Using chains or strings in elevator lift

Hi,I’m Liz.It’s my second year in vex and this is the first thread.
We are thinking of building a double linear lift for skyrise this year. i have already made some models. I heard people use strings rather than chains in it. Can anyone tell me the advantage of the strings?Or how people build it without chains?Im really puzzled…Any help is appreciated,thanks. :slight_smile:

first of all i dont think you can reach above 40" with a double linear lift you might need more stages to reach a desired height. for some advantages of string the strength will not break compared to chain. you can make the lift simply by just powering the up side(or even the down side). you can build it by using the winch pulley kit and just use the drum to wind the string or even make your own drum using standoffs and gears.
advantage
thinner then chain you can make your lift skinnier as in a pulley is skinnier then a sprocket.
tension strength of the rope is stronger then tension strength of chain
you can directly replace chain with rope you would just need a new gearbox. but the rigging would be the same and you would also need to wrap the rope in opposite direction like how chain is wrapped
if im not making sense pm me or post a reply and i can try explain myself

Chains are more common for linear lifts in vex. Actually more popular, probably because they look so cool.

Disadvantages of chains include: snap too often, take up space, so on.

However, chain has a great advantage. Because chain is a long cycle on a continuous elevator lift, you can use the lift to pull itself up in the air. Still need to mention 1103, see how they high hang in round up.

Since with ropes, you just wrap them onto a winch, the lift is not able to suspend itself, which is not a requirement in skyrise. But still, if you tip at a high position, your chain will be just fine, but your rope will more likely become messed up.

For skyrise, I don’t see how rope is a bad choice for elevator lift. But still, chain looks so cool…

Have fun building elevators! We look forward to some battles between scissors and elevators this year!

This doesn’t make much sense. Why would cable/string not be able to “pull itself up”?

This is false. There is no reason why it can’t stay “suspended” at a desired height. If the cable is wrapped around a drum partially, then the lift would stay at that height, unless the drum/motors could back-drive. Also, there is no reason your string/chain would get messed up if you tipped over as long as the string was tensioned properly.

That in general is the main drawback to using cable, especially in VEX where you have to use nylon rope. It is much more difficult to properly tension the cable and ensure that it is secure at that length.

If you can tension it well, then using cable is lighter, takes less space, and has less points of failure.

-Nick

Sorry If I did not make myself clear.

I am talking about an elevator lift powered by a winch that stores the rope, not a continual rope loop. If you desire a loop, maybe ropes won’t provide enough traction for the motors to power the lift, and in this case maybe chain is your better choice.

When I’m talking about messed up, I mean if you use a winch to store the rope rather than making a continual loop, you tip and your lift slides out, and the ropes will get loose or messed up.

No offense taken.

This still does not make sense. If you have a length of string fixed to the bottom of your last stage, rigged through your lift, and it is wrapped around a drum when you power up, when the motors stop spinning the drum, the lift would stay at that height, unless the drum spun back the other way (powered to run other way) or if the motors/gearbox do not have enough torque prevent back-driving on the drum due to gravity, etc.

If you have a hard stop on the top of your stages, then the lift cannot slide out. People usually have hard stops to prevent the lift trying to expand more then it should, and it would be a good idea to have a hard stop on each stage anyway because tipping could be more prevalent.

-Nick

Maybe I caused the confusion, but when I say pull itself, I mean you grab the upper top of the lift, try to lower the lift and hang the robot. An elevator lift made of winch and rope cannot do this. It is only counter acting its own weight. Yes, both types of lift can stop at a desired height if your motors have or provide enough torque.

The situation I described is, when the lift is not all the way at the highest position, you tip and the lift leans downwards. The lift slides out due to gravity, not necessarily causing the stages to detach themselves, but rather, if this situation ocurs on a winch elevator, the rope will get loose and possibly get off of the pulley track. If you have four or more stages, it might be a lot of work to fix that.

Sorry if I caused the confusion.

