# FRC Tutorials: Lessons in mechanical, CAD, and design basics

Hello FIRST Community!

Over the summer, I created FRC tutorials to prepare any person in mechanical, CAD, or design. In the past, my team has struggled to find enough time for these style of seminars during meetings, so these resources have been valuable for providing our members with knowledge that they can access whenever. As well, these resources will ensure that the team always has access to the basics of these technical areas in case knowledge hasn’t been passed down.

I’d like to share these resources with the Chief community for if they can prove to be helpful for anyone else. But also, I’d love for any feedback/edits/advice that can be made to make these resources better.

Thanks!

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I was checking out the motor selection tutorial and noticed you went through the selection manually, calculating the torque of the arm and motor all manually and planning for a 30A current draw on a BAG. Have you considered that such an arm would be pretty difficult to control, and on top of that, frictional losses may prevent it from moving at all?
Using JVN Calculator may save you a lot of effort going forward.
I like the images that you’re using for the slides, the Vex build kits are really good examples for a lot of this stuff.

Thanks!
We do use the JVN calculator for gearing (I’ll make it a bigger feature in the presentation). But I included the calculations because I figured it would be good for understanding how gearing works.

I believe I accounted for efficiency loss in my calculations. Would you have advice on other friction losses that I could include in the equation? Also, would you know of a way to predict control for the mechanism through calculations, or do you think at that point I should just refer people to use JVN?

It’s good to run through the calculations by hand, but yes I think having some mention of JVN afterwards would be useful.
I missed it the first time around- you do account for a 20% power loss through the Versaplanetary. In my experience the losses can be as high as 50% or more after all the bearings, gears, chains, etc. depending on which team is building. If you’re confident in your team, 20% is a fine number to use.
I usually recommend not to use more than 10% of a motor’s stall current in theoretical 100% efficiency calculations of torque to guarantee that the system is highly linear. That means keeping a BAG motor’s current draw under 5A, or keeping a 775pro’s current draw under 13A, then just making your system efficient enough that things don’t get totally zonked. For that reason I also normally use 775pros unless there’s a really good reason not to.
One other thing I missed on the first read-through is that you plan current draw of the motor based on breaker size. Running a BAG motor at 30A is on the “wrong side” of the peak power on the motor curve, so for the same power output it’ll be making a lot more heat. Running at 20A would take more gearing, but would move just as fast.

Great resource, thanks a lot for these tutorials!

Great resource, thanks a lot for these tutorials! Can you If it is possible to find a totalitarian how to design a conveyor belt:

1. Select a DC motor
2. Selection of belt geometries

Ahh I see what you mean now about the peak power curve. Thanks! I’ve updated the lesson with an explanation of that included + JVN stuff.

Would you mind elaborating a bit more on why one should calculate with 10% or less of a motor’s stall current? 5A for BAG and 13A for 775pros seems so little, and I can’t quite wrap my head around it.

The 10% thing is completely empirical, but the idea is that at such high reductions, latent friction in the gearbox (which is constant throughout an arm’s rotation) is almost as large as, if not larger than, gravity. This makes using PID to control the arm very easy. I’ve seen it used on several mechanisms with great success. Plus, because of the high reduction, acceleration is very high and you reach top speed and stop in less time, meaning you can get more precise controls with less tuning.
It’s a very low current, but it’s very safe and will also prevent you from killing motors.

Hi, and thanks! I’m sorry though, I’m a bit confused by your question.

If it helps, I have updated the motor lesson to include a brief explanation on DC motor selection. On the Mechanical page of the website too, I’ve included WCP’s belt calculator under Mechanical resources. You can find it here too.

For design purposes, I’ve found it useful to calculate POWER requirements to select a motor (or motors), THEN to work on a gear ratio to make it function at the appropriate speed/torque/force. OBTW, I have come to assume I’ll need twice the power I calculate for motor selection purposes. So far, it’s enough. (I have missed at ~150%.)

thank you for belt calculator under Mechanical resources
I thought if there was
Mathematical equation,
FRC group asked me to give a lecture
Engineering design
I have the book Flat- and Round-Belt Drives 883
Shigley’s Mechanical Engineering Design
I hope I have correctly explained my question

Hi!
Unfortunately, most of my experience comes from using the VexPro/West Coast Products belts and pulleys. I also don’t have a lot of experience designing conveyor belts. However, you might find this resource helpful for flat belts and pulleys: https://bit.ly/2Ry2Oap

Sorry for the late response!

thank you

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