Hey CD
We need help understanding how to calculate what gearing to use for our climber. We plan on using a versaplanetary gearbox that is interfaced with a Afterburner winch kit which gives an additional 15:24 gearing. What gearbox should I use that will give optimal speed and climbing power
A great place to start is JVN’s Design Calculator. Go over to the linear mechanism tab and put in your robot weight (including bumper and battery) in the applied load; select your motor, pulley diameter, and number of motors. Then change the gear ratio until you are happy with the speed. Make sure to look at your current draw per motor to keep it under the breaker you would use for the selected motor.
Key things you omitted:
I’m a big fan of John V-Neun’s mechanical design calculator and have already busted it out a couple times this season. This should help you see the amp draw for your climber options, to ensure you stay under the breaker limits; I’ve been modeling our climber as an intake.
That is very dependent on what motors you use and the sizable of the drum.
I personally would be targeting between 6in/sec - 1ft/sec.
Our team using a mini cim with 30:1 on a ~1in roller achieved ~7in/sec.
rough calculation…
We decided we wanted it to take a few seconds to climb. This is because we want it to happen quickly, but not so quickly that we can’t control it.
We figured our robot would be relatively light this year, because we are not playing with fuel, so there will be only two mechanisms, and we want the robot to drive relatively quickly. Wild guess, 100 lbs total robot weight with bumpers and battery.
The design we came up with (based on a student’s wonderful idea) results in a winch drum of about 3" diameter. That means we will climb about a foot with every rotation of the drum. If we need to climb 3 feet, this will give us a drum speed of about one revolution per second (60 rpm) for a climb that takes about three seconds, since the robot will be just under two feet tall, and the sensor is about five feet off the floor.
Looking at motors for this type of thing, we want reliability, and relatively low RPM, and lots of torque. Weight is not a big deal, since our robot will be relatively light. A CIM is the natural choice. (we only intend to have four of them in our drivetrain). To get the 4000 RPM (at light load) motor to turn the drum at 60 rpm, we need a reduction of about 60:1. We have some two stage AM gearboxes that give us about 12:1 reduction, so we need another 5:1 reduction. A chain system is easy to make, and we have some 60 tooth sprockets, and some 12 tooth sprockets…that gives us just what we want!
Now, we need to see if it will have sufficient torque to climb. Motor makes 1.78 ft lbs torque at stall, so at the speed we want to run it it will make about half a foot pound torque. With the 60:1 torque multiplication, that gives us 30 ft lbs. the 2" moment arm makes that about 180 lbs pulling force where the rope meets the drum.
now, to build it and see if it works.
I would strongly advise this. I highly recommend JVN’s calculator.,
To give everyone a level set here, in rough numbers robots are going to have to lift 150lbs of robot 3 ft (yeah, I know it isn’t exact but should really read more XKCD/WhatIf if you are getting your undies in a twist over such simplifications). 450ft-lbs which is ~600N-m which (thank you Mr. Metric) is 600Joules.
So… 600J.
If you want to do that is 3 seconds, you’ll need 200W of power. If you want to do it is 10, you only need 60W.
Now is where you should start thinking Motors and Peak Power.
Note ALL MOTORS, no matter how big or how small can easily provide arbitrarily small amounts of mechanical power. If you load them too lightly (gear ratio too high) then the motor spins fast but the multiple of speed times torque is still a small number. On the other end, if you load them too highly you get a bunch of torque but the motor moves so slowly that, again the power is a small number.
The HIGHEST machanical power is when the speed and torque are both at 1/2 their max. This is Peak Power.
So… window motors are amazing in a lot of ways but they only have about 30W of peak power. Which means you need 2 of them to lift your robot in 10 seconds (if you load them optimally).
On the other end of the spectrum are the 775Pros and the CIMs, both of which can provide north of 300W of power. So a single CIM or 775Pro might be able to lift your robot in under 2 seconds (with optimal loading and a current source that isn’t going to give out when you need it most – e.g. one with a circuit breaker in line).
Word to the wise. CIM’s are much much much happier about getting hot that 775Pros. Look at the locked rotor test here. You really cannot stall them for very long before they decide they would rather not be motors after all. I am not saying don’t use 775Pros to lift your robot. In fact, I suppose my team will do that very thing. Just be careful about picking drum diameters, gear ratios and how your coders control them (oh and of course you want a way to keep the winch from backdriving so the motors don’t have to hold your robots up after the climb).
Good luck.
Dr. Joe J.
So is a 60:1 reduction with a CIM good to lift our robot?
A CIM at 60:1 will let you climb in 13.73 seconds with a 1 inch diameter pulley. Seems like a good place to start. Why not use a 775pro?
I don’t know, because I haven’t seen your robot design.
Maybe if you post some sketches or cad model of your robot, or at least describe how it will work and what other parts it will use, we might be able to help you decide if it will work, or suggest something different.
Okay, so it is a 130lb design. The gearbox will interface with a 15:24 reduction AM Afterburner Winch kit, turning our climbing mechanism.
Probably so they don’t have to use a ratchet system to keep the robot at the top. You can stall the CIM for the last few seconds of the match, and not needing a ratcheting mechanism simplifies the design a lot.
Also 60:1 is way over the needed gear ratio. The optimal gear ratio for a CIM pulling up a 154 lbs robot on a 1" pulley is ~9:1, which would climb in 3.88s (using the same height as you used) at ~54A. If you want to get closer to 40A, you can use a 12:1 reduction for ~4s at 41A. 60:1 gives you a much lower current, at the expense of speed and simplicity.
EDIT: If you’re using the afterburner winch kit, you can use a 775pro and not worry about stalling it at the top. It gives a bit more power at less weight, but you’ll need a higher reduction before the afterburner. You’ll only need a ~7.5:1 reduction if you stick with a CIM before the afterburner to get the equivalent of 12:1 overall.
I can’t really imagine an easy way to get the rope into that type of winch…???
As for the ratio you need, compared to the system we are building, you have a winch diameter that is about half as large, so you’d need only half as much reduction, assuming you were using the same motor/gearbox we are using. But that isn’t really clear.
Because 775pro motors really do not like to get stalled at 12v, and depending on your design you might be stalling the motor every match.