# Planetary transmission: can this really provide the torque of 2 motors?

One of my kids has become obsessed with this planetary transmission system he saw in this Guru jpearman animation:

http://jpearman.smugmug.com/Robotics/Misc/17225808_v2Hjfz#!i=1829423358&k=hS24cJx&lb=1&s=A

The kid seems to think it will provide double torque when torque is desired, but I’m not convinced and I’ve found mixed forum comments concerning actual available torque. Some comments suggest that the max you would get is only one motor’s worth of torque, not two. Can somebody please tell me or point me to an explanation of how the output torque relates to the inputs of the two motors?

Im beginning to doubt myself again because I haven’t seen that animation before, but if I did understand it correctly,

your speed ratio, which is when the motors spin together, uses the power of 2 motors minus all the friction you’ve gained through the planetary gearing (from experience you lose more than you expect)

Your torque ratio however will depend. If there is lock(whether pneumatic or motor), then it will use 1 motor, multiplied by the torque ratio, minus all the friction gained. The downside is that if you don’t have a lock, then motor input 1 will backdrive. Because of this, though you have more torque, your system will give way before it reaches that torque, essentially lowering your speed without raising torque.

In this video, there is no lock. There’s nothing which rivals the blue gear connected to the green gear. So the gear will give way under stress

the gear ratio of the transmission between the input and output is dependent upon the direction of the 2 motors. Both motors input goes to the output. The planetary transmission system can switch between high speed low torque, and low speed high torque, just by reversing the direction of one of the input motors.

This type of transmission has advantages over a conventional transmission because it does not use a shifter such as a piston to switch ratios, it can switch ratios with the same motors that power the output, yet all motors are supplying full power to the output.

The problem with this transmission, is that the chain that is used as the “ring gear” slips when used in a drive-train to push something, rendering the “high torque mode” pretty much useless. That is the only flaw, if the chain were to have been an actual spur gear instead of a chain slipping on sprockets, then the transmission would actually be able to handle high torque loads required when pushing other people.

Basicly, yes it can supply the power of two motors.
However the transmission can’t handle it when the drive train tries to push something.

I never did build a prototype of that transmission. My animation was in response to the discussion in this thread from almost two years ago.
https://vexforum.com/t/4194b-drivetrain-differential-planetary-pneumatics/21482/1

We were all blown away by what team 4194B had brought to worlds in 2012, here is a photo of part of what they had built.

and also incorporated somewhere in this monster.

You do not get double the torque with that setup.

If you’re familiar with planetary gearsets, you’ll know that they work by locking different components and powering different components for each ratio (some electric drills).

I unfortunately don’t have time to go into the physics right now, but feel free to message me and I’ll get back soon with a decent explanation.

But basically, the planet carrier and the ring gear in jpearman’s animation are BOTH powered by motors, so they only produce a ratio of 1:1 when they are unloaded, but as soon as you apply a load, one motor will encounter more load than the other motor, so you run into the case when one motor will actually fight the other motor, up to the point when one motor is completely stalled and the other motor is transferring its full torque to the output of the system.

Without locking some part of a transmission, it’s not possible to use a different combination of motor directions to change ratios. I ran into this when I made my differential-based transmission at the same competition as 4194B showed off their awesome drivetrain. You’ll notice (in the linked video) that I had to lock part of the mechanism when I switch to low gear, and in 4194B’s transmission that they had pneumatic locks when they switched ratios.

OMG, I’m very glad I asked this question and I’m grateful to all of you for your rapid replies. Though I have an engineering background, I confess I haven’t been able to grok how this drive works so it’s been hard for me to give a nod to the design the kids want to incorporate (never mind the cost of more parts!). I know the kids will be disappointed to hear this drive isn’t the answer to all their worries but surely that’s probably better than letting them run down a dead end. Thanks again for helping us out.

