Drive Train Stalling

VEX V5 General Discussion
#1

Hi everyone, my team has been doing robotics for 2 years now and we are still struggling to build a capable drive-train. We’re running with the Cortex system since our V5s don’t come until the end of January. So, we still have a few competitions left with our current system. Our main problem is that we stall out quickly if we get in any pushing matches with other bots. We also have trouble climbing the platforms after an entire match.
In case the photos below don’t show up
We currently have an 8 motor high-speed drive which is fine for most of the match except the conditions listed above. The motors were previously geared for turbo since we thought for sure we would be fine with 8 of them. After doing this it burned out if any amount of resistance was enforced. So, we switched to high speed. Although they work slightly better we still feel that we are doing something wrong. The rear motors are direct into the wheels and the front ones are chained to the front wheels. We’ve researched the PTCs in the cortex, power expander, and motors. We have the rear wheels in ports 2-5, front wheels in our power expander, and our catapult in ports 6-9 since it also draws a decent amount of current. We’ve read about teams with 6 motor turbo drives and they don’t stall out so we feel we must be doing something drastically wrong.
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So far we have two theories as to why the current design may be so inefficient. The first is that the modules are sagging which causes extra friction. And two, the metal on metal gears may be binding in certain situations causing the entire wheel to essentially stop.

We have two things we’re going to try with our new drivetrain. The first thing is that we are no longer going to use the modules seen in the pictures. Instead, we are going to attach the motors directly to our C-channel to prevent sagging. We’re also going to use plastic gears to hopefully prevent the binding with the metal gears. The picture below is a quick CAD model of what we’re thinking about doing.
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I’m wondering how you guys are managing to create such efficient DTs and if our new design is anywhere close. Thanks!

#2

Check to make sure there isn’t excessive friction with the spacing. You should be able to freely spin all of your spacers. Also, try to avoid putting shafts through more than 2 pieces of metal at once, because vex uses square holes so each piece of metal will be slightly misaligned with each other.

#3

This will be true no matter how you build your drive train. Do everything you can to NOT apply full power while the motors are stationary. Electric motors have their highest current draw (and thus highest heat generation) when stationary, decreasing as they get going faster. I see you already have IMEs on your bot; I would write your motor code such that the power to the motors is limited based on their current speed. So say max power could be allowed at 67% of free-spin speed, one third power at 0% speed, or what have you. You will have to play with the numbers a bit to find something that works well for your bot.

#4

Also never use 12 tooth metal gears with other 12 tooth metal gears. For that I suggest 2 36 tooth gears instead.

#5

just to add on to the above suggestions…

A 8-motor turbo should work well.
We have been using this setup quite a few times, and no issue with stalling.

but do take note about pushing battles - it is more than just about the number of motors and gear ratio. you will also need to take note of the weight of your robot. you still will not win the pushing battle if you have a light-weight 8-motor turbo-drive robot (and you are going against a 4-motor but much heavier robot).

always remember this, friction = (coefficient of friction) x (normal reaction force)

the heavier the robot, the greater the normal reaction force. So if you are going against a much heavier robot, then you are also going against a big amount of frictional force.

as for climbing up the platform, you might want to check on the position of the wheels and also the amount of clearance for the robot to go up the platform.

#6

I see one issue with your drive train - the linearly geared setup.

This is what I mean:

Last year I tried a similar setup, and ran into the same burnout issues as you did.

The main issue is that when you do a drive train like this, it’s more like your friend pushing you pushing boxes, than you and your friend pushing the box at the same time. The motor power isn’t being used to its fullest potential.

I recommend you try the direct drive CAD, although it’s still techically linearly geared in that case it’s still better than what your current setup is.

#7

One way to drastically improve eficiency is by removing the bearing flats on the side the motor is on. There is no need to have a bearing flat because the motor itself has an internal bearing flat in it. I would not suggest 12 tooth gears peimarily because they have a lot of friction when it’s metal-on-metal. Make sure you only have 2 points where an axle goes through. You should have your gearing or sprockets be right next to the wheel integrated in the drivetrain, not as a separate gearbox because it would have more than 2 points an axle goes through, which is increased friction. I’ve taken this image from Here:

#8