Help with motor gearing selection.

[Red-Rx7]

Hello..

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

2000 RPM
400 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

[ed1380]

about the gear box. it doesnt matter how many gears you have in the middle. as you said 40:5 will give you the needed 8:1 ratio.

as long as the gear on the motor is 5 and the load is 40, you could put any gears in between, you'll just lose power by friction

[bulkhead]

RPM doesn't correlate with torque as with cars.  Here the "stall-torque" is the maximum torque output under "stall" conditions where the motor's shaft is stopped.  This is because with electric motors, peak torque occurs at 0 RPM, and decreases as RPM increases.  RPM readings are usually for "no-load" conditions.  So if what you need is X torque at Y RPM, you will need a motor with much greater than Y no load RPM and much greater than X stall-torque, since the motors torque and RPM ratings are for their respective ideal conditions (motor being stalled, vs at no load).  For some motors, you can find a datasheet that shows torque vs RPM plots, but with frictional losses from the gearbox, you probably need to get a slightly overpowered motor to be safe. 

Also, you don't want to build your own gearbox as it is a lot of trouble.  Try www.banebots.com for some of their gearmotors, which come with prebuilt gearboxes for a wide range of gear ratios.

What is this motor going to be used for?

[Admin]

Don't forget to account for efficiency losses from your gearing.

This might help you:
http://www.societyofrobots.com/mechanics_dynamics.shtml
http://www.societyofrobots.com/RMF_calculator.shtml

[Red-Rx7]

RPM doesn't correlate with torque as with cars.  Here the "stall-torque" is the maximum torque output under "stall" conditions where the motor's shaft is stopped.  This is because with electric motors, peak torque occurs at 0 RPM, and decreases as RPM increases.  RPM readings are usually for "no-load" conditions.  So if what you need is X torque at Y RPM, you will need a motor with much greater than Y no load RPM and much greater than X stall-torque, since the motors torque and RPM ratings are for their respective ideal conditions (motor being stalled, vs at no load).  For some motors, you can find a datasheet that shows torque vs RPM plots, but with frictional losses from the gearbox, you probably need to get a slightly overpowered motor to be safe. 

Also, you don't want to build your own gearbox as it is a lot of trouble.  Try www.banebots.com for some of their gearmotors, which come with prebuilt gearboxes for a wide range of gear ratios.

What is this motor going to be used for?

Thanks for the replies everyone.

This is going on a remote controlled Helicopter.  I want to create an on-board starter system, instead of having to use an external starter.

Take a peek at this picture:

Notice the motor, gears, and bracket next to the helicopter frame.  I would like to build the same style for the frame I have.  Because this is a flying craft, I need to be very aware of the overall weight of this.  Here is the build thread for the helicopter:  http://www.scalerchelis.com/phpBB2/viewtopic.php?t=8749

The RMF calculator looks neat, but I am not sure how I can adapt it to the usage I am looking for.  I think I will have to read through the Mechanics Dynamics page multiple times. 

The BaneBot stuff looks neat, but very heavy as well.  This is why I was wanting to create my own flat gear reduction. 

Thoughts?

[bulkhead]

In that case you will want to take a look at servo city's (www.servocity.com) selection of nylon 48 and 64 pitch gears.  You could probably construct a small gearbox using aluminum 1/8" shafts with flanged bearings seated in parallel 1/8" thick (at least) aluminum plates separated by aluminum standoffs.

Actually now that I think about it that may end up weighing just as much as the banebots motors...you would have to check.  The collars and spacers for holding the gears on could end up weighing quite a lot.

Perhaps, given the scope of your project and cost, you may want to look into higher quality, albeit more expensive, motor/gearbox combinations rather than these "cheap" and heavy ones from banebots.  Depending on how much time/money you are willing to spend, you could consider having a custom machined gearbox paired with an efficient brushless motor.

Also, it might help if you just got one of the banebots motors (or just any motor for that matter) to test to see if it has enough torque to start it.  You could perhaps vary the voltage input to determine the minimum power needed to start the engine.  Of course, this is still a really rough estimate since the banebots motors and gearboxes all have different tolerances and performance characteristics.  Still, it could give you a reasonable idea of what weight to look at for an entire motor/gearbox starting system.

[ed1380]

you could try for a worm and gear reduction

[Admin]

The RMF calculator looks neat, but I am not sure how I can adapt it to the usage I am looking for.

Hmmmm you didn't mention it was for a helicopter

The only way you can calculate anything is to have actual numbers - but knowing the required torque for a helicopter blade setup is near impossible . . . CFD in theory could do it, but probably not with good accuracy . . . its outside my expertise!