QUIET strong MOTOR and drive train question

[AweLucid]

Hi,
I am building a kinetic circular labyrinth / art project.
The motor and drive system must be relatively quiet, and I am having a trouble knowing what to order, your advice is appreciated.

Motor Requirements:
Each DC motor must must be able to turn and quickly change directions a large PVC circle (16 feet diameter max) with fabrics hanging from them. The circles (with fabric and spokes) weigh approximately 15-20 pounds and should spin at maximum of 60rpm, but will usually go a maximum of 10rmp. The circles are mounted horizontal to the ground and spin from ball bearings at their center.

How do i determine the torque and horse-power required for this? I imagine it depends on how fast I need it to accelerate and reverse directions, correct? a good acceleration will be if each circle can go go from 0 to 10rpm in 10 seconds.

I ordered a strong gear-motor for this to prototype it, and found that the gearing makes it way too loud. Ideally, I would like something that is as runs as a quiet as a fan, but i can compromise a bit.

Do i necessarily need to go with non-geared DC motors to avoid the noise? I plan to to drive the wheels with a simple pulley system, one on the motor and one large one directly attached to the wheel. I hope to increase torque and decrease speed with this system, will this be my best bet in terms of quietness? (I hope to utilize stretchy pulley cord, and a custom made pullies).

however the problem with this is that with most powerful motors, (without gears), are really fast I cannot create a ratio large enough in a small space without multiple pullies as a drive, is that what i have to do?

What kind of DC motors should I look for or avoid?
Where is the best place to find this. I hope to spend under $50 per motor.

thanks so much for your help.

Awe Lucid

 

[dknguyen]

YOur load is too large and speed is too low for any reasonably sized direct drive motor. YOu need to "gear it down" in some way. You can use gears, chains, belts, or timing belts. Belts are probably the quiestest of the bunch, though a good gear drive is also quiet (and expensive, easily exceeding the cost of your motor by a lot). And yes, you probably need to use multiple stages of belt/pulleys or gears to do your job. That's why we use multiple stages after all...doing everything in one stage takes too much room.

Brushless motors are quiet...but expensive and also need expensive controllers. However, I think you might need an expensive controller anyways because changing direction of such a heavy mass smoothly and quickly isn't easy. Honestly, I think $50 is way too low. Little fist sized motors for radio controlled planes are largely in the range of $70-$100 and good ones easily go up to a couple hundred.

Typically required torque and power for a motor are:

Torque = Force[Newtons] * Distance from center of rotation (meters)

Power = torque(Newton-meters) * angular velocity(radians/sec)
         = torque(Newton-meters) * RPM*2*Pi/60

HP = Watts/746

But your problem is a bit different for these calculations. YOur motor isn't actually lifting anything because you're spinning a balanced wheel. All the mass is balanced around the center counterweighting the mass everywhere else in all directions so technically it takes no torque to spin at a constant speed. But it does take force to accelerate the wheel. So the amount of torque you need depends on how fast you want to rev the wheel up and how quickly you want to be able to decelerate and change directions. The general equation for that is a angular/rotational form of F=ma which is:

Torque = (moment of inertia) * (angular acceleration)

The moment of inertia for a a solid disc of constant density rotating about it's center is:

Moment of inertia for solid disc spinning around center = 0.5*mass(kg)*radius(meters)^2

Angular acceleration is in radians per second squared. Assuming constant acceleration over the rev up period this is:

Angular acceleration = Change in RPM * 2 * Pi / 60/Time

ANd of course, after you figure out your torque and RPM, you can find power which you can use to find your motor. Then you can figure out what gear ratio is needed so that the power can be converted into the required balance between torque and speed.

Since you don't know the steady state torque reequired to actually spin the thing because you don't know the losses, you could estimate an upper bound for power by going

Upper bound for max power = (Torque required for reversing direction) * (maximum steady state RPM)

which is the worst case for both torque and RPM even though in practice the thing is probably never applying that amount of torque at that speed. I get about 20W. And maybe quadruple that number or even x10 or x20 it to take into account motor efficiency, friction, gearing losses, unforeseen factors, and the fact your wheels are probably not the perfect mathematical discs that we used.

Remember, that a motor will be smallest motor motor that will provide the power you need. Being the smallest it will also be the fastest with the least torque and will therefore require the most gearing. If you use a larger motor that provides excess power, it will also have more torque and be slower which means you don't have to gear as much (efficiency is a different story).

 

[dknguyen]

Not sure how loud these are or what your required noise level is. But the ones on this website are worth a look. THere are other faster ones that would require additional external gearing (probably belts and such). But you probably want a very hefty shaft (and longer) shaft to support the wheel rather than support it directly with the tiny motor shaft and motor bearings, so you're probably going to have at least one belt allowing some inherent gear reduction as well as some shock absorbtion during the direction change.

**broken link removed**

I hope you're good at converting torque units back and forth!

 

[bryan1]

Due to the size of the disk I would recommend a tapered bearing setup to handle the axial AND radial thrust those big disks will produce. A smal worm drive gearbox hooked to a DC motor driven by a suitable H-bridge arrangment would work nicely. As you don't fill in your location I don't what part of the world you are in but do look for the Varvel brand of worm drives. These worm drive gearboxs come in a full range of sizes and ratios and are suitable for running DC motors.

A picture says a thousand words so if you can provide a sketch of what you want to do maybe I can offer some assistance.

Cheers Bryan