Cable vs chain

My team has deiced to use a cascading elevator for a robot and i was wondering if chain or cable would be better for the 7ft height we are trying to reach

We used chain effectively on our cascading elevator, but I can tell you some of the problems we ran into. Basically we didn’t have prestretched chain and after running our elevator for like 5 minutes the chain was already loose. After taking out a link and testing again for 5 minutes, we had the same problem. Eventually we got sick of messing with chain so we made an adjustable chain tensioner using an idler sprocket. You could also make a spring loaded chain tensioner and “set it, and forget it”.

The reason we chose chain was because Adam Heard made it seem like a good idea in this video. He was definitely right (after we added the chain tensioner).

I think you could achieve an equivalent level of success with cable, but our logic in not using cable was mostly related to figuring out how to attach the cable to the 1st stage of the elevator.

If you can, I would use cable. It’s significantly lighter than chain.

Some of the spectra ropes that are available have an extremely high working load relative to their diameter. For example 1538 used a 9/64" diameter rope for their 2015 lift. If I remember correctly, it had a rated working load of over 1000 lbs, which is almost 3x the working load of #35 chain.

The main difference between the two is that chain gives you the ability to go down faster as a basic cable system relies on gravity to go down. You can change this by adding either a spring (think rubber band or surgical tubing) that stretches as you go up, or by adding another cable that will provide power to go down.

My FTC team has used multi-stage chain lift almost every year, and it’s usually the most reliable part of our robot (unless the lift is on an extreme angle, story for another time). FTC motors provide enough resistance to hold a 2 or 3 stage lift at height, but FRC lifts can’t do this. Instead 1023 used a resistance “Brake” in 2011 that worked really well.

Cable needs a spool. You have to build one yourself. They are easy enough but if you get it wrong the cable will bind.

Sprockets are available off the shelf. You can change the sprocket size quickly.

Spectra does not handle abrasion well. If you use rope you have to be careful of abrasion, sharp edges and heat. Also the stretch of rope can be a problem.
Inspect them often.

That said, they all will work and there is not a clear winner.

If you don’t have that down cable, you’ll be unable to retract your lift when you inevitably clothesline yourself on the Scale. Having a retracted lift will be very advantageous for either disentangling yourself or for letting your partners tip you back up, and for not blocking the field.

The “easy” way to do that cable is to have a second cable on the same drum, wound in the opposite direction from the “up” cable.

The “easy” way to do that cable is to have a second cable on the same drum, wound in the opposite direction from the “up” cable.

We are working on how to then lock the robot in place, last year we only had to go one way, this year it is extending our lift then retracting it to complete the climb before locking.

AM 4" performance wheel or WCP Colson hub. Instant spool.

You could also use some of the new VersaRoller Tube, insert two VersaHubs in the ends and fasten them with some 8 screws. A little more work, but allows you to make a spool to whatever width you want.

On option is using a single cable to pull your elevator up but then using a constant force spring for the return.

Did you have any problems with that rope during the season? Is it easy to work with?

Then you’re fighting both gravity and the spring to raise your elevator and maintain any elevation. I would not recommend this approach.

We had no major problems with it. If I remember correctly we used the same cable the entire year on both robots.

I do highly recommend doing two cables on the same drum (one for pulling up, one for pulling down). I say the same drum so that as one cable is being wound in, the other is feeding out.

While gravity is constant, I wouldn’t rely on it to pull your lift down. Especially considering the relatively low impact a second return cable has on robot weight and complexity.

I also wouldn’t recommend using a constant force spring. Typically these have a cycle life and when they go, it’s usually pretty catastrophic. I’ve heard a few stories of people getting their hand(s) sliced open when their constant force spring decided to quit.

You can also use belts. HTD belts can be purchased in almost any length, and you can splice your own.

We gave up on chain a couple seasons back.

My personal favorite is 3/32 stranded stainless cable with a plastic sheath. It is supple enough to go around very small pulleys yet has enough stiffness to prevent kinks.

Things to remember:
-Put thimbles in the ends using crimps.
-Strip the sheath off the cable where the crimp should go.
-Use a turnbuckle to get the proper tension. Be sure to use locknuts to keep the turnbuckle from loosening. Safety wire is an alternate method.
-When you go through a pulley make sure the in cable, pulley, and out cable are in the same plane.

Securing the cable
-One end can be secured to the robot by running the cable through 4 holes about 1/8" diameter spaced about 1/2" apart in a straight line. Run the cable through the first hole and adjust to proper length. Put the loose end through the second hole in the opposite direction and pull tight. Repeat for the next two holes. Cut to final length with 2-4" coming out the last hole

Do you have a good source for Spectra Rope? My team used some many years ago and any time I’ve looked for it since all I’ve found it paracord (with a much lower strength rating).

IMO, better to use constant force springs to offset the weight of your mechanism and pull it up and down, this lets you move a lift much faster and with less motor power required.

I don’t have the actual source right in front of me, but this could be it: http://www.go2marine.com/product/38345F/samson-amsteel-blue-rope.html

If it’s not the same source, it’s something very close. IIRC we played with 7/64" and 1/8" and settled on 1/8". I don’t recall what the reasoning was.

If you use the “organic” forms of cable (Spectra, Dyneema, etc.), you might get some stretch. The spec sheets for the Dyneema we used to make the Bridge Pusher in 2012 said the stretch was some small number but we had to make adjustments to our mechanism when we got significantly more. It was a late season addition so it didn’t get a lot of use so I don’t know if it would have continued to stretch.

I would use a multi-tasking chain elevator. The elevator could be used for both lifting cubes onto the scale and switches (or even feeding cubes to the vault.) It also could be reversed to lift onto the bar. A chain elevator would be strong enough to lift a robot * 3. A chain elevator used for both scoring and climbing was used on the 2000 National Champion.

Chain elevators with a chain tensioner and brake are fairly easy to manufacture.

why does having it be plastic coated, what makes it better than a regular 3/32 steel cable