What’s the most effective tool for calculating a single CIM’s effectiveness for carrying a large dynamic load (the weight of two robots in a climb)? I’ve got a 12.24:1 reduction driving a 2" diameter spool, so the rate of climb is roughly 3 fps. Am I looking for stall torque? I feel a little foolish asking this, but I’m stuck. Thanks.
Stall torque multiplied by your gear ratio and spool radius can indeed tell you how much force a CIM will be able to provide at the output stage of your spool. However, remember when a motor is stalled, its not turning (so you wouldn’t actually be lifting anything). And in general, the more load placed on your motor, the slower it turns and the more current it draws. Current is an important factor to consider, especially over sustained loads. If you draw too much current for too long, you will trip your breakers.
Vex has a full, empirically determined motor curve for the CIM (and almost all other motors) on their website:
Explanations for how to use interpret motor curves can also be found on their website:
Many teams used the “linear mechanism” tab on JVN’s design calculator to help them select motors, ratios, and spool sizes for their climbing winches last year. It can similarly be applied to your questions this year.
https://www.chiefdelphi.com/media/papers/3188
If you’re just looking to ballpark lifting time, I always use peak power.
337W / (140kg * 9.8) = 0.25 m/s. So right off, 3 fps isn’t even theoretically possible.
You want to safely maximize power output to lift as fast as possible, so you’re looking to operate near the peak power of the motor. But on the lower torque/higher speed side so additional load increases the power output instead of decreasing it. Peak power is 1/2 stall torque. Aiming for 1/3 stall is aggressive, 1/4 is usually conservative enough. PLus considering the current limitations as Sean mentioned.
Thanks to you both. I didn’t realize that the JVN Calculator had a linear tab :o
I guess I was on the right track, but you have definitely shortened my work time and frustration. I appreciate it.
Another question: do I want to include the 9.8 factor when using the JVN calculator? 1500 lbs seems like an awful lot. Or is that factor included in the calculator itself? Thanks again.
JVN’s calculator is in pounds (force) not pounds (mass).
Note that you only have to get up 12 inches (possibly only 10, if your bumpers are as high as they go). You have to account some for leaning, but at 3 fps, you’re basically talking about climbing in under a second.
Now, our team is planning on climbing in 3 ms (we have some plutonium** at the bottom of the robot that we’re planning to hyper-compress using three 775pros attached to a Harbor Freight Arbor Press), but I suspect you’ll still do really well if you climb at 1/3 fps.
(**It took us YEARS to get the US Department of Energy as a sponsor, but it has really paid off in spades.)
There’s the possibility you’ll run afoul of R08. But then, plutonium doesn’t last as long as mercury, so perhaps you’ll be OK. 
I haven’t run the numbers on your case, but based on memory of similar calculations last year, I’m pretty sure you’re going to need a smaller drum, lower gearing, more motors, bypassed breakers, or some combination to get two robots lifted.
9.8 is because everything’s easier in SI. 9.8 m/s^2 is gravity to convert mass into force due to gravity. W = Nm/s = (kgm/s^2)*m/s sooo:
337W / (140kg * 9.8 m/s^2) = 0.25 m/s
It’s a decent way of estimating power requirements for a system without whipping out Excel and the JVN calculator. I could’ve done it in standard units if I wanted to convert Watts into lbf*ft/s, but it was easier to convert lb to kg.
It may be difficult to judge if the bumpers are above the line of blocks if the ARCADE is a smoldering pit. Just sayin’.