Appropriate gearing for wrist

#1

Currently we are exploring options for articulating our intake on the end of our elevator with about 90deg of travel. While pneumatics are an option, we’d prefer to have variable control over the position of the wrist. So the next option we see is something like a bag or 775 motor. The issue we are running into is getting the rpm down to a manageable level in a relatively compact package. So my question today is what would be a good gearing option for a wrist like this? We’re probably looking at similar loads to wrists from power up last year. What setups did teams run on their bots?

Using the JVN design calculator, even a 256:1 reduction with a 775 will result in the wrist going 90deg in 0.25s (albeit at full speed/power). This seems rather fast to me. Thoughts?

#2

You don’t have to run a motor at its full speed rating. Essentially what that calculator (and motor speed curves) will tell you is the maximum speed you can travel at that given torque.

You can always run a motor slower than the rated speed at that torque, and in practice that is generally the best practice.

It’s like running a race. Just because you can sprint at some speed, doesn’t mean you should run the whole race at that speed because you’ll burn out pretty quickly. Instead, if you pace yourself and run at a slower speed, you’ll (hopefully) be able to finish the race.

Motors have similar reactions to being run at full power constantly

#3

For starters, you aren’t obligated to use 12V to power any mechanism. Assuming your chosen ratio provides enough torque, you can throttle down your motor inputs substantially under 12V to achieve the rotational speed you desire.

Secondly, I would carefully examine how much load you expect the motor to endure, if and how your system will be counter-balanced, and then assess which motor is most appropriate for your application. Pay particularly attention to the locked rotor tests provided by VEX to see how well a particular motor will endure being stalled at that load. Also consider what types of failure modes are appropriate for the task and if adding additional motor power is worth the risk of a more severe failure mode. A wrist may be an application that is better suited for a BAG motor than a 775Pro depending on your design objectives.

#4

I’ll add here that we always try to end our wrist/arm reductions in a tensioned chain run to give the device some ability to absorb the shock from hitting a hard surface (we’ve had issues stripping aluminum gear teeth and destroying planetary gear boxes in the past if they are the last item in the reduction). There are other ways of doing it, but it’s in your best interest to design your mechanism with prolonging its life in mind.

We’ve run reductions anywhere from 350:1 to 900:1 depending on the game and application.

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#5

My design calculator can probably be helpful to decide which gear ratio you want for your mechanism. It does the same stuff JVN’s calculator does, plus a number of additional useful features.

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#6

Also, note that if you only need a 10W mechanism, there’s no need to power it with a 200W motor. Sometimes an automotive motor, or even a servo, is the right answer.

#7

I also highly endorse ending your high-load rotation in a chain stage for shock absorption.

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#8

A 775 pro is about the lightest motor you can get regardless of power, but you need to consider the safe stall current if your wrist will have to hold an angle under load.

Design your gearing so that the motor draws no more than 8 or 9A (in to the controller, or ~30A 3V out) and you can stall the motor safely for ~30s with or without current limiting. Under this restriction, the 775 pro becomes just a 100W motor, so it’s by no means overkill.