If you can afford the parts and can find the time let them build it. Far too often we limit our students learning by telling them ‘that will not work’ before they try it. In NZ we believe you learn as much by failure as by success and the lessons learned last a lifetime as will their understanding of how it does work.
Let them build it and analyse it for themselves. You may need it in another comp in the future.

Usually I agree with this philosophy but we don’t have the extra money or the time to explore mechanical dead ends. The kids want to add something “special” to their robot before Worlds and they insist on doing something mechanical/visible (rather than better Programming Skills, which was my suggestion because it won’t cost anything but time and effort). I know they are going to hate me when I tell them this planetary drive won’t make all their dreams come true. I’m sure when I show them these forum comments, they are going to accuse all of you of being part of a global conspiracy to make them improve their Programming Skills.

I read your post and it brought back memories of my team from earlier this year. In August we embarked on a differential transmission. It was a good learning experience but it (along with some other challenging problems) took up about eight weeks of development time. The result was that this concept didn’t work as well as we thought and we scrapped the whole idea. As someone else stated in their post, there are a lot of losses in these systems.

With just a few weeks until the World tournament I would agree with your position to just try and improve programming skills. It will take a lot less resources and probably be much less frustrating. At this point of the year, I highly recommend a design lock down.

Good luck to you and your team!

Thank you for confirming my suspicions. It helps a lot for me to be able to point to forum posts to help confirm what I’m saying since my kids rarely believe my naysaying when they’ve got their hearts set on something.

I keep telling them that, but they’re dying to do something with their hands. And, alas, programming isn’t one of them. Don’t ask me why.

Thanks, we’re going to need it. For me, just to maintain sanity these next 5 weeks is going to take more than mere luck.

I’ll often phrase it as, “You’ve got experienced people with good advice telling you that it won’t work, and explaining why it won’t work. But so did the Wright brothers.”

Jason

Ah, great point! I love it.

we’ve done lots of prototyping as well with this after we saw 4194B at worlds!

https://vexforum.com/t/team-1107b-taiwan-drive-credits-to-4194b/21506/1&highlight=taiwan+drive

more specifically in this video, you can see the back drive issues unless you have a “lock”

A fascinating saga, that. It’s interesting how you and the other forum members worked your way through the design and its issues. To provide a little consolation to the kids who were devastated that this transmission wouldn’t make all their dreams come true, I’ve now had to promise to teach them about pneumatics. If it ain’t one thing. it’s another. :eek:

As one of those kids who loves mechanics and designs, maybe you should show them this: Vex Roundup 1508a Peaucellier vertical lift linkage robot - YouTube , and challenge them to do a de-scorer

On a competitive point of view, in addition to autonomous, it’s probably best if you make sure each system is tuned to it’s finest, strategy is precise to the step, and drivers are amazing. They may not realize it, but it’s small things which separate the great teams from the good teams.

I almost had them convinced to try a buckyball de-stasher a few weeks ago just to be different at Worlds, but the driver nixed it. I guess there’s less glory in subtracting points???

That fancy mechanism is amazing. I’ll have to file that away in my Vex to-try list.

They definitely do not realize this and it’s the hardest thing to even pester them about. I think they’re always looking for the “secret weapon” rather than going over their robot after each match to make sure the nuts haven’t come loose again, etc. Some things they only learn the hard way, I guess.

Thanks!

descore at 6:31:00

Interesting. It’s almost creepy how it licks out the buckyballs. :eek:

When I develop secret weapons. (although I didn’t do it this year) is to have it so that it can be removed from the robot within 30 seconds or so. I myself never modify a robot to implement a secret weapon, I ether build a new robot based on the idea or add to an old one.

I can say this from experience because out of the last 18 competitions I have went to (spanning 3 seasons) I have gone into only 3 of them with essentially the same robot as the one proceeding it.

I have built robots that are made so that I can easily change out a subsystem, but I usually do this when the robot has a hit and miss idea so that I can change it so something more standard if it doesn’t work out.

But my best recommendation for making a robot unique is to find your own method of winning the game and clearly explain it to the judges. also make your aton code work with the strategy.