Desperate Plea for Mechanical Help

VEX V5 General Discussion General Forum
#1

Hey everyone,

Since early July (after TSA Nationals), our team has been working hard to build any variation of a working lift for this years game. We’ve built 7 (no kidding) different scissor-lift prototypes, a 4-Bar prototype, and an elevator lift prototype. At this point in time, just one of them has worked, and it is ridiculously slow.

The working lift is a rack-and-pinion based scissor lift, where a rack-and-pinion works to push the scissor-lift up. The rack-and-pinion is mounted vertically on the side of the scissor-lift, and while it “works” it is incredibly slow and unstable.

Regardless, we are at wits-end with these robots. It seems everything we do we cannot do correctly. We eventually (desperately) tried to replicate a robot we saw in a video and even that one would not work, so we must be doing something horrendously wrong with our scissor-lifts. Members are even quitting because they are so discouraged by the failure we’re experiencing this year. It does not help that we did so well last year, either, haha.

So, I am reaching out to you all on the forums looking for any tips on general scissor lift construction, or preferably a link to a “guidebook” for that. I know this is a very general question and you all may not be able to answer, but you’re a great community and have pulled through for us before :slight_smile: I’ve linked some pictures of our current build in Imgur down below. Thanks everyone!

Our only working prototype: http://imgur.com/a/bd3lY
An earlier prototype (gear ratio too low to work): http://imgur.com/a/X91V8

#2

How slow is slow? ie. how long does it take for full lift?

#3

Try to space out your stages and add rubber bands to support some of the lift’s weight.

FYI: From what I’ve seen in my club, a 4-bar (or any x-bar in general) is usually easier to build than a scissor lift.

#4

From the pictures, it looks like you have a solid design and good build quality. Scissor lifts do take time to fine tune, so don’t give up.

I noticed you are using screws with bearings for the joints, which is very good. On the screw that connects the two pieces, make sure you are using a nylock nut, as this allows you to not fully tighten the screw while still keeping the nut secure. You want to make sure the screw is snug, but still turn relatively well.

To make your lift faster, you will want to use a larger gear on the motors to increase the ratio. However, this also gives you less power, possibly too little. What you will want to do that should help you a ton is use elastics (#32 or #64 from any store) or Latex Tubing from VEX. This way, you can balance the weight of the lift, ultimately you want to be able to take the motors off and the lift holds itself at around the middle.

#5

It looks like on your ‘only working prototype’ the gearing of the motors on the lift has some issues. From what I saw, you have your two motors on each side sharing an axle and with a 12 tooth pinion which is fine, but this is driving a 36 tooth gear that doesn’t have bearing blocks and is then driving the rack. If this is the case, not only is the 36 tooth gear totally unnecessary as it is not contributing any kind of gear ratio, but as it is mounted without bearings and the teeth are meshing at an odd place (what I mean by the teeth meshing, is instead of the line between the two axles the gears are on being parallel to the vex metal holes, you have it at an angle allowed by the slider bracket), it will be hindering your arm.
It does seem from this and the other link you have shown that either I’m blind or there has been some confusion when building on how different gear ratios are achieved :confused:
Try removing the 36 tooth gear entirely and have the 12 tooth drive the rack directly, or if you want more speed and less torque, have the motors drive the 36 tooth and remove the 12 tooth. Hopefully this makes sense and helps :slight_smile:

#6

You need to use elastics. NEED. no elastics + heavy weight = stripped gears

#7

There’s already been a lot of good advice from this thread, so I won’t waste space and repeat it.

Because you are using 4 motors on your scissor lift, you should have plenty of power. Driving the Racks with anything larger than the 12 tooth pinion will probably be too drastic, but I would highly suggest switching your lift motors to their speed gearing. This will give you a significant speed boost, without giving away all your power.

You might also look into adding vertical elastic assist, where you have elastics attached to the top of your sliders and to your motor pods. This will help a lot at the bottom range where the horizontal rubber bands don’t do as much.

It looks like it should be very stable. Perhaps a video could give us more insight if you need more help?

Good luck!:smiley:

#8

Thanks for the feedback!

For the prototype, the outside gear (without the bearing) is just a restriction. Without it, the motor pods jump the sliders. It doesn’t touch the driving gear directly, it just rolls up the side with the rack-and-pinion. We’ve just added bearings to that gear, however :slight_smile:

The second one was a 1:1 gear ratio, which was why it didn’t work. I just posted that to show some insight into a failed kind of build. We built that kind of lift with many different ratios but none worked.

We are currently in the process of making another four bar. Is everyone doing double-reverse? Also, we’ve seen double four-bar a few times, any success with with that?

Anyway, here is a video of the current working prototype build. We’ve noticed that the lowest X opens the most, and subsequent Xs open half as much, a fourth as much, etc. I’m pretty sure this is a huge problem with lift, but so far we’ve been unable to fix it. Trying to start the lift partially open hasn’t helped either, because the same thing happens. Also, this is the fastest build BY FAR. With cubes, it’s about 3/4ths as fast. The slow issue isn’t SO MUCH an issue here (although increasing speed would be nice) as it is on the other prototype, which takes about 11 seconds to fully extend without cubes, 13 with.

Video Link: https://www.youtube.com/watch?v=x37z-ir2Y0k&feature=youtu.be

Also, I uploaded two photos of our non-working build. This is one of the ones that we “copied” from a video, although I think this may not work because it’s based for an “even” scissor-lift, and ours is an “odd” scissor lift. We’ve adapted it for an odd, but still.

Image 1: http://imgur.com/SbJTKIF
Image 2: http://imgur.com/3fVY5lI

EDIT: We got it working this morning, but it is incredibly slow and unstable. It almost seems like one side is much weaker than the other. Here is video footage of it working.

Video: https://www.youtube.com/watch?v=PDgHD6xfCyY&feature=youtu.be

Also, we’re trying vertical elastics, although last time we think it was responsible for some gear skipping.

What is everyone doing for sliders? We just have rails with pillow-block bearings. I think it might be causing issues with pulling up, but so far we haven’t found a way to fix it (we’ve tried grease and elastics).
Image 1: http://imgur.com/MfubCsB
Image 2: [http://imgur.com/f6gvhnz

Thanks again everyone!](http://imgur.com/f6gvhnz)

#9

When I first started out with large lifts, I had no idea how much elastics they needed. I would say that if you haven’t exhausted the supply of latex in your shop, you should add more. Like about 5 times the tubing you have on in the second video. I think that’s the only thing wrong with it. The wobbling can be fixed with code (there has been a lot of discussion about that type of stuff on the forums lately). I wouldn’t worry about the speed just yet; get it working well first.

#10

You know if you have enough elastic is when you can no longer able to collapse the structure back to start position and then you start taking away elastics to reach the optimum amount.

#11

make stoppers on all layer of your lift so that it will barely (within 1/2 in) fit inside / under your rack gear structure. you wont have power so hard to get the bars un collapsed. you might also try powering it on the horizontal slider after doing this.

or the first or second middle joint kind of like in your first post, but with a substantial gear reduction. something like a compound 1:3*1:7 should work.

#12

You need to see the video made by Hai Sing Catholic School in Singapore. They are now using Vex hinges for their scissor lift. I looks very stable and they have some of the top teams in skills.

#13

The Singapore hinge solution is pretty good. However, when working with aluminum c-channels, use some c-channel couplers to reinforce the structure at the hinge.

#14

As for stability, one thing that our team has found helps is to put cross bracing c-channels at all of the rotational points on the scissor lift. This helps to keep the two sides level physically, however it’s not perfect. We also use a set of quad encoders to keep the lift fully level.

#15