DC Motor and Gearing
 

Hello..

I need a motor to spin a shaft at the following minimum requirements:

1500-2000 RPM
300 oz-in torque

I found a motor on Robot Market Place: http://www.robotmarketplace.com/products/0-MSJ.html

This motor produces the following specs:

Speed : 16,000 rpm @ 12V
Angular velocity constant: 1560 rpm/V
Amps @ nominal: 1.2 Amps
Efficiency: 71.4%
Peak Power: 0.36 hp
Stall current: 91.8 A
Stall torque: 78.7 oz-in

If I made a gear ratio reduction of a 8:1, the math would make it produce:

Stall Torque oz-in = 78.7 * 8
Stall Torque oz-in = 629.6
Speed RPM = 16000 / 8
Speed RPM = 2000

Did I caclulate that correctly?

Questions:
1) In my car experience, Torque to relative to RPM. Meaning, if this motor can get up to 2000 RPMs it would then produce 629.6 (assuming calculations are correct). My concern is the shaft I need to spin from this motor has a minimum torque requirement to make it rotate in the first place. The shaft doesn't just freely spin. The minimum is around the 300-400 oz-in. Would this still work?
2) In achieving this gear ratio, I simply would like to put a gear on the shaft that I need to spin, put a gear on the motor, and have some gears inbetween the two (to cover a distance gap and create the gear ratio). How would I do the gearing at this point? If it were just two gears, I could achieve an 8:1 with a 5 tooth and a 40 tooth. If I just used a 5 tooth, 40 tooth, and a 5 tooth; would this cancel out the 8:1?
3) Any other thoughts?

Thanks,
Mike

Re: DC Motor and Gearing
 

in a dc motor Torque and rpm are not related you get max Torque at 0 rpms
now i'm no expert so

as for you gears yes a 5 to 40 to 5 is the same a 1:1 ratio not 8:1 but the gears will spin in the same directions

your math looks good for rpms
what is this for ??

Re: DC Motor and Gearing
 

Yes in theory, but in practical terms it's not a big enough motor.

Gears: A 5 tooth gear is too few teeth. To get reduction you need compound gears. Try 10 tooth against 40 tooth, on the same axle (i.e., stacked against each other co-axially) as the 40 tooth another 10 tooth, then driving another 40 tooth, which is the final output. If it needs to be further, another 40 tooth would be used.

OK, torque: If you need maybe 400 just to overcome friction, that leaves a peak of 230-ish to do the work, and that's at stall. With the motor moving the actual torque will be much less - it IS linear, so with X as the speed axis (0 to 16000) and Y as the torque axis (0 to 78.7), draw a straight line from (16000, 0) to (0, 78.7) - that is your torque line. You can see that there's not much torque available at 16000 RPM, and the motor will slow down until the torque is enough to drive the load.

Overcoming that constant 400 oz-in of torque required to start the shaft moving is a considerable challenge, particularly for 2000 RPM and the small (=affordable) motor world.

If you can provide us more info on the application, we can brainstorm some possible solutions.

Don

Re: DC Motor and Gearing
 

Colin & Don,

Thanks for your replies!

I am trying to build an on-board starter for an R/C Helicopter. Currently, I have to use a starter (like this one: Hobbico TorqMaster 90) with a long starter rod to insert into a Hex coupling on the Helicopters starter shaft. When I power the starter, it spins the starter shaft. I would like a self-contained unit.

I am trying to find the smallest motor possible, due to weight reasons. The motor would only spin for a few seconds at a time, to turn over the Nitro powered engine.

I have clear horizontal access to the starter-shaft. I was wanting to build something like the on-board starter in this picture:



Notice the motor, gears, and plate next to the helicopter mechanics.

Thanks for all of your help! As a side reference, you can see the Helicopter that I am building here: MD 530F

Re: DC Motor and Gearing
 

Just a quick thought: If you add this motor and gearing, will the helo still be balanced, or will it become impossible to fly?

Re: DC Motor and Gearing
 

Lawry,

Good question. Ultimately, it depends on the overall weight. It is going into a scale helicopter, and the motor is powerful enough to carry quite a few extra pounds. I would like to keep the whole assembly near a pound or less.

Thanks,
Mike

Re: DC Motor and Gearing
 

good call don yeah a 5 tooth gear is really small good call

Re: DC Motor and Gearing
 

OK, that changes things a bit - I was thinking you had to overcome 400 oz-in of friction and THEN 350 oz-in of torque on top of that.

The motor you selected might work, but not at 2000 RPM. To start an engine of that type (at least, those that I worked with many moons ago) needed about 500-700 RPM to start, but (as you said) only for a second or three.

Looking at the photo, see how the motor gear is about 10 teeth, going to a bigger gear of about 40, which is stacked with a 20 tooth, going again to a 40? Just what you need.

For the $25 it'll cost to make it happen, I'd say try it.

If you can include some kind of clutch to disengage and engage the starter at will, you might consider a flywheel. A jet engine starter - thinking of it, a helicopter too - first spools up to speed with a flywheel, then uses the stored energy to turn the engine, effectively multiplying the torque considerably at the very first stage, where you need it the most. For this case, a 4 ounce 2" diameter flywheel on the fastest shaft will store considerable energy, almost enough to start the engine on its own. The motor will take a second or 2 to spin that baby up.

Don

Re: DC Motor and Gearing
 

is this for in air starts :ie it fall out of the sky o no or main starting
i've really been thinking about this and if you looking for emergency restart
a explosive charge or small pneumatics (co2) cylinder it would be lighter but it's wayyy out there

Re: DC Motor and Gearing
 

Alternatively, a LARGE explosive will prevent the aircraft from hitting the ground and breaking...:eek:

pretty spectacular, too. Not Recommended.

Re: DC Motor and Gearing
 

Thanks for the reply Don.

The starter shaft has a one-way bearing on it, so it completely disengages from the nitro engine once it is started. If I have gears resting against it, it would just sit there idle.

As far as the gear ratio itself goes, how does that work out? The 10 to 40 connection makes a 4:1 ratio. Then with another 10 tooth gear on the same shaft as the 40 tooth gear, connecting to a 40 tooth gear, is another 4:1 ratio. So do you simply add those up and come up with an 8:1 as the final drive?

Also, what should I connect the last 40 tooth gear too (meaning, what size gear should be on the starter shaft)? Another 40 tooth gear so it makes it a 1:1 at that connection?

One other thing of note; the motor specified shows a stall amp of near 92 amps. That means I need a battery that can supply 92 amps burst discharge - or it will never reach the implied stall torque we have been using as a baseline?

Re: DC Motor and Gearing
 

While this sounds fun, it isn't what I would like to use it for! I have too much $$$ invested to have it go "bang". :)

If the engine were to fail during flight, I would hope to have enough head speed that I could autorotation to the ground safely.

Re: DC Motor and Gearing
 

Yes, gear ratios add like you stated.
WRONG - Gear radios multiply. Oops.

You can use the last 40 tooth gear for the output, or another one of the same size (but why?) for 1:1 if you like.

The peak current from a battery may surprise you, but I^2R losses in the wiring will keep that peak current well below the theoretical 92 Amps. The 12 volt lead-acid battery FIRST uses can deliver 720 Amps for 3 seconds... So a small lead-acid will run your starter no problem.

Don

Re: DC Motor and Gearing
 

you could do a 10 to 80 as well it be bigger but possible lighter
how do you plan to make this mill, drill press??
do you know gear stuff like pitch diamter, diametral pitch, Pressure Angle you need to look at this stuff before ordering gears or making any parts

Re: DC Motor and Gearing
 

Don't gear ratios multiply?