NEED HELP! Scissor lift..

Hey all, somewhat new to the forums and now in my second year of vex as our lead designer. We went with a four stage scissor lift and a single claw(for now) in order to have the ability to score on the high posts. But we cant get the arm to lift… We have very little support from our school and kinda have to make do with what we have, although i was able to get in some new stuff this year. We first tried a 4 motor horizontal slide, which did not work. then with one 393 on each side, went to a 1:5 center gear on the bottom stage. no power and no meshing. Then a 1:35. plenty of power but no mesh. then to a 1:12 plenty of mesh and very little power and motors made a chattery shudder noise, so then switched to the current 2 269 on each side with the 1:12 ratio and now they instantly trip thermals in motors… What can we do?? first comp is saturday…
Here is a link to pics of our design: https://www.flickr.com/photos/114791075@N07/sets/72157649600740485/
Thanks in advance

I think the 393s were causing your gears to skip because your 36-tooth gears are cantilevered. If you extended the axle holding the 36-tooth gear to the outside c-channel, you’d have a better chance at using the 393s instead of the weaker 269 motors.

Also, having the 3:5 gear redution happen before the 1:7 gear reduction will help significantly in keeping the gears from skipping.

There are a couple of things that you can do. In terms of gear ratio, you would want something relatively high, 1:21 would probably be sufficient. To prevent the gears from slipping, you want to make sure the axle is supported by bearings on both sides, as close as possible to the gear. Also, if you are doubling the 84t gears instead of using a high strength one, cut off the little extension on the middle, so the edges are flush, giving a greater surface area.

Also, I would highly recommend using 393 motors, and at least 4 of them. I know you might be tight on money and not have the motors, but it is definitely an investment well worth it.

Also, use more elastics. You want to get to a point where the elastics take most of the weight, and the motors need to apply force to get the lift down.

Running the axle through both was one of my first ideas, but if we do that, the arm wouldnt go down far enough to be in the 18, we are about a 1/4 inch in on all 3 axis, so i have is so it slips up into the assembly.

Could you explain the 3:5 reduction? Our gear guy went to the air force academy and i’m kinda missing him…

I’m not so sure 1:21 is sufficient. Of course it all depends on what you’re planning on lifting. For me, three cubes on a 1:21 with many elastic a will simply not due. It is however pretty decent for two cubes and a skyrise robot. Simply one man’s opinion though haha

Have the motors power the 36-tooth gear, which then powers the 60-tooth gear, which is linked to the 12-tooth gear by an axle, which then powers the 84-tooth gear (mounted directly on the lift c-channel)

unfortunately, there’s not enough room for a 84 tooth in between our scissors, to again, stay in the 18.

What do you guys do in terms of lubrication??

Most people use White Lithium Grease on the slides, but it isn’t necessary on the turning joints.

I also find that normal light machine oil works fine, but Lithium Grease is probably a bit better.

If you don’t have the room to run the axle through multiple sections, you can add a small piece of metal that goes to the other side of the gear and supports the axle. I can post a picture of what we did later today.

As to Lubrication I believe we use a white Lithium grease in small amounts. and we would use it only on the sliders given your design.

Scissors lifts have always been a challenge for us to get to work smoothly. Our team is building one this year but still haven’t got it to motivate smoothly, getting it started from the fully folded position has always been our challenge.

One tip you might consider is adding an eleastic (rubber bands or stretch tubing) across the pivot points to aid the lifting. I see you have them to aid stability that looks like a great idea, and some down on the linear slide to help pull the lower portion in. can you add some to the midsection to help with lift force?

Cheers Kb

  1. More rubber bands. Enough that you have to make it power down versus fall down and you have about enough.

  2. Your connection points only use a single black spacer. So somewhere in there you have metal on plastic friction going on. Having two 1/8" nylon spacers (or a nylon plus a teflon little guy) gives a plastic on plastic rubbing. That lowers the coefficient of friction to deal with on the joints. Your gear train should be able to lift the weight of the scissor, it’s the friction you need to overcome as well. Also putting the teflon washers on the outer parts of the bolts gives that something to slide about (both screw head and nut ends). Teflon washers on the outer parts of the axles on the bottom to eliminate metal on metal rubbing there too. (not as likely given it is a square axle but you never know)

  3. Are the screws sufficiently loose on the connections of the scissor? There is an incredible balance for the tightness of the scissor joints versus free spinning of them acting as axles. They are axles here, not structure.

  4. (minor for now) the back bracket placement of the connection is far from the bolted on part. This creates a twist motion that may not last really well.
    https://flic.kr/p/qfp5b9

With the screws, I snugged them down then be ked out a half turn so there is enough movement and less lean.

This morning we took the top half of the scissor off to see if we could at least do something other than a push bot, and went back to 2 393s. With have more but on our drivetrain, and with me being a car designer, it’s pretty beast, the only good part on it, and I’d rather not take it apart. But the motors still made the noise. Here’s a video of it:https://flic.kr/p/qhVcAH

A shaft to synchronize the left and right sides will help with the lift too.

This way it will go up evenly

I found that even if you can just connect the bar on which the motors are mounted together by a c-channel or something, it helps tremendously by creating a similar effect the connected shaft. Both are easy changes, and they would make a world of a difference.