scissor lift help

Any help or pointers on making a scissor lift more stable?

We’ve got a lean and occasionally we can’t even get enough torque to lift the thing… I know scissor lifts aren’t full proof but the kids liked the design so we went with it…

thanks for any help

The lean means stabilizing left to right and reducing slop in the joints.

For lifting, the initial push can mean a heck of a lot of torque required. Look up some old threads from @vamfun on here. He had a very nice reference from the Navy written ages ago showing the forces as you fold down the scissor.

Rubber bands are your friend here. Lots and lots and lots of them. Unhook the motors and you know you have enough rubber bands when the scissor does not want to stay down,

any pics of how the rubber bands should go? I have thought about using them but i’m not sure where they should really go.

So I guess to fix lean I need to tighten up the bolts. Do you put a washer between the 2 metal pieces?

thanks!

There are many ways to put the rubberbands, but most people put them between the 2 outer joints of the “X” on each side. Also i have never made a scissor lift, but rather than a washer, i would personally put a the thinnest nylon spacer thats legal. It reduced friction. Also to help with the leaning problem, you need to make sure that each joint is the same tightness as the other one.

This is a really good video from Wingus and Dingus on a method they had to reduce the lean in a scissor lift. It was their submission for the 2015 Innovate Award. It also shows how they placed their rubber bands on the lift.
link text

thanks! i’ll watch that video. This is our design so far. Hopefully the rubber bands will help with lift… Somedays it lifts right up and other days I think the lean binds up some joints and we don’t have the torque we need.

A couple of suggestions. One, try re-arranging your motors and gearing so that you have one axle (might have to use axle connectors) running across the robot connected to both pinion gears. This helps keep both sides in synch. Two, run some bracing horizontally between the two sides of the lift where you can. It only needs to fold enough to be under 18". The horizontal bracing helps prevent lean as well. Three, add a little VEX legal grease to your linear slides. Friction bad!! I hope this helps.

i’ve thought about adding some wd40 just to see if it helped. i’ll try that thanks.

are you saying use 1 motor to lift both sides? I have thought about his but I would imagine the 1 motor would have trouble lifting this even with a high torque gear ratio… will it lift both sides with 1 motor? I haven’t tried it, just assuming the motor can’t handle it… thanks for the help

No, still use two motors. Just run one axle between the two sides, but power that one axle through gearing using two motors. WD40 is 1) not VEX approved and 2) isn’t truly a lubricant. You may use white lithium grease.

i’ll look into powering it with 1 axle and 2 motors. i like the idea. makes sense.

didn’t know wd40 wasn’t allowed, glad i didn’t use it yet. lol

You can also put potentiometers on xs and run a pid for horizontal stability.

The mechanical coupling of the scissor drive connecting both sides works quite well and generally adds to the stability.

Adding bars going left/right on multiple stages also helps.

Hanging with a scissor can be tough though. If you think it is tough lifting just the scissor part, then lifting the base weight too on the pole is even heavier than lifting from the ground upwards. More weight than you might think.

I started adding braces today… that does help the stability. that seems to have fixed the problem of lean. Now i’m going to try the 1 axle and 2 motors with high torque to see if it will lift all at once. thanks

Depending upon the style of lift, be careful about the lifting on the pole. We had some disastrous results with the 1x25 style of scissor lift by 81J in toss up.

Are you using bearing blocks/flats on your lift? I don’t see any in that picture.

on the actual lift there are no bearings, just bolts with washers and a nylon nut . The driven axle has bearings tho. I’m tired of breaking gears trying to figure out how much torque is needed… :slight_smile:

Metal on metal friction is a bear. Try and get rid of that grinding.

Start without additional scissor sections and get something working mechanically smooth first. Then add more sections on one at a time.

Do you think it would work better to have bearings and a shared axle?

Yes. Mechanically coupled is the way to go.

Any time you can avoid that initial force and overcome static friction to get you moving, the better. In a scissor lift, you have many touch points and they all add up as forces holding you back from moving. So multiply any force differences across all the touch points.

So let’s look at material properties of the available choices in our scissor…

Metal on metal friction is much higher than metal on Delrin plastic. So make sure the metal arms of the scissors do not rub. That eliminates a ton on its own. Then look at the bolt or shaft bar rubbing against steel (or aluminum).

Then there is the geometry of the square holes. Avoid a flat surface trying rotate off a flat surface. Lots of forces holding you back. A bolt gives a lesser touch point than a shaft in that square hole case.

Steel on steel & aluminum on aluminum reference:
http://www.engineeringtoolbox.com/friction-coefficients-d_778.html

That says the steel on steel coefficient is 0.4-0.5 (while lubricated drops it to 0.16, too bad you can’t keep it well lubricated all along your scissor) Aluminum is far worse in this case. 1.05-1.35. PTFE on steel is better. PTFE on PTFE is the best.

Delrin 100 on steel looks like 0.27 from the manufacturer:
http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DELDGe.pdf

Background on Delrin platics which is what these are made of (not sure the grade, probably the cheaper ones like 100 or 500)
http://www.dupont.com/content/dam/dupont/products-and-services/plastics-polymers-and-resins/thermoplastics/documents/Delrin/Delrin%20Low%20Wear%20Low%20Friction.pdf