Chain Bar Elastic Support

We have built our chain bar arm, and attached the intake. The arm can lift the arm + intake/hopper fine. When we start putting objects in the intake, the arm can’t lift the intake + intake/hopper + game objects.

The Chain Bar is geared 5:1 right now. I am going to change that to 9:1 tomorrow.

My question is, where is the best place for elastic support? I would like to have as many rubber bands on the arm as possible to not put a lot of stress on the motors. We will be lifting the arm a lot.


Hmm… maybe you could have them pulling the arm back, like attach them to the back triangle support bar, then over the top and onto the bar.

We actually have that right now. It is connected to a point on the top of the c-channel then a part on the back of the bot.

It works OK, but it holds the intake off the ground when we apply more rubber bands. It still can’t lift that many objects…

When you have the 1:9 the lift will be held down a little bit more just due to the increased friction in the gears.


I just want the elastics to do a lot or most of the “lifting”.

Yup! We actually ran into this last year. We used velcro to strap our lift down and then in auton we had our lift go up suddenly and it came undone

Well, the lift won’t stay down to intake the objects. That is the problem…

The force the elastic provides is directly proportional to the distance it is stretched, meaning that if it is stretched out a lot more when the lift is down and not as much while it is up, it only provides a lot of force when the lift is down and falls off when it rises.

In order to get the most out of elastic on lifts, you should position it so that it helps it pull up, but has a relatively small difference in the amount it is stretched between being all the way up and all the way down. This might take more rubber bands to get it off the ground, but the force should remain pretty constant all the way up. I do not have much experience with chain bars, but this works well on 4 bars between the bars when they are closer together.

Also of importance is understanding where/when you need the most force when lifting objects with a 4-bar or 6-bar. This is at the point when the bars are parallel with the ground (horizontal) and create a rectangle. This is the point where the weight of the intake/objects is directly perpendicular to the arm itself, so all of the weight of the objects is pushing down on the arm. Like Owen has already said, it’s important to position elastics in such a way that you get the support where you need it.

You’re also probably going to see a very large difference between a 1:5 and 1:9 ratio.

Good luck!

My team is thinking about using a chain lift. are there any pictures of functional chain lifts. what exactly are the benefits compared to gears?

I put together a CAD drawing of a compact Chain Bar here on our website:

Here are also some videos of our chain lift working.

Chain Bar Lift Testing (On Bot)
Chain Bar Lifting 3 Sacks (Steel Metal)
Chain Bar Lift (Geared 5:1)
Chain Bar Lift (Geared 5:3)

What do you mean by

I really like the chain bar because it uses minimal metal. It works very well for our strategy, and over time I think it will work even better than the 4-Bar + It looks really cool :smiley:

If you do build one, I would suggest compound gearing, or just a higher ratio. Go higher than 5:1. Go 6:1, 7:1, 8:1, or 9:1 (We are doing 9:1). Make sure when you build the chain bar the sprocket on the tower is attached directly to the tower so the sprocket cannot move. The same for the bottom. The sprocket is attached to the piece of metal that holds the intake. The gear needs a circular insert for the gear that is attached to the arm. That way the arm can spin freely around the axle. The axle on the bottom spins freely around the arm pivot point.

If you have any questions, send me a message.

This is awesome! Thank you so much for sharing

As an alternative to using lots and lots of rubber bands that do not move a lot, you can mount your rubber bands to your arm to minimize the change in vertical force on the arm as it raises and lowers.

You can try an up-turned 45-degree gusset sticking out of the back of your arm, over which you hook the rubber bands. As your arm lowers, the distance the arm sticks horizontally out the back increases until the arm is mostly raised.

If you used a 6-bar, but replaced the further 4-bar with chain bar, you could use rubber bands like this, which result in the most vertical force when the arm is raising up and horizontal. The idea behind this is that you want as much acceleration as possible when the arm is low so that it starts quickly, then can rely on the motors to keep it going quickly through the rest of the motion.

A little of topic but what is the use of the pistons in that picture.

See this thread for a complete description of that robot.

The image in question.

In short, the two most visible cylinders help to lower the arm during hanging (that robot being a Round Up 'rob), and the cylinder inside the lower C-channel of the arm (whose fitting is just barely visible) releases the rubber bands on the arm by angling the axles that the rubber bands hook over on the right side of the picture.

We have the chain bar working on a 7:1 ratio. We also have the intake attached. It can lift 4 sacks pretty easily, but we are trying to lift more.

Here are a few pics that way you can see our setup to better help us!

Looks good!

If you want to lift more sacks you will either have to allocate more motors to the lift, make a compound gear ratio, or figure out how to put elastic on the chain bar.

Do you have any problems with sacks getting caught in the intake?

Thanks. That is what this thread is about.

What is the best way to use elastics. I got a good idea from team 1024. Any others are appreciated!

No. We have had NO problems with sacks catching in our intake.

I’m surprised this is not giving you issues. The axle needs support through two bearing flats. It looks like it is just cantilevered through the C channel, or is there some other structure I can’t see.


It has given NO problems what-so ever. We did not have enough length to get all the way across, but I just picked up some longer axles from Team 1024! Thanks guys!

We will be adding longer axles next meeting.


I believe I figured out the right elastic placing.

Here is a video of the arm lifting 8 sacks (4lbs):

Thank Y’all for your help!