DR4B Tutorial by 333A

[333A]

DR4B tips from ITZ world champions Team 5225A:

@nickmertin

Here is an album of pics of our worlds ITZ robot, many of which show the DR4B. I’d recommend you take a look at the methods of attaching elastics, how we braced it, and the different ways of saving weight (depending on how you’re planning on using it). A major factor in our design was the thinking that we’re only using it to lift one cone at a time, which is fairly light, so we don’t want 5lbs of overhead; this thinking may or may not carry over to the current season, depending on what you’re thinking of for design.

Excerpts from: Tips for Stabilizing a Lift (Prevent Sway)

@400X

Obviously, you want to figure out where in the lift the instability is coming from and reinforce or tighten up that part of the system. … Instead of looking at the whole lift for instability, analyze it one section at a time.

There are a lot of ways to “stabilize” a lift, one way you can remove a lot of the slop is by using Single-Rotation Joints . That’s a video on common joint designs in VEX from Owen of BNS. I’ve linked to the last part of the video. In case anything was unclear, you’ll want to firmly attach a Screw to one of the bars with a tightened Keps nut. This will remove a lot of the slop and keep the screw perpendicular to the bar. Then you would run it through a bearing on the other bar so it can rotate, and then hold it all together with a Nylon Lock nut, just tight enough to keep it from coming apart. This joint only rotates in one place and you don’t have to compromise between a tight lock nut to remove slop and a loose lock nut to prevent friction. I would highly suggest using these wherever you have a joint using screws.

You will definitely want to run some cross supports between the two sides of your lift to prevent it from swaying side-to-side. Make sure the spacing is correct so you aren’t pulling them together or pushing them apart and adding friction. You want the bars to always be parallel between the left side and the right side. This isn’t referring to your actual linkage, there are some instances where the bars within the linkage could be non-parallel. I mean when you look top-down at the robot, not from the side.

Not as many teams do this, but I always like to sync up the two gearboxes when I build a lift. This would mean running a shaft that goes through the left gear train and the right gear train to forcibly keep them in sync. This helps a lot with tilting side-to-side. Its best to have that shaft on as small a gear as possible because x Degrees rotation on the smaller gear will equate to less vertical movement in the lift versus the bigger gear. …

I would highly suggest studying 118’s Skyrise reveal very intensely. A lot of teams have made great DR4B’s and DR6B’s, but 118 was one of the pioneers of this design. The Double-Reverse lifts had extremely limited use before Skyrise and 118 had an insanely fast and stable lift from the very first competition that year. Other teams may disagree, but as a South Texas competitor, I always see 118’s DR4B as the beginning of the era. They didn’t invent the design, but theirs was miles ahead of any other I’d ever seen before. A very important thing to note was how they used C-Channel Extenders to increase the stability of their joints and reduce torsion on the bars within their lift. Freeze frame here at 1:09 and look right above the 84-tooth gear on the left. They’ve put a C-Channel Extender inside the main C-channel and run the joint all the way through. 2587z was another brutal Double Reverse Lift that year as well as AURA in VEXU.

That’s all the specific tips I have at the moment. Keep in mind the different ways the lift can be unstable, and I’m sure you’ll be able to invent your own solutions as well. It can sway sideways, the lift can tilt sideways (One side is higher than the other), and then these could both happen forward and backwards as well.

Keep your joints tight and your COG centered, and I think your DR6B will be very successful.

@Owen

Just to add to that response, the best type of cross bracing between the bars in my experience looks like an X. This can keep the lift from flexing more than just a bar across, since it reinforces the lift in the direction that the metal is strong in. (it would need to lengthen or shorten to flex much with X bracing)

@anon4126930

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The main reason why my robot worked so well with one motor with 200 RPM and 1:7 gear ratio is because the lift, although looked bulky, was incredibly light (Seen in the reveal, they are using C-Channel Box bracing rather than X-Bracing, because it’s lighter and smaller) and the banding was tuned explicitly to be near linear with only two points. So, if you want your lift to be fast and only use one motor you need the following:

• Light field elements and in low numbers
• Lift that uses KEPS screw joints, in all places possible (Even through the gears)
• Lift has no more than 2 axles for its support, so you can track the position of the lift with a potentiometer

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If you lift up 10 of the cubes, you will be lifting up roughly the weight of TWO mobile goals, or 1 Mobile goal and 10-12 cones if you know the struggles from In The Zone. My cap bot only lifted 0.6 Ibs of weight. So if you’re planning on doing one motor you should be either doing 100 RPM 1:7 with one motor or 200 RPM 1:7 with two motors, and even that will likely be more or less a struggle for most teams.
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Delta IV Iteration 3 Close Up: https://youtu.be/UCfDbmOVanE

@technik3k mini-tutorial about building very stable DR4B with 2" long screw joints:

@technik3k

It is a well known “secret” among the veteran VRC teams that it is best to use single bearing screw joints when building 4B, 6B or DR4B/6B lifts. With them you can achieve minimum slop, which translates into more stable lift that could go higher with more precision and less wobble.

To reduce slop and increase precision further you can use longer (2") screws, They act both as a gear axles and rigidly connect sides of the DR4B mid-section.

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@technik3k

@Anomalocaris, that is a good observation about the top gear being offset forward - there are multiple reasons for that. As @AperatureLabs correctly pointed out, the main reason for the offset gear was to move top bars forward such that their top/forward joints could be fully in front of the tower.

Also, splitting mid-section in two, allows precisely adjustment of distance between the gears (@Got_a_Screw_Loose got it right), improves its structural rigidness, and makes it easier to build entire DR4B in two separate pieces and then assemble them together.

The focus of the original DR4B post was to show how long screws in the joints could improve stability and precision of the DR4B lift.
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Even more DR4B building tips:

@CSpencer

Here is my ITZ robot which was a cross between a dr4b and a 6-bar.

