Powering the bottom bar on a 6 bar?

Has this been done before? I have never looked much at 6 bar robots. I can see why the way the bottom bar transfers to a top bar later on could be a concern. So if I wanted to power the bottom bar of a 6 bar and not the top, would this cause issues?

I was under the impression most teams powered their 6-bars from the bottom bar? Anyways, we did this fine Gateway season without any problems.

Short answer No.
The power will on transfer on a parallel 6 bar from the bottom bar to the top bar.

The way most teams do it is by powering the bottom arm…

When making a six bar, my recommendations are to make sure all the joints move freely and easily while not being too loose, and I’m pretty sure it’s a good idea to have your elastics (if you are using elastics) on the lower stage of the six bar, or preferably, on both stages. It shouldn’t really make much of a difference, but I found rubber bands only on the top stage to end up being less effective.


Also try to make sure the friction and elastics on both sides are the same to avoid tilting. On my team we try to make sure the same person tightens the screws of all the joints because “a little less than tight” means different to different people.

great advice. this Eliminates most of the common problems we see in our club.

Well we have experience with 4 bars so we understand all the primary principles. I just didn’t know if powering the bottom bar was common practice or not for a 6 bar specifically. On our four bars we have never put on bearings because we didn’t want the screwheads for those bearings to force the bars to be further apart, is that what you guys do too or have people found ways to put bearings in while maintaining the almost 0 space between linked bars?

We just use screws with nylock nuts :D.
This is what most teams I have seen do.

I’m not quite sure what you mean with your bearing problem. If you’re connecting two c-channels for your linkage, you should ideally set them up like this:


with bearing blocks “inside” the C’s.

However it’s not terrible to face the C’s toward each other either and still have the bearing blocks “inside” the C’s. You just want to minimize the space between the two bars, but make sure that you have bearing blocks at the joint holes on every piece of metal that’s going to be a part of the linkage.

so no shoulder screws?

Yeah, on every 4 bar we’ve ever built we left the bearings out entirely purely for the purpose of keeping the two bars that are being axled together only a washer’s width apart. I guess it gains us that stability but throws out a little bit of the stability of the angle held by the parallelogram to do that. It has always worked out fine for us though.

We bought a pack but the not threaded part is way to large to be used in this instance.:mad:
We have yet to actually find a good use for them.

Add a few washers to fill the extra non-threaded space. You can adjust the snugness of your fit with the washers and you get the advantage of the larger, smooth diameter of the shoulder bolt. Also, no bearing blocks needed.


I don’t know about other teams, but my team always makes sure to power both, the bottom and the top bars, when we make any “multi-bar” lift. We do it mostly because it seems to help with axle twisting. If you want to see ours from the middle of the sack attack season you can see that on the 944B progress thread here: https://vexforum.com/t/944b-incredibly-late-progress-thread/22880/1&page=2

Also, we’ve noticed that on many 6-bars, the 2 sides are driven with motors mounted on the inside of the rails; hence no axle between the 2 sides. Some have a mechanical connection (like a piece of C-channel connecting the top bars of the 2 sides), but some don’t, and they seem to work fine. Does this mean that there’s programming to keep the motors on the 2 sides turning/lifting at the same rate?

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Not necessarily. We had no mechanical connection and no control and they went at the same rate anyways.

If you put both motors on the same button or joystick they will move at the same speed and power whenever the signal to move is sent.

The issue with connecting your axle on a 4 bar is that it wouldn’t add any significant amount of support because the axle could just twist. We keep our sides seperate intentionally, partially because it makes building easier, and partially because it makes troubleshooting easier. In gateway, we had an issue where the sides moved inequally, and I believe we tried to fix it at first with programming which didn’t work too well because of potent updating. We also didn’t have much coding experience then, and ended up just swapping the motors :o

This year, we just led the sides be slightly unequal, and had a hard stop “reset” them at both the top and the bottom limits of height and ground. This kept the sides from getting too unaligned, even with unequal motor strengths or friction. The rest of the advice in this thread is also pretty good, so I won’t add to that.

We powered both the top & bottom bars before and mounted our motors down low. It worked out pretty nicely.

+1. We powered both top and bottom for our gateway season. Powering both reduces the loads on the bottom pivot axle.