My team made a Planetary gearbox for our drivebase (that switches between torque and speed based on motor direction) based upon designs we found online. (see: team 62)
So we realized that the torque mode doesn’t really uhh… well, work—which was further substantiated by some old forum posts I found that verified this. Our gear ratio is such that both motors have a very similar gearing which means the two motors just fight over each other in torque mode and little torque is produced.
So I see a few options we can do to fix this:
Change the gearing for one of the motors which would decrease the speed of speed mode and (I think marginally) increase the torque in torque mode
Make a function to apply a holding torque to one of the motors (We would have to make our own function rather than using the hold brake mode for the right amount of current for our drivebase)
Figure out some way to add a mechanical lock in torque mode.
The motors shouldn’t be fighting each other, the output speed will be slower, but there should be more torque. Making bigger differences in gear ratio means your torque mode would be faster with less torque than before. If you used the design I think you did, the chain ‘ring gear’ is likely slipping or causing excess friction.
This is why planetaries are challenging in Vex. Pm me if you want to look into a different transmission modeled off of planetaries that I made (but not properly tested) over the summer.
Sorry for the confusion.
The title is actually a misnomer; our gearbox is a planetary gearbox that changes the speed ratio based on direction of the motors––we just call it a CVT.
2 years ago. Team BLRS made a CVT for the Make it Real CAD challenge, never found an actual competition use for it but it was still cool. Here’s the Link
It is great that you are exploring advanced mechanical concepts and research the theory behind them!
As you to correctly identified, this transmission could not deliver actual high torque in the “high torque” (read: slow speed) mode.
The best picture to illustrate the problem with such “planetary” design is, probably, this one:
Additional explanations could be found here: 1, 2, 3, 4, 5
This isn’t really an option, because there is only one gear ratio that guarantees equal torque distribution between the motors. If torques are not balanced, then you are wasting motor resource. It will be more efficient just to connect both motors directly to the output. In some sense electronic circuits sending variable power to the motors is your CVT.
That may somewhat work, if your goal was to conserve battery power, since V5 motors could deliver peak torque at low rpm or in hold mode, without consuming full power. However, main constraint in VRC is limited number of motors - so, once again, it would be better just to connect all motors directly to the output.
In theory, that could be done, but the limited choice of vex parts makes it impossible to avoid power losses and with complex designs you also lose reliability of a simple direct drive setup.
If your game strategy calls for extra drivetrain torque, then consider other ways of sharing the motors between subsystems to free up additional motors for the drivetrain.
For example, if you get an extra (fifth) drivetrain motor to make H-drive, then you could gear it for very high torque. This way you don’t lose any speed in the forward (in-track) direction but, while being slow going cross-track, gain much higher torque in the diagonal direction (by adding torque vectors).
