tight chains

[Micah Chetrit]

Hello,
We are using a banebot 1:256 with a mini cim gearbox to tension our rubber that stores energy to throw the ball. We are not direct driving the tension system, instead we are using #35 chains with a 1:1 gear ratio. The problem is that the bottom chain is overly tight, and the top is loose, and we suspect that the excess tension is causing us to loose significant power because (although this could be from gear ratios as well) it is significantly slower than when we had a 1:1.5 gear ratio that wasn't overly tight on the bottom. I am not a chain expert, and I am wondering how we can make the bottom chain less loose (and why is it tight), and if we could be loosing power from the tension. We tried oiling it and putting a tensioner inside it. Overall advise on chains is appreciated as well.

[magnets]

To have a chain drive that works you need to either tension the chains or get the exact center distance. If your chain is too tight, add another link, then tension the chain.

Search for previous threads for good tensioning methods.

[mikemat]

From my understanding of your mechanism, I'd imagine what you are seeing is the chain transferring torque to the gearbox. If you are using it to tension elastic, all the force the elastic has needs to go through the chain to the gearbox, and it only runs through one side because chain always pulls. If you chain up 2 wheels and pull them in opposite directions, you should see the same thing happen. It shouldn't be a problem because it's just the normal function of a chain amplified under the extra tension.

[Oblarg]

Worth noting that exact C-C distances on chain don't work quite as well as you might like them to, thanks to chain's annoying habit of relaxing by 10% or so during normal use. My rule of thumb is either to be able to move one of the endpoints of any given run of chain, or to include a tensioner. The former is preferable.

[cadandcookies]

Like Eli said, being able to move your motor mount would probably help significantly with tensioning.

You might also try increasing the size of your sprockets, which would probably help (both with making sure you have solid engagement on your sprockets and minimizing the necessity of tensioning).

[BBray_T1296]

This is what I am guessing is roughly the situation you are describing. If it isn't, I cannot imagine the concept is much different.

The slack is a result of the chain's slight ability to stretch, and in this situation is fairly normal, Unless your chain is very loose, even with no loads at all, in which case you should shorten the chain. In the setup pictured that slack can hardly be helped and has likely little effect on the performance.



Another beautiful drawing courtesy of yours truly :P

[Ty Tremblay]

it is significantly slower than when we had a 1:1.5 gear ratio that wasn't overly tight on the bottom.

This leads me to believe that your 1:1.5 ratio allowed your motor to run closer to it's maximum power.

A motor's rpm under load is determined by how much torque it needs to generate. Do the math and figure out what torque your motor is seeing (this will be based on the force of your surgical tubing, mechanical advantages, and gear ratios), then look up how much power it is generating at that torque.

You may do well to adjust your reduction so that it's running closer to peak power.

[Micah Chetrit]

Thanks for all the responses, I haven't used chains in 4 years and I guess I expected it to be a bit less taunt. We'll be sure to implement some method of tensioning in the next few days now that I understand what's happening.

This is what I am guessing is roughly the situation you are describing. If it isn't, I cannot imagine the concept is much different.

The slack is a result of the chain's slight ability to stretch, and in this situation is fairly normal, Unless your chain is very loose, even with no loads at all, in which case you should shorten the chain. In the setup pictured that slack can hardly be helped and has likely little effect on the performance.



Another beautiful drawing courtesy of yours truly :P

Yes, that is very close to our design. The force vectors really helped me understand why one side is slacking, thanks!

This leads me to believe that your 1:1.5 ratio allowed your motor to run closer to it's maximum power.

I just thought that the difference between 1 and 1.5 wouldn't be so significant, it went from pulling it with ease to barely making it.

[Ty Tremblay]

I just thought that the difference between 1 and 1.5 wouldn't be so significant, it went from pulling it with ease to barely making it.

You have to remember that it's 1.5 times the full reduction. Assuming you're using a ToughBox Mini, you're looking at 256*12.75 = 3264:1 reduction. Include a 1.5:1 reduction on your chain and you're looking at 4896:1. That's a big difference.