# Simple Torque inertia question

1. Mar 15, 2009

### thesnesinator

Hi I am designing a 3 axis robot but need help with one calculation.

How do you calculate the torque required to rotate a supported vertical load.

For instance the torque required to rotate a dead weight sitting on top of a thrust bearing. I am looking for some formula from mechanical design books but haven't found anything.

Sorry about asking such a basic question.

Regards,
Tim

2. Mar 15, 2009

### Staff: Mentor

Welcome to the PF, Tim. The equation you are looking for is:

$$\tau = I \alpha$$

Where tau is the torque, alpha is the angular acceration in radians/s^2, and I is the Moment of Inertia:

http://en.wikipedia.org/wiki/Moment_of_inertia

.

3. Mar 15, 2009

### thesnesinator

Thanks Berkeman,

If I'm using a Stepper motor that has an input pulse of 100Hz (90 degrees a second in this setup) is my initial acceleration 1.57 rad per second^2?

4. Mar 15, 2009

### Staff: Mentor

If the motor swings 90 degrees in a second, that is an angular velocity omega:

$$\omega = \frac{\pi}{2}$$ radians per second.

The acceleration would be during the time it takes to go from omega = 0 to omega = PI/2 rad/s. The angular acceleration is the change in the angular velocity with respect to time.

5. Mar 16, 2009

### stewartcs

Don't forget to include the frictional force from the thrust bearing.

http://www.roymech.co.uk/Useful_Tables/Tribology/Plain_Bearing Friction.html

CS

6. Mar 29, 2009

### Bolts

Umm, don't forget to in include the weight either T = F.R . Which would be the "torque required to rotate a supported vertical load" as described in the first thread. Where F is the weight of the item and R is the horizontal radius of the load from the pivot position (without acceleration).