Author Topic: Gear Ratio and motor selection  (Read 1324 times)

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Offline SilverFadeTopic starter

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Gear Ratio and motor selection
« on: August 17, 2013, 04:25:14 PM »
I am new to robotics and am having trouble with motor selection and gear ratios. I am working on a project where I need to move a cylindrical rod in a pendulum motion about 2' from the center point forward and backward in about 5 seconds/direction. The rod weighs approximately 60lbs and is about 3' long. I visualize this as the motor will be stationary with a gear attached to the motor shaft, the gear will drive several other gears (this is where I am trying to figure out the number of gears, their size, tooth count,...) the last gear will turn a shaft that will turn the rod.

My questions are;
What size DC motor should I use(the smaller the better)?
How many gears will I need?
Gear size, tooth count, and how to orient the gears in a suitable manner.

I know these are a lot of questions, but even being pointed to a proper website with this information would be greatly appreciated.


Offline waltr

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Re: Gear Ratio and motor selection
« Reply #1 on: August 17, 2013, 11:50:02 PM »
Long before trying to figure gear tooth count work out, calculate, the gear reduction ratio required.
Before this decide on the motion speed of the rod you need to move and the torque required to move this rod, mass, at what rate of acceleration.
Then you can start to choose a gear ratio and the motor.

Offline jwatte

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Re: Gear Ratio and motor selection
« Reply #2 on: August 18, 2013, 03:10:53 PM »
You probably want to make that rod counter-balanced, or you're going to have to fight gravity AND inertia at the same time.

So, 3' long, 60 pounds. Center of gravity is 18" out, but you're going to need some significant power to actually get a good acceleration curve on the velocity. Call it 960 ounces, and 36 inches, so about 35,000 ozin or 245 Nm of torque. The rpm is small -- sounds like about 75 degrees of movement over a few seconds, so even 1 rpm would be sufficient on the other end of the gearbox.

So, now you know you need a transmission that can take up to 245 Nm of torque. That's car-level forces -- you'll need a beefy transmission or some kind of industrial gearbox to reliable transmit that. Now, select the motor.
Let's say you find a motor that wants to run at 8,000 rpm. Because you only need 1 rpm, you can use 8,000:1 gearing (assuming you can find a gearbox with that much reduction) which means the motor needs 245/8000 = 0.0306 Nm of torque *before the gearbox*. This is a pretty big motor, if you compare to most hobby projects.

Hopefully this gives you enough numbers to go looking at various motion control vendor websites.

Offline jkerns

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Re: Gear Ratio and motor selection
« Reply #3 on: August 19, 2013, 09:07:43 AM »
Is this essentially going to be a free swinging pendulum that just needs some assist to keep it going or are you trying to make it swing to a commanded position on demand?  Two significantly different problems.
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