Reverse Double Four Bar Tips

Hello, I have been looking into building a reverse double four bar. However, i was wondering if there were issues with this lift that are harder to see.

I know it can have stabilization issues. We plan to fix that with multiple supports between each side. Also, we plan to have a program to keep the sides equal through potentiometers.

I think i remember there being a few weird things to do with powering it, but I’m not too sure. We plan on having 4 motors powering the lift from the second vertical section at a 1:7 gear ratio and tons of elastics.

What are the best ways to mount this type of lift? We were thinking to put the towers in the front of the robot. Are there any common issues to this?

If there’s anything I’m being vague about or that i have missed, please let me know. If you couldn’t tell, I’ve never built a RD4B before so thanks for the help!

It’s a good idea to mount the motors at the middle vertical bar, and be sure to make the metal bars sandwiching the gears as close together as possible to prevent gear slippage, aim for 3 holes of space.
Mounting it shouldn’nt be that complex, just attach it to a 18" tall tower. People use 5 width c channel most commonly, and just be sure to support it with a strut.
With what you have described it looks like you’ll be successful.
Lastly try to use really long screws and holed inserts in the gears instead of axles when possible, it will reduce slack and lean.

It sounds like you have a great start, to your DR4B.

Powering motors from the mid_section.
4 Motors
Mounting in the front(not sure you can mount it anywhere else)

Be sure you save all the space you can between the towers. You will need “every” space you can get. Many teams shave there c-channel just to get enough room for the intake/cube conveyor.

Good luck!

I’m confused on what you mean by this. Do you mean to bolt the large gear to the linkage bar?

I believe he means have (all) your joints using (long) screws and bearing blocks, including your gears.

that is, use 2" bolts going through your gears in the middle vertical tower with the circular inserts, rather that using axles and the square inserts. The bolts will be a better joint, and will also force things to be close together with minimal slack.

If you’re like me however and using doubled up low strength gears, you’ll most likely use an axle as trying to file out the square holes to fit a bolt won’t be accurate. I’ve got my gears on axles with 2.5-3" between bearing blocks, but because all my other joints are VERY solid, there is practically zero lean forwards/backwards (but still some side to side as I haven’t added bracing yet)

Hopefully this helps, and don’t forget to locknut your joints :slight_smile:

How much height do all of you get out of your reverse double four bars?

You can easily score the top skyrise cube.

RD4B height varies from about 4 to 4.5 feet. This is not the actual height of the robot, but the change in height from all the way down to all the way up. If your object manipulator moves cubes up that last bit, then, yes, you can score on a fully built skyrise. Otherwise, you should be able to score on the tallest posts and either 5 or 6 skyrise sections, depending on how well you build your lift.

Let me tell you some things that I’ve done on our reverse double six bars. Double up the high strength gears attached to the arms. This both increases tooth contact and reduces slippage while also spacing the two arm sets a little further apart. I find that this extra half inch of clearance is extremely helpful. Doubling up the gears, unfortunately makes it so that you cannot use a 2-inch bolt as the axle. Fortunately, Vex came out with high-strength axles for this year. In my opinion, these are better than using bolts. Simplicity in moving parts is key to strong, reliable systems. Since you are using a 1:7 ratio with only 4 motors, place the pinion gears (driven by the motors) on the top and bottom of the lift arm 84-tooth gears. This minimizes friction as well as keeps all the gears meshed together. As always, make sure everything lines up well and axles spin freely before putting rubber-bands, motors, or any weight on the lift. On the high strength axle bearings, we drilled them out to a slightly larger size so the axles would spin more freely. When I say slightly, I mean the next size of drill bit up that is as close to the diameter of the inside of the bearings. You only want to make them a tiny bit wider.

Here is picture showing most of what I have said.

Is this doable without high strength axles? we currently have nothing to do with them. To get them is going to be a big order too.

Also, is doubling up the HS gears necessary?

Doubling the high strength gears is not necessary, however doubling the normal gears was the previous version of high strength gears.

I don’t think you need high strength axles, especially if you bolt the gears directly to the metal, which is much stronger than using axles.

This is something our teams are wondering too. Most are getting near driveable build.

Some teams have opted for HS shafts in the smaller follower gears as the force is large.

But the weight of those big metal bars are heavy!

This isn’t the purpose of the high strength axles. I was assuming that the gears would be bolted to the metal.

The high strength axles increase torsional stability significantly–there is a larger area for the load to rest on as well as HS axles being extremely difficult to bend.

A RD4B is doable with regular axles. We had them on a RD6B before. It wasn’t as stable as it is now, but a RD4B doesn’t need it as much.

I realize now that with a RD4B you don’t need the extra space provided by doubling up the gears because there are not any extra linkages on the arms. While doubling the gears isn’t necessary, it’s still a good idea.

What are thoughts on this gearing arrangement? Ignore that it is a reverse double six-bar. I was experimenting.


Something I learned today is that an RD4B suffers tremendously when the weight is infront of the top arm. We had a light weight skyrise intake out front, and the whole lift was thrown off. Keep the weight a little further back.

a lot of excess gears I think. it will still work but I think the best way is to gear the top and bottom lift sections together directly and bar the top gear up to power the second section. its not a bad way to go though.

What is your intake, because that actually effects your dr4b a lot not just in the gearing but the building.