Heavy DR4B

I’ve got another question. How would we be able to directly attach the gears the c channels? There is a screw in the way that is holding the bearing block.

If I understand what you’re saying, you would just need to space out the c-channel w/ the gear from the bearing block.


What @Battlesquid said

Ya know I never even thought about that…

Yeah that’s probably where the strain is coming from so you might want to find ways to reduce it’s weight.

Oh boy. There is a lot wrong here. I’m gonna address certain things step-by-step.
First, the rubber bands:
To simplify the concept of triangle rubber banding for you, here’s a pic of what I think your rubber bands should look like:
The purpose of triangle rubber bands is to give you A. an extra bit of rubber band length without making the force direction too close to parallel with the c channels and B. Just generally introduce a bit more tunability. (Also, in my opinion, this notion that triangle rubber banding will effectively linearize force output is to some extent false, so in my opinion there’s no real reason to do triangle rubber banding unless you can’t do linear rubber bands (which would just be the hypotenuse of the triangles I drew in the above picture)

Next up is your joints. They don’t actually look too dysfunctional, and although they could be a lot better I think they’re plenty okay for now as long as you’ve locked the screw on the inner side of the c channel with a keps nut and as long as your spacing is fairly correct.

The other thing that obviously needs an improvement is that in this system you only need two gears; you only need that singular method to transfer power from the bottom stage to the top stage, and the other gears are simply doing nothing. I think if you remove your upper gears and put the C-Channel directly on the joint the stress your upper bars are currently facing (which is causing the standoffs to warp the c channels and gears) will all but disappear. If you continue having issues, I recommend placing a c channel on the gear that is connected to your top stage in order to stiffen it, as it’s the weakest part of the upper four bar. You can see the screw destroying it in the picture you sent.

Lastly, your rd4b is not very heavy, but if you have a lot of load on the end of your rd4b and you don’t think you can lift that much load, remember that rd4bs function as essentially a very, very long lever. In this case, the lever would be a bit more than the lever of a 50-hole-long regular four bar. Sounds a bit wild to put that much weight on the end of such a long lever, right? Think about shortening your bars. If built right, an rd4b that consists of two 25 long c channels can reach around 40" high. There’s only one tower that requires such a height, so you probably don’t need to go for it until very late in the season if at all.

Now, remember, the fixes I’ve listed are meant to minimize change to your gearbox design. These changes should get you functioning quite well on this robot, but you won’t be near a peak. Keep this in mind for your next robot, and make sure to learn as much as possible about strong joints and bracing!

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Thank you for sharing the photos. I wonder how the screw joint is used in the DR4B. It seems the gap of the middle tower is 4-hole or 2 inch. And the longest screw is 2 inch. I saw two kep nuts at each end. How do you make the screw go through the middle tower to support the arm with gear in Screenshot_20181017-113442_Gallery. Thanks

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The gap isn’t a perfect size. The channel that is the bracing on the bottom in that picture is filed inwards so the gap is actually slightly less than 2 inches so that the 2" screw can go all the way through.

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Wouldn’t the rubber bands gradualy loose strength over time?

Thanks. Why do you need two kep-nut inside of the bracing c-channels? Should there be on on the outsider of bracing c-channels on the opposite side of screw?

Ya but u only have to replace them like once every month depending on what brand of bands ur using

You can also remove the bands when the bot isn’t in use.

Tbh I think the rubber bands are bending the arms of the dr4b, bc they have so much tension in then

No, OP said they have a heavy attachment on the end of the DR4B.

No like this.
If the arms bent because of weight on the ends, they would be bending.
In these pictures, the arms are not bending, but are twisting. This implies the force is not on the ends of the arms, but rather on the sides, where the rubber bands are placed.
Further evidence of twisting is in the 2nd image. The reflection of the top bar compared to the lower bar suggests that it is twisted to face up. The inner face is being twisted to face down, and the rubber bands happen to be mounted on the inner side
The first four bar does not bend, because the c channels are supported on both sides, whereas the upper 4 bar is not.
Although since the standoffs on the lower bar are also bending, there is most likely a lot of weight on it.
Btw I was talking about the upper bar not lower bar.

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If the “twisting” of the arms was actually caused by the rubber bands then the arms would twist towards each other, but both arms have “twisted” in the same direction.

The reflection in the image shows that the upper bar is twisted to face up while the 2nd one is not.
If there was bending caused by weight, both would bend in the same direction, creating similar reflections.

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The bottom bar is also “twisted” if you look at that 2nd picture. The rubber bands do affect it to where the bottom bar is “twisted” less severely than the top, in which case double siding the bands would be helpful. But the weight is also playing a significant factor in it


Yes, I agree that the weight is causing the lower bar to bend as well. I wouldn’t really call it twisting, more so it’s being pushed down.
Double sided banding is a good idea though. Itll fix the issue and give you even more tension

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Nor would I, that’s why I added quotations marks to “twisted”

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