Gearbox issues

[PortugazD.Ace]

I am trying to develop something and it requires a REALLY REALLY small gearbox. The motor I'm using can get up to over 7000 rpm and it has a Dynamic Friction Torque of 1.35*10^-5mNm. The are going to be a total of 4 motors in my development. This adds up to 5.4*10^-5mNm. I need it to move a weight of ~12Kg. Can anyone help me figure out what kind of gear ratio I need to get this to work?

[Chris is me]

How are you moving the load? Is it on a lever arm? Are you pulling it with a winch and string, or are you lifting it with an arm, or are you driving a robot with it?

[PortugazD.Ace]

To the moderators: Please forgive any possible profound language.

Reply: If you have ever read the manga Air Gear, or watched the movie Jackass, then you should understand what I say when I say motorized rollerblades(i.e. Air Trek). If you haven't, then read more manga.

[JamesCH95]

*profane... and telling people to go read more manga is not a good way to get help.

Do you have any speed/acceleration/gradeability requirements for these motorized roller blades?
Is a person going to be attached to these roller blades?
How big is the motor and how small does the gear box need to be?
Can you sketch up what you're trying to do?

[Tristan Lall]

Originally Posted by JamesCH95

*profane...

It sounded more interesting as "profound"....

Anyway, the "dynamic friction torque" sounds like a measure of the resistance in the bearings, not the torque output of the motor. (Look at the magnitude—it's tiny.) So, you probably don't need to worry about it for now.

In addition to all the other questions, we really need an idea of the power of the motor. You've given us the (free?) speed, now you need to give us the power (e.g. in watts), or the output torque (so that we can calculate the power).

What kind of motor is it (DC permanent magnet like a FIRST motor, or something else?), and how do you plan to control and power it.

Are you planning to strap these gearboxes to the side of a rollerblade? How much money do you want to spend, and what sort of fabrication capabilities do you have?

Without knowing more details, we can't be sure—but I think there's a rapidly emerging consensus that this is going to be more difficult than you anticipated.

[DonRotolo]

Said a different way: We really want to help, but need a little more information.

No sense in expending effort to solve the wrong problem, eh?

[PortugazD.Ace]

At the bottom there are two attachments. One is a PDF with the motor data, the other is a PDF image with a sketch of the base of my design(please forgive me if the sketch is badly drawn.)

Now to answer your questions. The system will speed up in proportion to the users kick. These things are essentially just going to be roller blades with motors attached. Yes a person is going to be wearing them. the gearbox needs to fit in a cylinder of aproxamately 8cm in diameter and .8cm in depth(it has to fit inside of the wheel). The motor itself is about 2cm in diameter and I dont know how thick it is. There is also an attached file that has draft designs for the tire rim.

Attachment 9214
Attachment 9215
Attachment 9216

[Tristan Lall]

You're going to have a power problem: those motors only output 0.3 W at their maximum power point. By comparison, the CIM motors that drive an FRC robot are good for around 340 W mechanical output each. With less than one thousandth the power of a CIM motor, I don't think these are the right motors for the job. (Those little motors are used in precision machinery—they're great, but not designed for this sort of application.)

The relationship between (output) power, speed and torque is simple: power = rotational speed × torque (in the proper units, [W] = [Hz] × [N·m]). Since you can figure out the rotational speed required (based on the wheel diameter and the linear speed you want to travel at), you'll find that either you won't get very much torque at a reasonable operating speed, or you'll get your torque at a very, very low speed. Plus, the bigger your wheel gets, the more gearing you'll need to slow the output down to something manageable.

Also, those motors are of the brushless DC variety—it's actually a 3-phase motor that uses a special speed controller and a series of sensors to indicate to the controller when to energize the coils. And because of their design, simply applying power to the terminals won't run the motor. We don't use them in FRC (yet), so you'll have to familiarize yourself with a slightly different technology.

So, can we interest you in a more powerful motor (or a smaller rollerblader)?

By the way, maybe it's just my copy of Acrobat, but there's a big black box where your CAD rendering should be in the third attachment.

[PortugazD.Ace]

1. I was thinking that I would need a more powerful motor. My problem is that I can't find one. If you have any links or recomendations for this while still being a workable size for my concept, please post.
2. There is a black box where the CAD rendering should be. I'll have to figure that out and get back to you.

[PAR_WIG1350]

electric screwdrivers

once you cut down the excess material (move the batteries and controls outside of the casing) they are very small. I would recommend the black & decker as-600. It runs at 130 rpm and has 2260 mNm of torque available (20 in-lbs).
The AD-600 is similar, but the casing isn't as easy to modify (on the AS-600 you simply remove a pin and the transmission can be easily removed and then all you have to do is pull the two halves of the case apart).

without the transmission, the motor in the reduced shell is about 2-2.5 inches long. You can use the transmission supplied (2-stage planetary) or you could build a custom transmission, a hex-shank flexible shaft would easily allow you to mount it almost anywhere at the cost of some power loss.

[JamesCH95]

Originally Posted by PortugazD.Ace

1. I was thinking that I would need a more powerful motor. My problem is that I can't find one. If you have any links or recomendations for this while still being a workable size for my concept, please post.
2. There is a black box where the CAD rendering should be. I'll have to figure that out and get back to you.

What are your power requirements? Do you have a top speed or acceleration in mind? If you can give us some parameter to work with we can help determine how much power you will need, which will determine what size of motor you should get.

Drill motors are a good way to go, they can pack a ton of power in a relatively small package. Gearing most motors down is going to require a bit of space, so using a flexi-shaft as PAR_WIG1350 suggests might be a good way to go.

I would immediately surrender the notion that you will find a motor that can fit inside of a roller blade wheel and supply any meaningful amount of power.

[PortugazD.Ace]

In terms of acceleration: the system accelerates in proportion to the users kick. the motors are just assist motors to make you kick last longer. I have already figured out where I'm going to put the gearbox to gear the system down. the gearbox is going to fit inside the wheel. I can manage 2" of space for the diameter of the motor, and 3" of space for the length. But I can't manage any more space because I also need to fit control electronics, brakes, suspension, air cusion systems, etc...
Any ideas?

[JamesCH95]

An average roller blade rider can get going a little over 20mph, with the roller blades going slightly faster than that, say 25mph, which is about 37 ft/s. Get your wheel circumference and calculate how fast you need the wheel to spin. at max speed. Go to robot market place and pick out a geared motor that will fit in your space requirements, then figure out if it can apply any appreciable amount of power. Your average human can do about 350watts sustained, and up to 1700watts at a full sprint.

[Chris is me]

Originally Posted by JamesCH95

An average roller blade rider can get going a little over 20mph

I would say 20mph is much higher than an "average" roller blade rider. 15 mph is a lot more accurate.

[PortugazD.Ace]

But don't forget, there are also people who can reach street speeds on rollerblades(beleive me, I've seen them). The assist motors are meant to take your kick, multiply the would-be resulting speed by 1.5-2, then use that result as the output speed of the wheel. So, in short, your speed gets increased by 50%-100%. Get where I am going so far?