VEX Transmission Recap/Explanation/Tips

Recently, this post showcased a cadded model quite similar to the drive/angler transmission we have been devloping and improving upon since the summer, so adding on to the prior topic we’d thought we would explain how we implemented our transmission in practice, what we learned from it applied practically, and some tips for teams interested in building something similar in the future (full robot explanation videos are attached at the bottom of this post).

trans1 trans2
Overall, our transmission offered two main advantages: being able to give more angling torque (totals out to be 1:14 100rpm from the 4 green motors, which allowed us to angle 11 cubes quickly with ease), and being able to save 1 extra motor to use for other applications. Note that I estimate this would be able to angle up to 13 cubes easily.

There were 2 main non-negligible downsides that we noticed using this implementation of transmission. This first being friction losses, which made the robot have less pushing power while driving (comparable to x drive). However this was slightly mitigated by applying white lithium grease. We did not have any issues of motor overheating whatsoever the whole season, just make sure the gears free spin easily while building. Another downside was that we were not able to angle and drive at the same time which saves time during skills runs. Team 81K was able to develop code being able to power the drive and move the transmission simultaneously during their itz season, but under the heavy load of 11 cubes it isn’t able to supply enough torque due to losses as covered in this post.

Some helpful look-outs to building this transmission effectively in practice is try not to cantilever your gears, as this caused us skipping issues. It is also preferred to minimize the number of gears that are being used to minimize losses of friction and to decrease slop on the wheels - essentially try not to make the drivetrain unnessecarily complex. The aluminum on the four bar part of the transmission should be full c-channels, as the half cuts on our first iteration eventually snapped due to metal fatigue accumulated over a few months.

In summary, I would recommend this implementation of a drive/angler transmission if you have a definite idea of where you are going to use the last motor. Something like an rd4b would be an interesting idea. It is possible to fit a cube lock on our transmission robot, but it would have been placed on the tray (similar to 7k), which would be slightly harder to implement.

Anyhow as promised, here are the full robot explanations, feel free to ask if you have any questions!
(there are more random drivebase tests and videos of the robot in action on my channel if you’re interested)

Final Explanation (current iteration)

Robot Reveal (current iteration)

Second Explanation

Initial Explanation

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If I have anything to add, having seen the evolution of this entire system is this one important notion that this takes a lot of time, effort, and resources.

If you’re looking at this as an idea, please justify to yourself the huge hour-cost associated with trying different build techniques to integrate such a design, please understand the trade-off that you’re necessarily making not just in mechanical power and flexibility, but also in the fact that you’re very likely going to cut the amount of time you have programming or driving the robot by half, if not more.

Transmissions are also very difficult to repair, unless you have a hot-swap drive system (@40404A). This means that should something snap in a match, there’s a lot of pressure on the pit crew to repair and rebuild the entire system, and you’re likely looking at a repair that lasts around 30 minutes.

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Thanks for sharing the design! It is going to be easier to do next year with many people sharing details of their robot with great explanations.

@ranOOm your mentioning of hot-swappable transmission units got me intrigued. Now I will start thinking about it a lot…

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For screw joints on gears, did you use the circle inserts for free spinning gears, or screw a screw into the square axle inserts?

It’s the circle inserts. From what I know it’s impossible to put a screw in an axle insert

Well I can say I have done it…not easy though. I wasn’t sure if you needed the rigidity though. The screws are still somewhat loose in the circle inserts.

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Would the looseness be an issue?

Not that I know of. The circle inserts fit very well with the screws with no slack at all. Sometimes I even have to screw on the inserts

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I’ve had similar experiences

While green inserts are about the right size for an 8-32 screw, there are small variations in both insert inner diameter as well as screw outer diameter that may depend on screw manufacturing process, coating material, insert batch, or insert age.

The bottom line is that, if you have a mix of old and new inserts, you may want to sort them by how tightly they fit on the screws. You don’t want them to be neither too tight nor too loose.

If the only type of inserts that you have are too tight on the screws (gears cannot freely rotate), then you may want to carefully widen them will 11/64 (~4.36mm) drill bit, because extra friction will negate any possible advantages of the differential motor sharing.

If the only type of the inserts that you have are too loose, then you may want to try screws with different coating or from different manufacturer to see if you can find better fit, because when gears are too wobbly it may result in additional power losses.

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We started out early season in Tower Takeover working with 5-ish teams to develop one single robot. We all started out with this robot, which was later scrapped because the tray kept moving while driving around. Keep this as a note: if you are wanting to build this, you may need to slow your drive speed down to ensure the tray does not move, which will result in power loss.
This is the 6 motor drive / 4 motor tray mech, which was disbanded:
image image image image image

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Nope. So far circular green inserts have been working pretty well for me. They fit tight enough and are pretty low friction on screw joints.

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Hmm, we never had this issue before. We would always be able to run the motors max speed with the tray always staying in its rested position. It would only move backwards passively due to gravity when im driving around with it raised.

I have a feeling this is a friction issue…

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