hello, I am designing a chain elevator, I have seen that sometimes when you demand a lot of force to the elevator the chain comes off, I saw that the thrifty elevator uses 2 chains, this helps to support more weight to the elevator? or what is the purpose of having 2 chains?
We also have access to laser welding, do you recommend welding for the elevator or gussets and rivets?
I assume two chains would allow you to lift more weight, but I think the larger concern is racking. I don’t know that the chain itself is really a limiting factor in that scenario compared to the rest of the elevator elements.
Can you post your design? I recommend a 2-chain lift from the left and right sides to prevent the middle from going crooked anyway.
Just make sure that both chains move together at exactly the same time (the term racking used by Brendan usually refers to one side/chain of the elevator moving at a different speed than the other, causing your elevator to bind).
The easiest way I know to do this is by linking both sides mechanically, such as a single shaft driving both sprockets for your chain.
A much harder way to do that IMO, is to have an individual motor controlling each chain, and using software to ensure they stay in sync. This option in my eyes is near impossible for most FRC teams to pull off successfully, but is technically possible in theory.
Edit: I stand corrected about controlling 2 sides of an elevator independently, apparently that is now possible with some crazy witchcraft out of CTRE (well, witchcraft to me anyhow 
).
You can put a chain in the center. Racking will be an issue. That can be fixed with structure, but that will be with added weight.
You can use 2 chains or timing belts on the side. Best to time them mechanically. I see it supposedly can be done with software. But that adds workload to the programmers. Doing it mechanically is Wiki-simple. I would a good reason other than it is shiny to do it in software. Also with mechanical timing, you could run it with one motor.
We used a chain for our elevator in 2018. We were able to lift 500 lbs off the ground using a single loop of #35 chain without the chain going slack (our buddy lift was originally designed to lift all 3 robots rather than just 2). The elevator was a cascade design such that the final stage was moved using a second single loop of 25 chain.
I seem to recall that we had some issues with the system at our first competition that was caused by flexing of the output shaft from the gearbox (but my memory is a little fuzzy on this point). We were able to fix that before our second comp and the system worked pretty much flawlessly for the rest of the season.
The chains were located such that they pulled each stage from roughly the center. We did not have any issues with racking causing binding in the elevator. We did have issues with the elevator system racking if we were lifting a second robot on one side without a 3rd robot on the other side to balance the weight. A second chain run would not have helped that problem - it was the result of uneven load on the elevator during the buddy lift rather than uneven load from the chain drive system. Our final design had a buddy lift off the back of the robot such that the load was centered and we did not have any racking issues with that design.
You will want to scrutinize your design to look for places where flexing of the structure under load could cause the chains to go slack.
I have no experience with using chains for a continuous elevator. We have used Dyneema rope for continuous elevators and we did see quite a bit of slack in the rope. However, this was because of the geometry changes in the rope pull angles that occurred as the rope would wind onto and off of the spools. You would not wind chain onto a spool, so the chain system should not have this problem. I’m sure there is a way to do a continuous elevator with chains (I have seen plenty of industrial elevators that use chains for a continuous rigging elevator. However, in my experience with FRCFIRST Robotics Competition, continuous elevators tend to use either rope or webbing/belting and cascade elevators can be chain, rope or even belts.
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