# Torque needed to mantain a disk rotating

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1. Jan 31, 2015

### Cuervo

Hello, first post!

I'd apreciate some help on finding out the calculus for knowing the motor's torque needed for mantaining 2 vertical disks ø46cm, 30kg each at 300rpm, taking into acount 3 generic bearings friction that hypotetically is 0.5Nm friction torque

I'm having some trouble trying to figure it out, cause I see that for starting it up needs much more torque than for maintaining it turning at that speed, anyways I can give it a hand and start it by giving it some push, but what I'm looking for now is for the late torque needed.

2. Jan 31, 2015

### Simon Bridge

Welcome to PF;
The torque needed to maintain the motion depends on the friction and other losses in the motion - alone.
Per your information, that would be 0.5Nm - which will be the kinetic friction perhaps.
Starting up you need to overcome the static friction, which is usually much higher.

IRL the friction will be speed dependent... so you start off with the torque to overcome static friction, which is more than enough to overcome kinetic friction so the disks accelerate ... but the kinetic friction increases with speed (though it may initially decrease as the lubricant warms up and gets less sticky) until it is equal to the applied torque and you maintain a constant speed from there.

That should help you understand what you are seeing... then you can look up the references for common models that may help you. What you actually use depends on the specifics of your requirements.

i.e.
Tedric A. Harris, Michael N. Kotzalas Essential Concepts of Bearing Technology, 5th Ed. ch10.5
(probably overkill - most people would use the steady-state number for NOC.)

Last edited: Jan 31, 2015
3. Jan 31, 2015

### CWatters

If there was no friction then no torque (or power) would be required to maintain a constant rpm. To maintain a constant rpm the motor only has to overcome friction. Is that 0.5Nm per bearing? If so the answer to the question is just 3 * 0.5 = 1.5Nm.

The torque required to accelerate is:

Tmotor = (Moment of Inertia * angular acceleration) + Tfriction

So you need to work out the moment of inertia of your discs and the angular acceleration required (in Rads/S2).

Edit: Cross posted with Simon.