Is a Cim motor connected to a 50:1 planetary enough?

Our plan for the climb this year is to use a cim connected to a 50:1 (10:1 first stage and 5:1 second stage) versaplanetary that has a 1/2 in hex output. I keep hearing teams using at the least 70:1 with climbers, so i am kind of worrying about if our setup is going to be enough if our bot is going to be near the 125lb weight limit.

That depends on a few different factors. The most significant is the radius of your winch/sprocket/gear, since the torque from the motor in ft-lbs needed to lift up the robot is going to be equal to weight (in lbs) × radius (in ft)

To figure out if 50:1 is enough, you can either look at the motor curves (motors.vex.com) for a CIM to determine the current draw at the torque you need, or you can use something like the JVN Mechanical Design Calculator (if you search for it in Chief Delphi, you should be able to easily find it) to do the calculations for you. The key to remember is that the CIM has to be on a 40 amp or less circuit breaker, so if the calculated current draw is more than that with a 50:1 reduction, you need to gear down the motor more.

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Seconded on the JVN calculator for this. Just look at the elevator/linear calcs.

One thing we’re optimizing for is lower current draw on our climbers and a < 5 second climb. We’re thinking we will be using a lot of the battery during matches this year, so using less amps on the climb helps protect against brownouts at the end.

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Thank you! I used the JVN calculator, one last question though. Is the pulley diameter just the spool diameter?

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Yes, it is.

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It’s been years since I’ve used a VersaPlanetary, but are the 10:1 stages still having the issues from years past of binding up? I know I’ve seen a lot of posts recommending not going past 7:1 or so, but haven’t followed along as much this year. (Which would still get you to 49:1 in two stages, which is very in the ballpark.)

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I would not use a 10:1 in a high torque application. A 9:1 with a second stage after would probably be ok though

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The problem with that is that we already bought the 10:1 3 days ago, is it going to be a big deal if we keep it near the motor shaft and not out on the second stage (suggested by others who got 10:1s)? if it is then we’ll just get the 9:1

If you’re going to use a 10:1 make sure you’re geared down a whole lot. This will help ease the strain on that tiny sun gear. Like 2x-3x full robot weight at least. (figure 155 pounds all up with bumpers and battery)

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You’re going to break the VP based on what the load ratings say so… I’m more concerned about that.

10:1 tend to die in high load situations too so…:man_shrugging:

http://link.vex.com/vexpro/pdf/VersaPlanetary-LoadRatings

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Actually in your chart 10:1 is a failure and acceptable ratio.
If you put the 10:1 as stage 1 then a 5:1 you’re fine. Your highlight is for the 10:1 being the second stage (further from the motor)

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True but the 49:1 and 70:1 isn’t.
And I was pointing out what would break according to the ratings for some of the ratios metioned.

And even if the rating sheet says it’s ok, I wouldn’t trust a 10:1 on a climber period.

would you think a 9:1 first stage and 7:1 second stage would do fine?

I do not recommend 7:1 or numerically higher stages. They too are fragile. I will not use anything above 5:1 for a high load application again.

We are using a full cim with a 10 to 1 first stage and a 4 to 1 second stage. The way you place stages is important. So far so good.

Personally, that 35:1 (7+5) is as much as I would do for a climber using the cim. If more power were needed, switching to 2 775 pros on the 35:1 would certainly do the trick.

I’m 99% sure that the real issue with the 10:1 is that the tolerances are so snug that shock loads become intolerable, especially if they’re on high torque stages, but maybe even on lower stages. Here’s the math:
VP gearboxes have 0.5 module (half a millimeter pitch diameter per tooth) and 72 tooth ring gears, meaning that the ring gears have a pitch diameter of 36mm. For a 10:1 stage, the sun gear has 72/(10-1)=8 teeth, and the planet gears have (72-8)/2 = 32 teeth. The centers of the planet gears are on a (8+32)*0.5mm = 20mm diameter circle. The planet carrier is an equilateral triangle, so the planet gear centers are sqrt(3) * radius, or 1.732mm * 20mm / 2 = 17.32mm apart. The 32t planet gears have a pitch diameter of 16mm, but an outside diameter of 17mm. This means that the teeth of the plane gears move in opposite directions from each other only 0.32mm (.0126 inches, or ~1/80") apart. It’s not too terrible to imagine shock loads crossing that boundary.

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I’d advise against using 775 pros for your climber. You can do it if you are very careful with it and have a good ratcheting mechanism (which you will probably need anyways so you can stay up for 5 seconds after the match), but you also will need to use some sort of sensor to ensure they stop before they reach the max height. If you don’t have sensors on it, you could end up stalling the motors, and 775 pros (and Redlines, they are the same motor) are known to go up in smoke in seconds if you stall them.

The other options would be to use a third stage on the Versaplanetary gearbox, use a different (probably three stage) gearbox, or use a smaller winch. In general, if you need a gearbox that is going to be handling a large load, you’re probably better off with a different gearbox than the Versaplanetary. The Versaplanetary is amazing for most uses, but it isn’t designed to handle as large of loads as other gearboxes like the CIM Sport or Banebots BB220.

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At least with 775pros (with about a 1/3 the torque) we’ve run 70:1 or so with a 1 drum on climbers in the past with success. When using 10:1s make sure they are closest to the motor as it’s the torque that causes failure not speed. In general tho I’d probably listen to the load rating guide. But with a 1’’ drum and a CIM I’d say 50:1 is very likely safe for one robots weight

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