard to lift the scissor up. This makes sense because for force is only partially being applied to upward movement (less force the farther down the scissor is) However if you drive your scissor by pushing up on one of the centers of an “X” you will find that you don’t run into as many issues.
Scissors also have to be built to be structurally sound in the side to side direction. Usually people double up 2 scissor lifts and link between them to combat this as the team you talked about in your original post did.
Torque is an issue… The less “extended” the scissors are, the more torque it takes to lift. Here are some torque equations to help size motors/gear ratios/etc:
Note that a scissor-lift is a sequence of square 4-bars.
4-bars are reported to work better when more of the joints are powered.
Most scissor-lifts have completely unpowered joints.
Scissor-lift have a large variation in transfer function from input force to output motion, which makes it hard to efficiently match to the motor power.
Where:
F = Force provided by the jack-screw or hydraulic ram,
W = combined weights of the payload and load platform,
Wa = Combined weight of the two scissor arms themselves,
A = Angle between the scissor arms and the horizontal.
This is the big problem with scissor lifts. A few ways to combat this are to add a spring assist or you can change the way you raise the scissor lift. If you push up on a joint you will have better luck because in general you aren’t going to be lifting your scissor as far up as it can mechanically reach so you are in the power-house zone for this lift method.
You could put latex tubing between the top left and right parts of each “X” and that would assist the lifting. Thats the easiest way to do it with vex parts.
To prevent the problem of inconsistent force, has anyone tried using vertical linear slides to power a scissor lift? It could be between any two (or more) levels of the lift, and it would maintain constant vertical force (I think).
It could be mounted like the rightmost arrow in that sketch so it’s only pushing one level (instead of being mounted on two moving joints).
It would be just like any other rack and pinion-powered scissor lift, but the rack would be vertical.
Even if the rack and pinions were “floating” between two levels, it still wouldn’t matter too much, as long as the joints are strong.
This is the type of lifting I was talking about earlier. It is good but not great. You don’t get consistent force however I have found that for because you don’t usually lift your scissor all the way up to its mechanical limit (when the “X” becomes more like an “I”) this style of lifting a scissor is better suited to scissors. Just as the other style of lifting is horribly inefficient when trying to lift the scissor the first few inches when it is fully down, this style of lifting is horrible inefficient when trying to lift the last few inches.
In FRC one year we used both methods of lifting with great results. We used some springs for the vertical linear force and a pneumatic piston for the horizontal linear force.
How does it lose force when the lift gets narrower (higher)? As far as I know, the force should be constant because when the linear slider extends n inches, the lift extends n*number of rhombi (or triangles or whatever) in the lift.
A scissor lift tends to be more stable and does not bind. You also don’t need to worry about chaining or using lots of rack and pinions. It’s basically just easier to implement and should give you less problems (if done right).
I personally prefer the linear lift, just because it’s smaller and I have plans for the whole volume of our robot.
