Double Reverse Four Bar Lift Help Needed


Long story short my team has Regionals tomorrow, our robot doesn’t work, and we don’t know why. We don’t think it has anything to do with the programming or the weight, but we aren’t so sure about that (though we did use a lot of Aluminum). Attached are a few pictures of our robot. We are planning on tomorrow (at Regionals) switching the gears in the front (for the bottom layer) from three medium sized gears (high strength) to big little (with the motor in it) big. If anyone has any idea what else we could do to possibly make this robot work, or as to why it doesn’t, we would be hugely appreciative. Also, the claw will be going on the two (on each side so four) aluminum bars in the middle of the bot. Thank you.

~Zoe and 5447B


Attachments_201536 (4).zip (292 KB)
Attachments_201536 (3).zip (292 KB)

What doesn’t work? Does it drive?

Sorry. The wheels are fine but the lift doesn’t work unless you do it manually.

Does it try to lift?

You need more rubber bands.

Check that each motor is moving.
Check that each motor is turning the correct direction.

(also, not related but check your dimensions, it looks very close to 18", check the axles are not too long on the drive).

yea axles reach about 17.5 and we have to shave the top off of the highest point of the robot (like 18.2) we tried adding more rubber bands but it just lifted the lift up and held it there and we had to use the motors to get it back down. Also all motors have been checked (one burned out - had to be replaced) and are going in the same direction.

Your lift does not appear to have a gear reduction between the motor and the lift gears, at all.

A vast majority of teams use a 7:1, with the fastest I’ve ever seen being a 5:1, maybe. You are essentially running 1:1. Those motors don’t have any where near enough power to run that kind of ratio, and if they did, the lift would go up in around a third of a second to full height.

I would highly suggest looking at some other successful double-reverse four bars and definitely add a much larger gear reduction. You should also look into supporting your shafts on both sides as cantilevered gearboxes can be prone to skipping.

With such a small time-frame, the best solution would be to either try and use the Vex turntables as you can get a decent reduction out of them and not have to make a custom gearbox, or playing to your strengths and moving some motors to the drive train to be stronger at pushing.

Good luck at your regional tomorrow and I hope everything gets fixed.

Thank you very much. We intend to switch out the gears tomorrow morning with the larger high strength gears and a small one on the motor shaft. We do have a backup plan (our states robot unattached from the base) that we saved in case this didn’t work, but seeing as it can do two (maybe 3) skyrise sections and that’s it, we feel as though fixing this is our best shot


One of our middle school teams is also trying to get DRFBL to work as well. The two areas of concern mentioned earlier are gearing and structural support. Our team has the structural component worked out, but gearing is taking a little longer than anticipated.

I might stop by to see your final solution.

Good luck today!

Update: competed today with a non-working lift. Apologies to everyone we were allianced with :(. That being said, we still went 3-4-1, which we dont think is particularly aweful for having a not totally functional robot (go floor goals). Unfortunately, the lift is vary unstable, and Im not sure how to fix that for tomorrow, especially because i only have short pieces of metal with me (I brought the robot to my house overnight to try and fix it). So any stabilization tips? It oftens leans to one side for no apparent reason, and stops going after it reaches a certain height (nothing is in the way)

Any advice would be appreciated as we have to compete again tomorrow and i feel terrible for our alliances, as this is a regional championship and world qualifier, and id hate to drag anyone down further. Thanks


Originally it was entirely a strength issue. Is it able to lift now? What is the ratio?

In terms of leaning the best way to fix that is cross support across the lift. Several teams have had success with small X’s across the lift using 1 bar. These are light and cheap so if you have a few you can try that. Also just standard cross supports will help as well. Also connecting the axles together on the middle tower should help bind the lifts together ensuring less lean.

A video of it trying to lift could help us diagnose it.

1 last thing
I am finishing up an essay for my college classes so I am not there volunteering but several of my teammates are. Greg Carlson will probably be happy to help give advice if you can find a moment when he is not busy volunteering.

Most of the volunteers are college students who have done VEX for years and just like giving back so if you can corner one (maybe at the skills fields) they should be able to give better advice because they are in person.

Thank you very much - before it was a strength issue, now the ratio is i believe about 5:1, which works a lot better. I am currently working on stabilizing it, but thats very difficult to do as i dont really have any long metal with me. The axles in the center are connected, and i would post a video if i knew how to, but i dont :(. Also, if you have a second to describe it, where exactly did the x’s go? Thanks


X’s connecting the 2 sides of the upper arm. It binds the left and right side of the lift together well.

In terms of videos the best way is usually just to upload to youtube and then post the link here.

Also more pictures of how it looks now might give me some more ideas.

Ok thanks, and ill upload some more current pics in a few minutes. Thanks


Nevermind. I discovered i cant upload pictures from my phone to this site, so I cant provide any pictures. Thanks anyways :confused:


Maybe just email to yourself and do it from a computer or even email to me if you want.

I discovered that I can message myself pictures from fb and I went from there. Here are several pics of the current bot. I have like two or three more pics but I guess there is a 5 pic limit. Thanks again ~Zoe




2015 Regionals Robot 6.jpg
2015 Regionals Robot 1.jpg
2015 Regionals Robot 2.jpg
2015 Regionals Robot 3.jpg
2015 Regionals Robot 3.jpg

Since these pictures were taken, the lift has been stabilized well, though it did add on weight :(. Therefore, the lift can barely get to half of its potential height. Before, it could reach maybe half to two thirds potential height. It also angles and occasionally leans/ tilts hardcore to one side for no distinguishable reason. If anyone has any idea of how to fix this or about what is wrong it would be greatly appreciated,as we had hoped to to use this robot for Nationals in addition to the second half of Regionals tomorrow ( well technically today). Thanks for the help.


one thing I noticed is you have cantilevered lift arms. Although you have an inner bar to tie the axles together (which is good), this bar seems to be missing bearing blocks. I would suggest you extend the inner bars and secure them to the lift towers with beams to improve the arm stability. With the addition of bearing blocks and non cantilevered arms, friction will be reduced on the lift drive shafts. Also, have you tried using more rubber bands? Are all your lift motors geared for torque (ie 100 rpm)?