Our team is experimenting with building a flywheel launcher with the same specifications as launcher #2 detailed in this thread: 4 “turbo” geared 393 motors (2 per wheel) with 35:3 external gearing and 5" wheels. One of the flywheels spins up easily, but the other one will either begin to spin up, but stop accelerating within 10 seconds, or will jump slightly when power is applied but then refuse to do anything. We’ve tried swapping out the motors, and both motors spin up fine when detached from the flywheel. We’ve also tried connecting the motors to a different port on the cortex and (in a moment of desperation) different motor controllers, all with no success. Has anyone else experienced trouble getting motors to spin up? If so, how did you alleviate this problem?
I wouldn’t recommend using turbo geared motors. We had 4 of them on our drive in skyrise with 2.75" omni’s, direct drive. This ratio is about the same as speed motors with 4" wheels, but our drive burned out a ton even with the lack of pushing other robots. I would just increase your external ratio. Hope this helps!
My kids discovered that their motors would stall or soon overheat if their external gearing couldn’t spin freely for at least about 10-15 seconds after being revved up by hand (with the motors physically disengaged). They now use that as a kind of criteria for when they try new assemblies. You really need to get the friction as low as possible. My kids found that their system couldn’t tolerate axles if they had even the slightest bend in them or if the support structure warped and twisted any amount. The motors “see” friction at the flywheel magnified by whatever gear ratio you have. So if there’s 0.05 inch-pounds of frictional torque at the flywheel, it might look like 1.25 to 1.75 inch-pounds to the motor.
As others have said, friction is most likely your problem.
However, it may not be the PTC fuses in the motors that are tripping. Each 393 motor can run with a .9 amp current continuously without tripping (about 20% torque) its PTC. As you might know, the cortex has two PTC fuses, one that protects ports 1-5 and another that protects ports 6-10. A cortex PTC can only handle 3 amps continuously (total current of all the motors it is protecting). Now if your motors are only drawing .9 amps, you can still trip a cortex PTC if they are all connected on the low or high side port banks. I.e. if you have all your motors connected to ports 1,2,3 and 4… then you will trip the cortex #1 PTC since it will be seeing 3.6 amps which is > 3 amp threshold and may trip the cortex. So good design practice is to have two of your motors connected to say ports 1 and 2 and the other two motors connected to say ports 9 and 10.
A simple check would be to unplug the motors on one side of the shooter and see if the other motor still shuts down. If it does then it is likely a friction problem.
As a final note… if you are not sure whether your motor PTCs or Cortex PTCs are tripping first, put a light load in the same Cortex bank as your motors. When the motors shut down, if the light load is still running, then the cortex is still ok.
Edit: J.Pearman pointed out that the spec is 4 amps for the cortex PTC. However, testing has shown this value to be nearer to 3 amps.
I really don’t like correcting my friends, but the spec for the cortex is 4A not 3A. spec
Here are some tests I did a few years back, you will see that even though the spec is 4A the PTC can trip in a little as 30 seconds. PTC performance measurements
However, the substance of Chris’s post still stands, even 2 motors can theoretically trip the internal cortex PTC, it’s more likely the motor PTC will trip first but it all depends on what else in on that group of outputs.
Get used to the fact that this year teams using flywheel shooters will be running their motors at the edge of their capability. Managing the current going to these motors will be one of the factors that differentiates the top teams from everyone else.
I went back and read my original post, it seems we did conclude that, although the spec is 4A, I could not duplicate that using the parts I had bought with the constant current bench supply. I did find that these PTC devices seem to have a wide range of tolerances, 3A is certainly a safe value, 4A should work but I couldn’t prove that, perhaps I had a batch of PTCs on the low end of the curve.
Thanks everyone for the advice. After further testing, it seems that our problems were caused by a slightly bent shaft. Swapping out the bent shaft for a straighter one, in combination with very slight lubrication with some 3-in-one oil (permitted in competition by <R7e>) solved the issue.
EDIT: After rereading the thread, I’d like to add that the tip for determining acceptable levels of friction submitted by FullMetalMentor:
was extremely helpful in our diagnosis of the bent shaft and subsequent swapping out for a straighter unit.