Low Cost Planetary Gearbox Source...

[Joe Johnson]

Originally Posted by The following is a redacted version of a PM I just received. FYI

i'd like to use a pair of the 16:1 gear boxes mated with 2 FPs to run a <secrect mechanism> but after several posts (namely, the ones that said they can't handle continous use), i'm worried about using them. do you think that they would survive in this application?
<more secrets removed>

This is my redacted reply:

This is not true. The FP motors CAN take continuous use. They CANNOT operate near 12V stall or even at 70% of 12V stall (a.k.a. below 30% of their 12V free speed when applying 12V to the motor) for long periods of time without turning into a stinky, smokey blob of copper.

You do not give enough information to know if the conditions you are talking about are reasonable.

At 12V, the FP motor puts out .4 N-m at stall. With 2 stages of 4:1 each @ 70% eff. you will get Tgearbox stall = .4 X 4 X 4 X .7 X.7 = 3.1N-m.

I recommend that you operate at about 1/4 of this number to run continuous and happy all season long (not that you can't muscle through more than that for a short time - a few seconds say - but that you should not need more than this for 10s of seconds at a time).

Can you keep the continous torque on each motor to below .75N-m?

Let's assume that you have 4" pulleys on your <secrect mechanism> This works out to a radius of about 50mm = .05m. That means that you could get 15N (3.4lbs) on the <secrect mechanism> continuously.

Is that enough? It is hard to know without my getting into the nickers of your <secrect mechanism> design.

By the way, the <secrect mechanism> would be FLYING at almost 4 m/s (12ft/sec)* when driven by a 4" pulley with a tangential load of 15N.

If you don't need a <secrect mechanism> that is so fast, use a smaller pulley or a higher gear ratio gearbox.

Joe J.

*I made a mistake in my calc. my initial posting mistakenly claimed 25m/s which was just whacked. Sorry. Even so, 4m/s is not exactly turtle speed.

Here is my calculation for review and perhaps further corrections:

16000 RPM @75% (i.e. 1/4 stall torque) = 12,000 RPM (@ motor)

12,000 RPM (@ motor) / 16:1 = 750 RPM (@ Gearbox)

750RPM = 79 Radian / Sec = w

V=w X R
79 Radians / Sec X .05 m = 4 m / s

[Richard Wallace]

Originally Posted by Joe Johnson

This is my redacted reply:

This is not true. The FP motors CAN take continuous use. They CANNOT operate near 12V stall or even at 70% of 12V stall (a.k.a. below 30% of their 12V free speed when applying 12V to the motor) for long periods of time without turning into a stinky, smokey blob of copper.

You do not give enough information to know if the conditions you are talking about are reasonable.

At 12V, the FP motor puts out .4 N-m at stall. With 2 stages of 4:1 each @ 70% eff. you will get Tgearbox stall = .4 X 4 X 4 X .7 X.7 = 3.1N-m.

I recommend that you operate at about 1/4 of this number to run continuous and happy all season long (not that you can't muscle through more than that for a short time - a few seconds say - but that you should not need more than this for 10s of seconds at a time).

Can you keep the continous torque on each motor to below .75N-m?

Let's assume that you have 4" pulleys on your <secrect mechanism> This works out to a radius of about 50mm = .05m. That means that you could get 15N (3.4lbs) on the <secrect mechanism> continuously.

Is that enough? It is hard to know without my getting into the nickers of your <secrect mechanism> design.

By the way, the <secrect mechanism> would be FLYING at almost 25m/s when driven by a 4" pulley with a tangential load of 15N.

If you don't need a <secrect mechanism> that is so fast, use a smaller pulley or a higher gear ratio gearbox.

Joe J.

I never intended to discourage anyone from using the FP motor. As Joe J. points out above, there should be no problem with running it continuously at loads that are less than ~25% of its 12V stall current. When using the single-stage 5:1 BaneBots gearbox with an FP motor, I would not be too worried about running continuously at up to 15 Ampere, because the FP has about 50% higher stall current and is therefore a more capable motor than the one that comes with the gearbox. The gearbox itself will probably wear out faster at FP continuous loading than it would with the standard motor, but that is probably not an issue for a mechanism that will be used for one FRC season.

My recommendation above was just to avoid continuously loading the BaneBots gearmotor at currents greater than about 10 Ampere (about 25% of its 12V stall current). I really don't like to see smoke coming from motors, and this one started smoking at 20 Ampere.

[Joe Johnson]

Originally Posted by Richard

...See above...

I am just having fun with ya'll and while I am at it, trying to give examples of the way you can calculate reasonable numbers for motor loading.

Joe J.

[jonboy]

Just got our Banebot transmissions and noticed that the air duct of the motor near the output shaft is blocked by the Banebot transmission. The FP plastic transmission has some holes to i assume let cooling air flow. The question is at what loads and duration at room temperature is this detrimental?

[Kevin Sevcik]

About the pinions splitting. I think it's only a problem with the 5:1 pinion since it's so small. I split the first one pressing it on the shaft and didn't even notice till the tranny wouldn't work under load. My solution was to grab a new pinion and a .1247 reamer I had from the NBD mod we did last year. I ran it through with a hand drill and it pressed on fine after that. If you can actually chuck up the pinion in a lathe, I'd use a .1250 reamer, as that's what I ended up using on the FP last year when I could chuck the dewalt pinion.

Second comment. Has anyone else gotten one of the trannies with a crooked keyseat on the shaft? We've got one that takes a distinct jog to the side halfway down the keyseat.

[Andy A.]

Originally Posted by jonboy

Just got our Banebot transmissions and noticed that the air duct of the motor near the output shaft is blocked by the Banebot transmission. The FP plastic transmission has some holes to i assume let cooling air flow. The question is at what loads and duration at room temperature is this detrimental?

No data that I am aware of for exactly the effect this will have on the lifespan of the motor. Obviously, it will limit airflow, as a good deal of air does flow out of those front vents.

My suggestion may come to late, but if you can permit some standoff between the front of the motor can and the transmission, obviously airflow would be better. By using longer screws and some very small spacers it would be easy enough to move the motor back. This would require that the pinion be hung out at the end of an unmodified motor output shaft, and you'd probably want to support the end of the motor can to keep strain off of the very small and very weakly threaded attachment screws.

As for 885, we are just hopping that the motor will make it through. In the end, the way FP motors die pretty much negates air flowing through the front vents. If a FP motor dies, its because it stalls and there is no fan moving air period, and all the venting and heat sinks in the world won't help you. A slow, gradual death from heat build up is probably a lot more rare.

Give the motors time to cool between matches, and invest in some canned coolant.

-Andy A.