VersaPlanetary load rating questions

Our team is looking into rebuilding our climber, and we have a few spare CIMs and a VersaPlanetary laying around. Just had a few questions before we put orders in so we don’t strip mid-match (always fun!)

Looking at the load ratings chart, it looks as if 63:1 is the sweet spot for a single CIM. The problem is, we already have two 10:1 stages. 1. If we ordered a 7:1 stage and went with 70:1, would that do the job?

  1. How much headroom is there with these ratings? I noticed they are for the motor’s maximum torque, surely a standard ESC can’t even output the amps to get a CIM to max torque…

  2. In the past we used a 1.5" radius drum/spindle/spool whatever you want to call it… could we still use this on the 1/2" hex shaft or do we risk stripping the planetary?

Thanks for responses in advance. I’ve seen a lot of threads about stripped planetaries, I don’t want to be the next victim

Where are you getting that 63:1 is a sweet spot? 63:1 is a ratio that is said to be out of the load ratings. While the load ratings are conservative, I wouldn’t go much beyond them for risk of damaging your motor.

Either way, the point of these load ratings it that you should be able to bear up to your stall torque without damaging the VP.

Sweet spot as in maximum recommended for a CIM with a 1/2" hex output.

Now that you mention it… is there a way I can calculate the maximum “acceptable” gearing ratio for a specified drum/spindle/spool radius? I’d assume that would factor into reaching the motor’s stall torque…

You should look at this and see if it works for you?

The maximum acceptable is the one that meets your design goals best. Unless we’re not looking at the same chart, the highest allowable ratio for a CIM is 64:1, and that’s only if you use three stages (4 x 4:1). With two stages, 50:1 is the maximum.

I would recommend taking a peek at JVN’s calculator and seeing if that helps you determine the ratio you want.

If you truly do want to know the physics behind it, the higher the spool diameter, the more torque on your shaft.

Cool, thanks for the info.

Note: I am taking your 63% at face value, and did not check your numbers.

  1. If you’re worried about having 11% too much torque for your gearbox, just limit your throttle on the climb to 90%, or use current limiting.
  2. There’s probably enough headroom for some minor shock loads, which is probably no more than what you’ll have when the climber hits the davit. I wouldn’t plan to use that headroom for the main load.
  3. The smaller drum that you use, the less reduction and less torque you will need. If you just wrap the rope around a 1/2" shaft, you can probably use half to a third of the reduction you will need at 1-1/2". This will be much easier on your gearbox!

I’m just concerned about speed. I feel like if we lower our drum size too much we’ll climb way too slowly.

Also, watch the rope routing path and make sure any drag across that could increase friction is minimized. Make sure there isn’t anything that can ensnare the rope/belt/Velcro whilst climbing.

Try to provide a soft landing for the davit button and a cushion to allow the forces to be spread out - plus provide reaction time for the driver to stop the climb (and/or monitor the climb motor current and build in a gear life insurance program with or without an driver override feature).

Good ideas, I think we’ll wind up using a 1/2" ratchet wrench as our gear insurance :slight_smile:

The ratchet wrench is great insurance from falling after the power is shut off, but it won’t do a thing for the gear system in the climbing direction.

Don’t the sustained breakers in the PDP max out at a continuous 40A? That’s well under max torque for almost any motor I can think of

Played around with the JVN sheet a bit, and threw together my own sheet as well. How viable do you think using a 775 pro with a 100:1 dual stage and just the 1/2" hex shaft as the drum would be for climbing?

Using the numbers from http://motors.vex.com/ assuming a constant 35A delivery from a VictorSP to a 775 pro I calculated 9 in/s climb speed and 250 lbs of pull.

Sorry, to clarify. I do not think current = torque, just using charts from motors.vex.com to estimate motor torque output at specific current input values.

This sounds really, really slow. Last year we stripped a VP on our low goal shooter, so I doubt one can lift a 120+ lb robot in a reasonable time. I’d look at MiniCIMs (at least) and an AndyMark gearbox.

After saying that I see people climbing with a VP here, so maybe you are okay.

We end up saying this a lot:
Look at the breaker spec sheet.
40A is the MUST HOLD rating. It will never trip at 40A. At 200% overload (80A), it takes 1.5-3.9 seconds. At 300% (120A), it takes 0.5-1.1 seconds. A motor at stall is going to develop and apply pretty much full torque for atleast 0.5 seconds. Breakers are NOT to protect your mechanical devices (or speed controllers) from death. They’re strictly to prevent your wiring from catching fire.

My team, and hundreds of other teams, are using 775pros with one or multiple versaplanetaries to lift their robot. It’s proven to work

What value did you use for rope diameter?

What ratio did you use for power_out/power_in for the gearbox?

I didn’t factor in rope diameter… I didn’t think it had a significant impact in comparison to the gravitational pull of the 120 pound robot.

As for power_out/power_in, do you mean torque in/out? Are you referring to something specifically in the JVN sheet? In the past I have only done programming so I’m kinda clueless when it comes to this stuff :confused:

To calculate I just used net torque = F * r, where F = mg and r is radius of drum, and net torque is the torque of the motor at 35 amps * gearbox reduction