Guide to Six-Bars

We’ve had this written for awhile but it got left in the editing pile and then we forgot about it. Anyway, some teams have already started building six-bars (notably S^3 who used it and won the first Gateway competition of the season (except they call it a 7-bar for some reason)), but some people are still a bit confused about how it works and why.

We’ve posted a guide about the advantages of six-bar over four-bar, and some things to watch out for on our website here:

Feel free to post below if you have any questions or are confused about anything - we’re happy to answer, and will edit the guide if anything is wrong.

So yeah, I’m really not sure why I was calling it a 7-bar before. If anything it would be 6-bar or 8-bar, but for some reason I had thought 7-bar one time, and it stuck. o.O

Anyways now thanks to Andrew from Aperture, we’ll be calling it the “extended 4-bar.” :smiley:

Awesome guide, by the way!!


Its technically not even an 8 bar :stuck_out_tongue: I can see 6 though :stuck_out_tongue:

What’s fun is when you get to the offset 4 bar systems to complex for competition level robotics : D

For the six bar, is a good ratio a 12 tooth high strength gear to a 60 tooth high strength gear. Or is that too weak. Should we go with a hs 12t to two 84 tooth gears? :confused:

it depends how heavy your arm is
it also depends on how many motors you have powering the arm

It really depends on your situation. My team attempted a 5:1 torque gear ratio and it didnt work for us, with 4 motors.

One thing that i didn’t mention when writing this was about the use of rubber bands or other elastic to help the lift. If you use elastic to help pull your arm up, it will make it much easier on your motors, and on the way down, even though you are fighting the elastic, gravity will mean that you arm will still be able to be lowered easily.

I agree. Elastic is EXTREMELY helpful. I wouldn’t even test your 4 bar too much before you put latex tubing or rubber bands on to assist in lifting it. You will not believe the difference until you see it.

No such thing as a linkage too complex. A complex linkage makes for some pretty simple software.

how would you attach this elastic to the lift?

You can either attach the elastic to the back of the arm, behind the pivot point pulling downwards, or between the parallel bars of your arm. Attach the elastics at a diagonal so that they stretch when the arm is down, and shorten when the arm is at a fairly high position. Also, the tension is usually set so that, without motors or game objects, the entire arm will feel weightless at a mid-high position.

sorry to bother, but our team has not even built a four bar linkage, so i don’t really understand. This 6 bar is going to be a challenge for our team. A picture might helped.

Ok, so I attached a sketch of a simple 4-bar link with elastic stretched across two points. When the linkage is raised, the elastic has a small amount of tension because it is short in length. However, when the linkage is lowered, the length of the elastic increases causing tension to increase. Under tension, the elastic is trying to spring back to its original, shorter length. This forces the 4-bar link upward. You can vary the amount of force applied by the elastics by increasing and decreasing the starting length of the elastics (when the link is highest) or by adding more elastics. Optimally, you want to make the force of the elastics great enough to counteract the force gravity has on your arm to the point at which it requires an equal force to push the arm down as it does to push the arm up (we like to call this making the arm “neutrally buoyant”). Using elastics relieves a lot of strain on the motors and allows you to use a faster gear ratio than you normally could. Hope this helps. If you have any further questions, feel free to ask.

  • Adam
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Thanks so much, i understand now. Also this elastic would be around two stand offs? and four the 6 bar linkage it would be around both of the four bar parts, so like two elastics on the 6 bar, or only one? :slight_smile:

We put our elastics usually on two standoffs. I suppose there are other methods, but we find standoffs are easiest. On a six bar you can place the elastics on both the front and back parts if you want or you can just do one. We have found that it doesn’t really make a difference.

Thanks, and by the way is 3057 a rookie team? cause i didn’t see you guys in math and engineering divisions at worlds. :confused:

In short, this is our second year as a team and we were in the Engineering Division last year at worlds and lost in the quarterfinals. For the long version see below.

I’ll give you a complete team history. Mendham Robotics started in 2007 with Team 3053, Occam’s Engineers, who won the World Championships as the alliance captain in the FVC game Hangin’ Around. They competed again in 2008 in Quad Quandry and qualified for the world championships again. In 2009, my freshman year, the school added Team 3054. As an all freshman team, Team 3054 qualified for the world championships in VRC Elevation. In 2010, those freshman (myself included) moved to Team 3053 and qualified again along with Team 3054. That year we also added Teams 3055 and 3056 however they did not qualify. Last year, we took members from Teams 3053 and 3054 to create a fifth team called Team 3057. We combined to create a pretty experienced team and qualified for worlds again along with 3053, 3055, and 3056. At worlds, Team 3057 was in the Engineering Division (along with Eagle Engineering, I believe you guys won the divisional Excellence Award, congrats). We did not do well in qualifiers because of some alliance members who did not show up and we had some motor problems. Luckily, the Number 1 seed noticed our robot and picked us as their first alliance partner (everyone was a bit shocked by that given our ranking). Unfortunately we lost in the Quarterfinals because one of the motors burned out in our gear tower for our lift. We hope to have better luck this year at worlds and we are designing our robot to be more easily maintained than last year.

But basically, we have switched teams so many times that we are not very well known even though we have had a lot of success and this year have a lot of experienced people on our team. Now we are trying to get our name out there through the forum and with our newly created website.

Wow, you guys were the team that got picked 1st. Im pretty sure i scouted every team in our division, i must have missed you guys. You guys got picked 1st by 2460, and i was confused. I didn’t know how good you guys were, but i though since you got picked first you would have to be a good team. Though you guys got beaten by the eighth seed i still think you did pretty good, good luck next year and hope to see you at world.

Thanks. Yeah if our motor hadn’t stopped working we may have advanced to the semi finals, but the 8th seed was quite strong. They made it to the finals and lost to Free Range, so we definitely didn’t lose to a bad team. Hope to see you guys at worlds too.

ya keep telling yourself that. it still doesn’t make the pain go away. lol :smiley: