Hi, we’re a new team and we need some help improving our scissor lift. At the moment it will lift but once it lifts up we cant get it to go back down again and the motors cut out. Were using a turntable with 2 high strength metal 12 tooth gears and 2 motors geared for torque not speed. Please look at the pics below and please give us some advice:)
Team #7348B
I think there are two main problems you have right now, friction and how you power it.
I would never mount the motors on the very bottom or top joints. Try to fit in the middle X joint, this will give you a lot more power when lifting. As the turntables are quite large, you can also try using a 12 tooth gear rotating around a 84 tooth gear.
One of the biggest issues in scissor lifts is friction, which need to be reduced as much as possible. First, make sure you put some bearings on all your joints, and then make sure the screws are tight but still turn well. Also, mount your linear slides flat, instead of at 90* like they are now. With the current setup, all the downwards force will want to make them tilt in the rail and bind.
Hopefully this will help!
I have actually been thinking quite a lot recently about scissor lifts, even though we won’t be using one. After much thought, I came up with a completely new idea, that at least to my knowledge, has never been built before in VEX. As soon as we have time to prototype it, we will get a video up on our YouTube channel, which will hopefully be within 1-2 weeks.
Isn’t there always so much more that you’d like to do than you have time to do? I feel that way constantly…
I haven’t tested this, but to the extent of my knowledge, you’ll need a gear ratio twice as high on that lift as the one you’d need on a center-powered scissor lift, so the 7:1 ratio (I think) on the turntable bearing won’t be enough to power the scissor lift. Other than that, I don’t see issues with powering the lift from the bottom joint.
Other than that, the plastic inserts on the slides will probably have a bit less friction if they’re placed with the screw holes facing up rather than sideways. Good luck on the scissor lift!
1:7 with four motors would work fine if done right. It would just need a lot of fine tuning and elastics, so as it is a bit tight I would recommend going with something higher.
Unless I am thinking about it wrong, at the bottom joint you get half the power. Think of it that the turntable moves half the degrees as at a middle joint. The issue is that you also need to get all the slack through the 3 stages, and that adds up as well.
Friction is a tricky mistress. Reduce friction wherever possible is the key to a good scissor.
Did you put plastic washers in between every joint? No metal on metal connections or friction goes up. A touch of lithium grease can help too.
For the joints, make sure the screws are not too tight. Fingers should be able to turn the screws a bit with the slightest tug. Otherwise there is friction holding that joint tight. This is a tricky one because too loose means unstable.
Are there bearing blocks for every joint? That extra room for the screw can be bad.
The number of rubber bands needs to be increased dramatically. Have you looked at how it moves without the motors? Does it move smoothly? Does it naturally want to stay up versus down? That’s when you know you have enough rubber bands on a scissor.
We have a team trying to use turntables too, but that 5:1 ratio may be too small. They’ve even geared it up an additional 3x by stepping into the turntable with limited success.
Thank you very much for all your replies. The team will meet this weekend and we will read all the posts carefully. We’ll send an update when our lift works better. 
Team 7348B
Thanks for the help we have decided to put spacers to decrease friction between the metal. We are also planning to put more rubber bands in order to have more power. In addition to our rubber bands, we will move the turntable to the first intersection (instead of the bottom) to have more power in the scissor lift. Thanks again
Team 7843B
You should move the turntable more towards the middle of the lift.
Here is our team’s scissor lift from last season. we were able to score buckys in the cylinder consistantly and set the big balls on top.
We used :
We worked had to keep the friction equal for both side to go up and sown evenly, but the obvious improvement would have been adding encoders to sync both sides.
