A friend of mine is trying this out. Will it work? Pros and cons compared to having a middle bar? He’s mostly doing this because he doesn’t have too much metal to work with.
That won’t work the way you’d want it to. The moving towers are there because the upper 4 bar goes below its pivot at bottom and it needs the extra height to utilize its full range of motion. If you were to do what you’ve described then you’d have to leave the lift halfway up to keep it from hitting the ground. In short: If you’re doing a dr4b you need the middle towers.
The more I think about it, the more it seems to be a double chainbar. He’s got chains running along both bars and the higher bars are on the inside, would it work like that?
Not necessarily. As long as the upper and lower beam pairs are offset, you could still cross them.
The middle section is rather an artifact of the gears connecting the upper and lower 4-bars - bigger gears - taller middle section (the only thing that really counts is axle-to-axle distance). If you use smaller gears, your middle section would be shorter. If you used tiny gears (and they could transfer the torque, which they won’t IRL), your axles would be very close.
If you went all the way to the “zero axle-to-axle” distance (i.e. having two beams on the same axle) and still managed to get them turn in opposing directions (hard but doable), it would still behave as rd4b.
Now, double-chainbar could work in a similar fashion if you synchronize the movements. The chain basically replaces the four bars. Now, if you drive them independently and synchronize them in SW, you could have extra possibilities - when synchronized, you’ve got rd4b. When intentionally driven independently, you’ll gain an extra degree of freedom for which the rd4b-based designs usually need an extra manipulator - chainbar of 4-bar ah the top of the rd4b.
(As with everything, there is the con side - reduced maximum stack height for the same size of the robot. But it might be enough for your goals)
The double chainbar should work, but there will still need to be a small middle tower for the sprockets to mount to.
Having just taken apart a very functional double chainbar, I can say this is not true. The second arm section pivoted on a shaft that also supported 2 sprockets screwed together to keep the end parallel to the starting sprocket. Worked pretty well, no middle bar at all, just not fast enough for us.
Oh! would you happen to have pictures I could look at? Just to understand the concept better
Oh
Hold up. All mechanisms mounted to one axle?
I dont have any good ones on my phone of the comp one, but here are some I took of the prototype a while ago. A little hard to see perfectly, so I will attempt to explain a little. the "shoulder is pretty simple, normal chainbar. The “elbow gets interesting. In the JANK pic (taken and sent to a friend, lol) you can see the parts a little better. Only the 12t sprocket on the very end is spun by the axle. The 2 18t sprockets are screwed together and drilled out, the HS 84t has green inserts. So the shaft goes through all those, then transfers power backwards on the end to power the 1:7 ratio. On the comp arm, the 84t was switched to a 36t sprocket, and the motor moved back really close to the shoulder and was chain driven 1:5 up to the elbow. This was because the 12t pinion shaft limited the elbow travel too much, so it would not fit in the 18”
Hope this helps!
