SparkFun Forums

[(((G)))]

Hello,

I am looking for advice on which type of motor to use in my project. Here are my requirements:

Quiet
Variable speed via potentiometer
Low speed… around 1 - 60rpm; most common usage would probably be 5 - 20 rpm
High torque
Ability to implement a start / stop toggle switch; preferably this could default to the “stopped” state on power-up
Full speed (or close to it) at press of the start button, i.e., it doesn’t have to “ramp up” to rpm set by pot

I’m afraid I don’t have specific figures for torque, but the motor will turn a group of four interlocked 6" diameter, 1" thick Baltic BIrch plywood gears, if that helps.

From what I have read, it sounds like a stepper motor may be the best choice, but hopefully someone can make some specific recommendations. If at all possible, I would like to avoid using a microcontroller or motor driver; I know this is possible with a stepper motor, but I don’t know it’s feasible. For example, could I limit the maximum rpm without a microcontroller? Or implement a start / stop?

If I do need a microcontroller and / or motor driver, could you please make recommends about what would work well with my project? Or perhaps there is another motor that would work better?

Thank you so much for your input!

Greg

[lyndon]

Look at the Firgelli Automation gear motors. Lots of torque and pretty cheap.

[(((G)))]

Thank you, Lyndon! I have been thinking I need to focus on the specific torque requirements before I move forward.

So my project has four 6" diameter Baltic Birch plywood gears, each with 36 teeth. They are lined up in a row and interlocked, so turning one turns all four gears.

Each gear will carry a small amount of weight, and each of the four gears will have to overcome a small amount of resistance created by a small flexible tine as it rotates (one for each gear).

So, is the procedure I have outlined below correct for determining torque requirements?

1. Weigh the gears and the amount of weight carried by each gear. Let’s say each gear is 1/2 pound, and each will carry a 1/2 pound, for a total of 4 pounds.

2. Attach one of the tines to the edge of a table like a little diving board, and tie a string with a weight to it. Add weight until the tine flexes enough to clear the impediment. Repeat for all four tines, and add the weights together. Let’s say each tine requires 1 pound to bend, for an additional 4 pounds. So I need a motor capable of moving 8 pounds. I will arbitrarily add 2 pounds to make sure I have a motor that is powerful enough.

3. If t = r x F, and I had a single 6" wheel that had to move 10 pounds, I think I would need a motor that had a minimum torque of 3 x 160 = 480 oz-in. Is this correct so far?

4. Now, here is where I’m a little confused. How do the 4 gears affect this? Since the gears are identical, the gear ratio is 1:1, so there is no increase or decrease in torque. So would I just need a motor capable of producing at least 480 oz-in.? Or do I need to take into consideration the radii of the other 3 gears? It seems like it would be mechanically advantageous to put the motor on one of the two inner gears, but I’m not sure if this is correct. Will the torque be equal across all four gears even if I put the motor on one of the end gears?

Thank you very much for your assistance!

[lyndon]

The simplest way by far is to assemble the system you plan to build and at the point where the motor would go, attach a torque wrench and see how much torque is required to accelerate it up to the speed you want.