Gearing a Rotating Arm

[kitare102]

Our current robot design includes a dual-flywheel shooter on an angle-adjustable arm which we are currently trying to figure out how to rotate. For reference, the arm can be approximated by a mass of 3.5kg at .3m from the pivot. Our question is what would be a good way of gearing the arm such that it is responsive. To make another approximation, let's say "responsive" means traveling from horizontal (0 degree) to 45 degrees in .5 seconds, which should really be taken as a guideline over a restriction for the design.

The current model involves a versaplanetary (motor and stages undecided) followed by a sprocket stage which is necessary for the placement of the gearbox but can be made a reduction anywhere from 1:1 to around 4:1. Is a versaplanetary a good choice for this situation? If so, where to go from there and if not, what other options have teams used?

[asid61]

JVN calculator is my go-to for rotary stuff. Just plug in your ratios and it spits out speed.
For your application, I get a speed of 0.27 seconds to go 90* if I used 2 miniCIMs on 100:1 versaplanetaries (no extra reduction).

[pmangels17]

Just a heads up, that speed sounds like it could be quite fast if you are looking to control the arm accurately, and you might want to play around with it a bit. I don't know the specifics of your mechanism, but you could probably mess with the chain reduction part of it to get it to a quick controllable speed in any case, and that would probably only require the minimal maintenance of changing sprockets. This is by no means a must-do, but it might be something you'd want to keep in mind.

[JohnSchneider]

No arms please

[waterbott]

Originally Posted by JohnSchneider

No arms please

Out of curiousity why would you suggest against that? Our team is looking on doing something similar

[Jarren Harkema]

Originally Posted by waterbott

Out of curiousity why would you suggest against that? Our team is looking on doing something similar

Two teams wanting to work with severed arms? Wow... Where are you sourcing these spare body parts?

[GeeTwo]

If you can swing that heavy an arm that fast, why not just remove the dual-flywheel shooter and its motors and throw the boulder a bit faster?

[kitare102]

To answer a few questions and raise some more...

The arm is relatively short, and only rotates about one axis. The ball is contained by the arm as it moves, so the ball falling out isn't a concern.

Now, according to the chart here: http://content.vexrobotics.com/vexpr...e-20151221.pdf (page 25, miniCIM simple load ratings) a miniCIM is not recommended for a versaplanetary at a reduction of 100:1. How should I interpret this?

Thanks for the help so far, everyone.

[Ginger Power]

Originally Posted by waterbott

Out of curiousity why would you suggest against that? Our team is looking on doing something similar

Their 2015 Robot could have something to do with it...

I think the teams in question are dicussing a very different kind of arm though. If I'm picturing it correctly, it's just a pivoting intake/shooter on a lever arm. A very simple concept as there is just 1 range of motion to worry about.

[Joseph Smith]

Back in 2011 when we had a long arm, we used gas springs on moment arms to balance the weight of the arm. If you do it right, the arm will stay at any position you put it without having to stall the motor to hold it there. A sprocket or gear bolted directly to the arm, driven by a miniCIM in a 100:1 planetary with a decent secondary gear/sprocket reduction should get the job done.

[Joseph Smith]

Originally Posted by JohnSchneider

No arms please

The Second Amendment begs to disagree

[JesseK]

Originally Posted by JohnSchneider

No arms please

Woah woah woah! You guys were the masters of arms last year!

[philso]

Originally Posted by Chris is me

Arms can be tricky!

I would avoid driving a long lever arm with a live 1/2" hex axle, ...

If you must motorize a long lever arm, it's probably best to do the final reduction for the arm using chain, with the sprocket bolted to the arm to transmit torque. 973 RAMP has some good videos on how to design a simple durable arm power transmission system. The chain can deal with some of the shock loading the arm will face a bit better than a final gear stage could, as there is some flex in the chain and the sprockets are engaged at multiple teeth versus 1 or 2 teeth for a geared system.

A team here in Houston twisted some 1/2" hex so that it looked like part of a wrought iron railing last year by transmitting the torque from the sprocket to the arm through the 1/2" hex.

Originally Posted by JamesCH95

3) Arms can usually stall to hold position, but you must make sure this current is acceptable (see JVN calc for this). I've designed arms that stalled RS775s to hold position with no issues, use your judgement here

The RS550 and RS775's rely on fan cooling. If the stall current is high enough, the windings will start smoking pretty quickly, possibly causing permanent damage to the motor.

[MrForbes]

We made a wooden cam last year to raise an arm, I think the rate and load would be similar to what you're trying to do. The students come up with some neat ideas!

[Sperkowsky]

We plan on running a small arm on a cim geared by a versa planetary 100:1. The arm should be done this week and will definitely be tested next week. We may tone the gearing down and if it seems like a situation of durability we may substitute our P80 in. But, It looks like it will be fine. One thing I will see is do not directly drive the axle with the motor. Use chains, belts, or gears. We plan on using gears.