Vex scissor lift slider function

Hello, I am making a scissor lift for the season and it seems like there are many ways to make the slider function in a scissor lift. But, I have never built a scissor lift so can anyone tell me different ways to make the slider function. Also, the majority of the robots are using bar lifts so would you recommend bar lifts or scissor lifts?

for your first lift i definitely recommend a bar lift, as they’re easier to build well, while scissors are extremely hard to make well, especially with tuning and gear ratios. you can make a functioning scissor, but it probably will have alot of friciton and slop.

I agree that it is difficult to make a scissor lift good on your first shot, but it will be rewarding and a great learning experience. For M.A.R.A.'s lift we went with a joint powered lift at a 7:1 ratio. What we did to connect the lift to the slide rails is to connect a a pillow block bearing (https://www.robotmesh.com/vex-robotics/vex-pillow-block-bearing-lock-bar-pack) to the inner linear slide trunk. The method we are using does use 8 total linear slides, so that does get kinda costly(insert plea for individual slide rail sales to vex). I attached a picture on how we have done it, but your team shouldn’t copy what we have done because we are using college sizes.
20170815_151002.jpg

Scissor lifts are an interesting building experience, but can be some of the hardest lifts to tune. Don’t let the forum discourage you from trying one, but if you get hung up for a long time it may be easier to switch to another type of lift.

You are correct in saying that there are many ways to use slides in one. To name a few ways you could:
-Attach the front bar of each side of the scissor via a screw or axle to the base, and let the back slide forwards on a linear slide, resulting in the weight and your load shifting forwards. (Recommended if you’re lifting via gears attached directly to the scissor.
-Attach the back bar of each side of the scissor via a screw or axle to the base, and let the front slide backwards on a linear slide, resulting in the weight and your load shifting backwards. (Not recommended, as your intake of choice would move internally as you lift.)
-Lift the scissor in the middle using two linear slides going vertical, using linear gears to pull yourself straight up. Then you don’t have to attach either side of the scissor directly to the base, but let the ends slide on linear slides inwards. This allows the scissor lift to always stay centered. One issue with this is that the lift will struggle at first, but go faster the higher it is.

How did you power it?

Rotationally, from a joint. As well as linearally.

Linearly, with the slide moving horizontally (parallel to the ground)?

both that and vertically

I believe that powering the scissor about a joint should require constant torque, and not get easier as the lift goes up.

It’s been a while since I’ve thought about a scissor lift, though.

The effect is also due to rubber bands, however its also alot in how its powered. For example, the effect is almost non existent in a rotationally powered joint, however it is still there, as evidenced by the speed graph that my team used to have based on real world performance, and graphed using robotc outputs

You guys have it backwards. A scissor that is powered by rotating around a joint requires more torque at the bottom and less at the top, while one driven from a vertical linear slide in the center will require constant torque as it goes up.

acutally, none of you are right, as they all require more torque at the bottom. as proven by experince

I care less about experience, and more about the physics behind the lift.

I’m in class right now, but some rough figuring:

Powered horizontally by linear slide:
Undoubtedly requires more torque at the bottom, as the motors begin by pushing essentially parallel to the scissor bars. As the lift goes up, the motion of the motors become more perpendicular to the lever arm, and less torque is required to have the same motion/speed.

Powered vertically by linear slide:
As far as I can tell, exactly the same as an elevator lift, and thus, constant torque required all the way, as the lever arm, and angle between the lever arm and direction of motor motion never changes. However, I don’t see a benefit to doing this with a scissor; you may as well just do an elevator lift. (I’m not a huge fan of scissors, so there are probably benefits I’m not seeing.)

Powered rotationally:
This one I’m unsure about. The angle between the motion of the motor and the lever arm it’s mounted on never changes, but as the scissor goes up, the motor is pushing across, not up, and does not have to fight gravity as much, so it may require marginally more torque at the bottom. Again, I’m unsure on this one.

This supports what i am seeing too. Horizontal slide is harder at the bottom and easier at the top, vertical slide is even the entire way, and unsure on rotation. I will see if i can do math to say one was or the other for it.

+1, ask @400X. He used a scissor lift in skyrise with a vertical slider, giving him a constant power curve.

Old but great thread…

The critical thing is getting that scissor started.

Read this old thread… And @vamfun has a nice blog entry about it.
https://vexforum.com/t/scissor-lift-calculations/26338/1