Hey everyone, hope you’re all having a great summer. I just have a quick question in relation to the kit motors and gearing basics. Gearing has always been our team’s weak point so I figure I may as well get started early while I can. So, my question is simple: when doing gearing calculations which specs do I use motor-wise? Do I use the free load speeds, max efficiency speeds, or some other number?
This is a good question. For most motors we use in FIRST, the max efficiency is at about 1/2 of the free speed. With this in mind, some designers size their reductions for this speed and have a robot that drives well.
For instance, they gear their 'bot to run at top speed when getting an input speed from the CIM motor at about 2800 rpm, or about 1/2 of the 5500 free speed rpm. Then these teams make their robot, get their drivetrain as efficient as possible, with little friction, and it drives too fast. The top end is too out of control, tripping circuit breakers and the low end is not torquey enough. Usually, they end up gearing down their robot, slowing it down so it is more controllable and doesn’t trip the circuit breakers after running at top speed for more than 5 seconds.
From my experience, I use a “working rpm” top speed of about 75% of the motor’s free speed rpm. For instance, when designing drive trains with the CIM motor, I assume that I can get 4000 rpm (again, 5500 is the free speed) at the robot’s top speed. This difference of free speed and working speed is because of efficiency losses in the drivetrain and rolling friction losses of the robot.
It all depends on what you’re gearing your motor for anyways. Ideally, you should know how much load your motor will have to move. If you know this, then you can maximize the speed of the system by gearing it for the “maximum power” torque. you’d match up your output torque of your gear system to the torque required. I did this with our winch for lifting our robot last year.
If you’re talking about gearing for a drive train, that’s an entirely different story. You’d have to figure out the resistance in your drive train and plan around pushing other robots around, etc. I’m not as experinced in this area, so I can’t say really. I’d stick with planning around the maximum power point and pick a gearing that gives you a decent output speed and torque. Definitely don’t plan around free speed, since you’ll never have a fricitonless drive system.
Anyways, that’s my 2 cents. I know there’s gonna be a load of better advice coming up right after this post, so I’ll stop here.
Thanks for the help. If I was to calculate using approx. 75% on the CIM and 85% on the drill would it work in practice or would the numbers only be usable in theory? For instance: if I wanted to get to 150 RPM I figured I could gear the CIM from 4200 (1->28) or the drill from 16800 (1->112), now if I was to use this in actual practice would it work well or should I expect torque issues?
When using two different motors, you should always make sure that they are geared so their free speeds match. This means you should be using the same percentage of each motor In this case, the ratio between the ratios for drill to CIM should be 19500/5500 or about 3.7. (112/28 != 19500/5500). This is probably a bit hard to understand, but there are numerous posts about this all over the forum with much longer, and well written explanations of why free speeds need to match and such…
John V-Nuen has made up a rather extensive spreadsheet to help with all sorts of things, one of which is the combining of different motors. I haven’t looked at it recently, but it has gotten fairly good reviews.