My tips for you are that you need to keep everything consistent, both sides need to be cross-braced or connected together in some way or form. This year if you are expecting to lift more than a few cubes you will need 2 100RPM motors and probably a 1:7 ratio. I would personally power the lift from the middle section much like the ITZ robot I linked you instead of powering it at the bottom like the other video.

If you have high strength shafts I would use them especially this year because of the weight being carried by the lifts, just be sure not to rely on them and instead rely on x-bracing.

@Jarred

A few simple mechanical things that might be the issue:

• Make sure the rubber bands on each side are equal
• Make sure the pivot points are the same (correct) tightness
• Make sure there is not binding or extra friction in the gearing or pivot points
• Slightly pull the axles from the motors and move the lift by hand to try to find specific bad spots
• Make sure you’re using all the same gearing in your motors
• Make sure everything is symmetrically balanced.
• Make sure your entire lift is square (the arms are 100% parallel)

@Team80_Giraffes

Cantelevering is not a good idea on a lift most times. Shoring up both left and right sides of the lift with another braced piece of metal will go a long way.

The clicking may be alleviated by using rubber bands to assist the lift. As you lift higher and the lift get vertical, the weight distribution changes a bit. The first joint where you have the big motors carries a bunch of weight. You could also be getting some torsional twist on those points as you lift up and that coupled with the cantelevering could be causing some separation of the gears.

Remember the gear touch point is pretty small. It’s just along the face of the gear teeth. You need good contact there to transfer the force.

@biglesliep

• in each segment of your lift you need cross-bracing. Just bracing once in one segment isn’t going to do it.
• on our lift, my team has 1 or 2 c-channels spanning the gap (in different areas) as well as 1x pieces that cross in an X shape, attaching left side to right side. The 1x pieces are great because they’re lightweight and flat, and are still effective.
• my team also has “self”-bracing on each vertical segment (from chassis to first gear; from 2nd gear to top gear), so that we have teeny 1x “X” bracing between the 2 c-channels comprising each side of the lift. Example: there are 2 c-channels going up from the chassis on the left side of the robot, with the gear sandwiched in between them. Where we can fit it, we have little 1x "X"s connecting these c-channels.
• rubber band triangle won’t help your lift go up straight, but it will help your lift go up, period. The concept is that the triangle method provides continuous tension through the whole range of motion of the lift, not more when it’s folded & less in the middle. My team had to add a little appendage to one of the c-channels for the top point of the triangle; you’ll just have to play around with this to find a setup that gives you “neutral buoyancy” – the lift stays wherever you put it, without motor help.

@Jasper_MCC

In general though, friction is your enemy. You can counter-act a little extra weight with more rubber bands, but friction is never good.

Symmetry is very important, not just weight left-right, but also the length of the bars. If you have a longer bars it can cause the geometry of the lift to change as it moves, making the lift “non-linear.” An ideal dr4b has all 4 bars identical lengths. Changing the top 2 to be shorter or longer than the bottom 2 will cause the lift to “drift” forward or backward as it goes up. Longer top means forward, shorter pulls backward.

Another issue to watch is wobble. Now this seems like tightening the joints would make it wobble less, and it does, but it also make the lift get “stuck” leaning, which then get worse the more it tries to fix it. So by making the joints looser, the lift can fix itself easier, which overall decreases wobble. And cross bracing is important, of course, but not always easy to do if the middle of the lift has to be open.

Those are the 3 biggest things to watch IMO.

@technik3k

… the best place to put motors on the full size DR4B is in the middle section.

The weight of the motors is not going to be a large factor, given the total weight of the rest of the lift. That is usually taken care of by rubber banding the lift such that when there is no game objects it would take almost no force to bring it up, because rubber bands would counteract the gravity.
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Example of the well crossbraced and rubberbanded DR4B from this thread:

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@josh_siegel

… there is always room for improvement so I will suggest some stuff at the end. The reason why it would be leaning to one side could be due to many reasons but will most likely go away if you employ these strategies:

1. Make sure your lift is as symmetrical as possible. I noticed you are using aluminum angle as the bottom bars on 1 side and c-channel on the other. I would change that to c-channels on both sides. Also make sure your angles are exactly what you want them to be (ex. 90 degree angles for a rectangular gearbox, if it isn’t than the holes won’t be exactly lined up causing more friction).
2. Speaking of friction try to eliminate that as much as possible. Any little difference between the sides can cause one side to need more power and therefore sway. This could be caused by the symmetry difference, bent gears, bent axles (even the slightest bend can cause a huge difference), etc… what I like to do is disengage the lift from the motors and feel by hand how smooth it is… it should be extremely easy to lift with just a finger. Also once everything is extremely smooth I add some white lithium grease just to make it even smoother (you would be shocked at how much this helps)
3. On the bottom bars I noticed you have a random 84 tooth gear on the bottom most bars on each side. There is no need for them so I would just take them off.
4. I also noticed you are using axle joints on those bottom bars, you should always use single rotating screw joints. There are a lot of forum posts already about these but essentially one piece of metal has the screw directly attached to it so it only rotates with the metal and the other piece of metal has the bearing so the screw only rotates in that bearing keeping it low slop and low friction. Make sure to keep joints as small as possible also.
5. Take the other’s advice with the X-braces! Those are SO helpful! I attached a photo of an old robot of mine showing the x-braces to see what they are but they really stabilize the lift. You can try linking the gearboxes but these actually help way more.
6. Once you are done mechanically stabilizing… very important that it doesn’t actually need this but as a backup I used to like to use encoders on each side of the lift to sense its position and correct for it using independent PID for stabilization, but this is not necessary and you should only implement once it is mechanically stable as a backup.

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