oz-in torque on dc motor

[thejoe]

Hello,

This is my first post on this forum and I’m not sure if this is the correct section to post but it seem to fit.

I would like to know about DC motors. I need to find a motor for an application I’m building. I need a DC motor to run on as little volts as possible, the diameter should be less than 1 inch, the motor should have an RPM at about 120 240 RPM.

I found all these requirements in some motors the only thing I don’t understand it the Torque or oz-in torque. Can someone explain oz-in torque and how this value would be would be used to determine a motor?

Thanks

thejoe

[DerekZahn]

Could you rephrase your question? Torque is the "strength" of the motor, like if it can put out 1 oz-in of torque (one ounce of force at a distance one inch from the shaft), you could stop it with your fingers, but 1000 oz-in of torque you could not stop.

Torque is highest at 0 RPM ("stall"), and lowest at maximum RPM (no-load), and is roughly linear in between. So if you put a load on a motor, it slows down.

I have a feeling, though, that this is not quite what you are asking.

[thejoe]

What I want to do is connect a 1" long by 1\4" round shaft adapter to a dc motor shaft. The end of this shaft could spin any number of attached objects like a golf ball, solid brass wheel, etc.

I'm trying to finger out how much weight or torque a motor would need to turn these objects without stopping?

[thejoe]

here is the motor I'm looking at.

http://www.lynxmotion.com/Product.as...&CategoryID=11

[DerekZahn]

If the thing you are spinning is centered on the shaft, it takes very little torque to just make it spin in the air. That little motor is plenty to spin a golf ball.

[ViperTX]

Well on the motor you show with 4.6 oz-in of torque as the stall torque.....this basically means that if you had a 1 inch long level attached to the end of the motor that anything greater then 4.6 oz would cause the motor to stall out (not turn).

[DerekZahn]

I'm pretty sure the 5 oz-in is the "rated" torque. That is almost certainly the popular "60:1 Copal" motor, about which more data here:

http://robotcombat.com/marketplace_ant_motors.html

50 oz-in stall torque.

[thejoe]

This is a simple drawing of what I want to make.

http://users.mn.astound.net/jokersdesign/steel.jpg

The shaft adapter is a custom machine milled peice that I had a local company make.

The steel balls that screw on could range diameter and the weight could be from 1/4 ounce to 4 ounces.

So if a DC motors specifications says Torque = 4.86 oz-in, then the motor should be able to spin the a steel ball with a weight of 4 ounces at X RPM?

Robert

[Al_The_Man]

One other factor to consider is the inertia factor which is calculated to provide whatever acceleration/deceleration is required.
Al.

[ViperTX]

Good point Al. I noticed that they recommend that you not hit the stall torque because you'll damage the gear box....so, I suspect that you can get it going, but you may have a hard time reversing direction or stopping at any number close to the stall torque.

[One of Many]

I am reasonably certain you would need to calculate the moment of inertia for a given mass to qualify a torque quantity. That can get complex for irregular shapes.

Do a web search for calculating moments of inertia. Your question relates to many principles of several foundations. The mass and how it relates to inertia in order to obtain a torque value to move that mass. Once it is motion the dynamics change.

Here are a few:

Solid Mechanics-Dynamics

Inertia to torque calulators and charts
Angular acceleration

Hope this helps some.

DC

[Al_The_Man]

There are also many free downloadable programs where you can plug in the motor/load parameters in to calculate inertia, Kollmorgan, Allen-Bradley Electro-Craft Seiko-Denki etc.
Al.