designing a three motor gearbox

[1683cadder]

Originally Posted by EricH

Anyways, if two motors are different, matching their speed will work for a quick solution. But to optimize, you probably want to match their available power at a given RPM, which will require slightly different gear ratios than just matching speeds.

How do you do that?

[Tristan Lall]

Originally Posted by 1683cadder

How do you do that?

The speed/current/power vs. torque curves are shaped similarly for DC permanent magnet brush motors (i.e. all current FRC motors). Motors differ in the magnitudes of these quantities. You plot those curves (either experimentally or theoretically), and then decide what ranges you'd like your mechanism to operate in. Then you scale the curves using gear ratios (to multiply the torque by some factor). Look at the JVN DesignCalc spreadsheet (here, here and here) and Andrew Kesic's motor curve spreadsheet (here and here) for examples.¹

If you want the motors to share the load proportionally to their maximum output power, then you want to gear them so that the speed vs. torque curves of all motors are superimposed (or that at least they're as close as possible within the relevant range). If you want motors to share unequally at different loads,² then you can orient those motor curves arbitrarily.

¹ I have some incremental updates to these I've been meaning to complete and post. (Mostly updated motor lists for 2012, and minor formatting/functionality improvements.) One of these days....
² That's a pretty rare requirement. Maybe one motor is a can motor with a fan that has a minimum effective speed, and you are willing to load it more at high speed, in order reduce its load when high torque is demanded (at low speed when cooling is ineffective).

[pmangels17]

My apologies. I forgot for a moment that the MiniCIM and CIM were different RPMS. However, going from all MiniCIM to all CIM wouldn't be a problem. You should definitely use a slight reduction on the MiniCIM, though I am not positive on the exact ratio required.

[cgmv123]

Originally Posted by pmangels17

My apologies. I forgot for a moment that the MiniCIM and CIM were different RPMS. However, going from all MiniCIM to all CIM wouldn't be a problem. You should definitely use a slight reduction on the MiniCIM, though I am not positive on the exact ratio required.

1.1676082862523540489642184557439:1 or 620:531 or 62:53* or 6:5*

*Approximate

[1683cadder]

How close do the gear ratios need to be?

[T^2]

They don't. Slower motors will act as loads on faster motors. If you're incorporating CIMs and MiniCIMs in the same gearbox, you can use 12-tooth pinions on the CIMs and 11-tooth pinions on the MiniCIMs (or 14t and 12t respectively), which will get you close enough. In all honesty, you can probably use the same pinions on the motors, and they'll balance each other out without too much stress.

[EricH]

Originally Posted by 1683cadder

How close do the gear ratios need to be?

This question doesn't make sense in the context of the current discussion. I believe the question you're asking is, "How close do the RPMs need to be?" unless you're looking into what the different stages of the gearbox will be.

The answer is, they WILL be the same, eventually, due to the single gear they both go to--it can't travel at two different speeds at the same time. However, having the speeds wildly different is going to really annoy one motor or the other or both. Given that you're talking about 5K RPM and 6K RPM motors, I'd shoot for within 100-200 RPM difference; that should be close enough to work with and within the rated speed tolerance.


Just in case you were asking about the different gear ratios in the gearbox, though, the answer is "it depends". Depends on design, depends on designed speed(s). Each reduction stage, as a rule of thumb, should be less than a 10:1 reduction by itself (total reduction of the gearbox can be a LOT more).