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# Identify gear and motor requirements given known working point

## Identify gear and motor requirements given known working point

(OP)
Hi All,

I'm a software engineer trying to design a simple belt driven system. I've identified my working point requirements but I'd appreciate if someone can help figure out the following:

- How do I identify the requirements for my motor given the unknown gear setup?
- What's the maths to convert the output from the motor to the working point in order to establish the gear setup?

Charles

### RE: Identify gear and motor requirements given known working point

You need to know the rod moment of inertia and then make estimates of the drive elements - it's an iterative process.

T = I*alpha where I is the moment of inertia relative to the axis of the rod. See http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html for more information about such calculations.

### RE: Identify gear and motor requirements given known working point

(OP)

I worked out that given the total inertia and acceleration, my required torque is 2.75Nm.

Now, I could pick a motor that matches this requirement but considering my mechanism is belt driven, how can I use the ratio between the pulleys to assist in achieving these figures?

### RE: Identify gear and motor requirements given known working point

That's not the total inertia. You didn't include the belt or the pulleys or the motor rotor inertia. You need to make estimates of all of these and then see if there is a motor that works. You'll also note that the torque from some motors changes with RPM so the acceleration isn't sustainable - otherwise RPM would become infinite.

This seems like an odd problem. No one needs a rod to accelerate without connection.

### RE: Identify gear and motor requirements given known working point

(OP)
It's the total inertia for everything attached to the working point (top rod). Given that I don't know which pulleys or motor I'll be using it's impossible for me to take them into account.

Perhaps I should rephrase my question; Based on 2.75Nm of required force, how can I calculate the effect of different size pulleys on the torque and rpm's?

I'm not sure I understand your last point, where would you expect another connection? This is a simplified drawing.

Ps: I posted another question related to this project here which might contextualise this question.

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