Our team is looking to use a rotating arm this season, and we are hoping to use versaplanetaries. The ratio we need is above the recommended specs, but if we have a ratio within the planetary gearbox that is within the range (100:1), then use an external reduction (3.6:1) to get us the rest of the way there, will this still ruin the gearboxes?

One thing to note is that for the VersaPlanetary (VP), the 10:1 stages are by far the weakest since they have the smallest gears. If you think you need a 100:1 VP I’d heavily recommend you go with a smaller reduction (aka replace one of the 10:1 stages) and then increase your external reduction.

Another note, should you end up using any 10:1 in your VP, it’s recommend to have the 10:1 early in the gearbox, i.e. closer to the motor than too the output shaft.

As to whether or not it will work, that really depends on the length and weight of your arm.

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So what would be a good way to test whether the reduction will work? Or if there isn’t a good way should we just use a custom gearbox/ off the shelf non planetary gearbox? And if so, what would be some good examples of a wrist gearbox for a longer arm? I’m thinking 1619 or 1538 in 2016, but I can’t find specs for that gearbox. Thanks!

You can do it mathematically using the JVN Mechanical Design Calculator and the VersaPlanetary Load Rating Chart. Look for the rotary tab at the bottom. You should be able to enter type of motor, # of motors, an estimated weight and length of your arm, and then input the VP reduction and external reductions. The calculator will tell you the speed, torque, and the amperage you are pulling. A good rule of thumb is to design for around 25 Amps max, which gives you headroom for unexpected behavior so you’ll remain below 40 Amps. This is important because the biggest fuse you can use on a FRC robot is 40 Amp. Theoretically you can pull 40 Amps for a few seconds before blowing a fuse, but you should design such that you are well below 40 Amps. Also max amperage for an arm is when the arm is horizontal since gravity acts on it the strongest then so your motor has to work the hardest.

Edit: Someone feel free to correct me if I’ve gotten anything wrong.

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The outside reductions won’t affect the load rating. You can use JVN’s calculator or mine to figure out if your total reduction is correct for the load you’re applying. But assuming your VP reduction is safe according to the Load Rating Guide*, changing the outside reduction will not make it unsafe.

That being said, the 10:1 reduction stages are the most fragile and prone to breaking. I wouldn’t recommend making a 100:1 reductions with two 10:1 stages, even if the Load Ratings Guide says it’s safe. You can get the same reduction with two 5:1 stages and one 4:1 stage and you will have a gearbox far less prone to breaking. Or you can stay with a 2-stage 81:1 gearbox (2x 9:1) and adjust your outside reduction accordingly, and you will also get a stronger gearbox.

*I didn’t check this, make sure you do

My team used a 270:1 ratio in 2017 with a bag motor for our climber, we had no problems with it.

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This is sound advice, and put the 4:1 stage closest toward the output. And then go with an external reduction from there. Also, keep your gear or sprocket close to the body of the VP, and/or support the end of the output shaft with another bearing.

One other option to look at is the AndyMark CIM-sport, which has gears cut from solid bar rather than sintered, and they’re a good bit larger. But, you lose the integrated encoder option that the VersaPlanetary gives you.