Torque calc question

I understand that if you have a 1 foot diameter drum with a rope raising 1 pound 1 foot high means you need 1 ft/lb of torque to do the job.

I'm planning a work-positioner for a friend who does welding.

What about ROTATING a load on a platter? A 10" dia disk is on a vertical 1" dia shaft (think of an old LP record player!). The vertical shaft (sitting on a trust bearing) will have a large diameter timing belt pulley/belt going to a small diameter pulley on a BLDC motor - controlled by an Arduino. The load sitting on the platter, centered, would be 100 lbs.

We are looking for LOW speed - under 3 rpms... Now - what torque rating would I need for such a job? :astonished:

I'm thinking the torque increase thru the small-to-large pulley should let me use a modest sized motor... Just can't quite get the torque for this 'horizontal' motion.

=Alan R.

Now - what torque rating would I need for such a job

Alan, I think you have two formulas to solve...

1) Stiction Torque - The Torque needed to break the initial Friction to start the table rotating

Can you measure the Force needed to break stiction with a spring scale tangentially at edge of 10" disk rim?

2) Acceleration Torque = I x A ...

Shaft Torque = Moment_Of_Inertia * ( Angular_Speed / Time ) + Torque for Bearing Friction

I think ... Your Shaft Torque is the greater of the two torques above. Your Motor Torque = ( Shaft Torque / Gear Ratio ) + % OverEngineering

How to calculate "Moment Of Inertia" for different shapes:

google: "Calculate the Torque Needed to Accelerate a Spinning Disc"

Thanks - great info! I usually do well with Google searches but didn't think of that phrase... :grin:

Will also check you the links you sent - prob just what I need.

=Alan R.

Harrzack: I understand that if you have a 1 foot diameter drum with a rope raising 1 pound 1 foot high means you need 1 ft/lb of torque to do the job.

Minor point - but the above would be true for a drum that has a radius of 1 foot. And the height of lifting is irrelevant to the torque.


Yes - in thinking about it I can see the distance itself has no bearing... :D

Have you actually built the device ? If it exists, the easiest way is to wrap a piece of string around it, and see how much force is required to make it start moving.

If you don't have a pocket luggage scale or some other means of measuring the force, you can run the string over a pulley or smooth rod and see how much weight pulling down on the end of the string is needed to make it start moving.

We are still in the planning stage, and that idea will be tried. I suspect we won’t need a huge motor…

If friction is the limiting factor then calculations aren't really that helpful, friction is too complex in a real system, depends on surface, bearing quality, off-axis loading etc. There's also two main kinds of friction, static and viscous (depends on speed), large objects will exhibit noticeable viscous friction from the air around them, for instance.

I'd look for a NEMA 23-size stepper. A medium-size one will give you around 10lb-ft of torque, but with a belt/pulley you'll of course be multiplying that. If you select a non-stepper type of motor you'll wind up with a less reliable speed or will need to include some type of encoder to keep the speed steady. I'm sure it can be done with the right mix of motor and gearing but you won't be saving much cost in doing so; the price for a stepper + driver is around $50.

A 1" mounted bearing (UC205 - the common type for mounted bearings in cast housings) is capable of ~600lbs of axial force. I can't imagine needing more than a couple lb-ft of torque to spin anything on them.

Stepper motor is overkill and rather power hungry - there are lots of gear motors with large reduction ratios on eBay to choose from. Anyway there's no point choosing any motor till the torque requirement is known and the toothed belt ratio is known.

I presume there is no requirement for positional accuracy, just a smooth slow rotation?

Why did the OP mention BLDCs BTW? Nice to have but you'd need a sensored BLDC and a BLDC driver board, DC gear motor is easier.

Actually there is another source of torque - if this is a "work positioner", might there be torques applied by whoever/whatever is working on it? That could be way larger than anything else and would suggest using a worm drive.

Here's an example of a work positioner in use:

The positioner is used starting at 7:15.