This makes more sense. All it would take to remedy this is a second length of string rigged the opposite direction through the lift mounted on the same drum, so when you spin the drum, one string length unravels while the other winds up. Like I said, making this effective all comes down to tension.

How does a lift slide out, yet it is not dis-attached?

EDIT: Oh, okay you mean when you tip over gravity (or rather forward momentum on its way to tipping over) would cause the lift to partially expand without the motors/winch causing it to expand at all, so the string would become loose. Yes, that could be an issue. So just don’t tip over. It will be difficult to tip something back that is 60" long/tall.

-Nick

Thanks for reply. i am a little bit unsure of the hard stop you mentioned. What is a hard stop???How do people build it?

I did a winch and pully system for sack attack (dont ask why :wink: ) it worked well with the teensioning issue, and was much more powerfull that the entire 98:1 lift even though it was 1:1 driven. I like the rope lots, and find it fairly easy to tension, if you know the right knots :wink: the only disadvantage is the wierd spacing and torque from having the rope pull at a single point rather than chain pull at a wider area because the chain is wider.

I tested 2.75" traction tire wrapped around 60T high strength gear, and it had pretty perfect traction if the rope was wrapped around 3 times. Also, it usually takes 4 stages to reach 60" for an 18" robot.(2.5" average overlap).

Thanks,guy.

Thanks for the details, but how do you do this???:confused::confused::confused:

You can just pull the tire off of the grey wheel base. You can use some thing to pry it off if needed. Then flip the tire inside out, and apply pressure to get it on to the gear.

Many teams used something like this for big ball intakes last year. Search some pictures if you want.

EDIT: Check out the green egg’s version.https://www.youtube.com/watch?=1&v=Iw5mjxtPvaE

How I did it is oil the tire first (with vegetable oil), then try to get it on the gear. You need to thoroughly wash the oil out at the end, or the tire will fall off.:D:D

I was literally doing the exact same thing today. :).

As I did practically the same thing, I will try to explain what my team is trying to do. We used an inverted tire from the traction wheel, (like many teams used as an intake for large balls) around a gear. This would then simply act as a high traction pulley, which can be powered by a motor. The problem is that it does not get sufficient traction wrapped around only once (as you typically would with a pulley), and with multiple wraps (as Tlin used), the rope will inevitably push itself off the pulley or overlap on itself. The trick is to get the most surface area on the pulley as possible, without the rope touching itself because contact between ropes always in our testing resulted in loss of alignment and/or friction. We had the best success so far with multiple separate pulleys to maximize contact. When the pulley is held fixed with a bar lock, I found that the force the system could hold before noticeable slipping (unlike chain, there will typically be some slip, you just want it to be negligible in a good system), depended largely the tension of the rope. What we are trying to maximize is the force of friction, which is the coefficient of friction (based off surface area and material) times the perpendicular force (tension). Fortunately for us, tension on one end of the rope is typically proportional to the weight to be supported (and therefore required traction), but it would probably be worth trying to add some tensioning system for the other end of the pulley.

Practically, it would be possible for a light system, although if you are trying to power a heavy multi-stage elevator (we aren’t), I would prefer a different system. One other option I don’t think anyone has mentioned which has the same effect as a powered pulley is two separate spools, one that releases a rope and one to pull one in each on the same shaft, but wound in opposite directions.

It is true that multiple wraps will cause the string overlapping, which will give you a slightly variable lifting speed (and force), but with lexan circles bolted to the side of the gear and elastic pulleys somewhere in the pulley system, the rope will not pull itself off the pulley. The variable gear ratio is a factor to consider when wrapping the rope around something multiple times, but all systems using the vex winch have very variable ratios (the winch is bigger in the middle), more variable (circumference is 0.272" longer in the middle) than a rope wrapped over itself around an inside-out traction tire. Vex 1/8" rope actually compresses down to about 3/32", so that adds about 19/64" to the circumference of the pulley, or 0.147". That will add tension to the rope, but with elastic pulleys I am optimistic that it will handle the increased tension.