More maths..

[Pigeon_Droppings2]

Think your original maths is OK...the radius is used because you are measuring from the centre of the shaft to the load (or counter load).

Is the gearbox applied directly to the central shaft (ie attached directly)....or is the shaft geared for example?

 

[Brad93]

Moment of Inertia is what you need to know.

½mr^2

so,

0.5 • 100kg • 0.5m^2 = 12.5kg/m

You’re not accounting for friction, so alpha=0

you need to ascertain a rpm you want to achieve, so for welding a 1 meter part you probably only want 0.3RPM or so which is 0.03radians/s

also ascertain a time in which you want the part to reach that speed. Let’s say 1 second for simplicity.

so if we put a scientific calculator into radians.

I*omega/time = torque

0.375nm

by my calculations.

I’m no engineer or maths person, calling @Dr.Al or @Munkul

 

[Brad93]

If you think about it a 100kg disc on a frictionless system would literally just take a push with a finger to spin at 0.3RPM.

I’m sure somewhere, there must be an article on one of the bearing sites like SKF or FAG listing the coefficient of friction you could expect from a bearing.

you could plug that into an equation, and get a realistic figure. Remember that’s a uniformly distributed load perfectly balanced on the centreline

 

[Pigeon_Droppings2]

Thought it was a point load at 1m radius...didn't realise it was a uniform disc?

 

[Brad93]

Thought it was a point load at 1m radius...didn't realise it was a uniform disc?

Why would you want to rotate a point load on a welding positioner? Even if you’re rotating pipe you’d balance the pipe.

 

[Pigeon_Droppings2]

No idea...but if the disc is uniform then I think you are right....very little force would be needed. I thought it was a disc with a load attached but maybe I misunderstood.

 

[Agroshield]

Have a look at:

https://khkgears.net/new/gear_knowl...ears/know-about-gear-transmission-torque.html

Exercise 3-5 is exactly what you are asking about. Substitute your own numbers. Note their given factor for worm gear efficiency (0.3). The torque you have calculated is correct.

 

[hotponyshoes]

Why would you want to rotate a point load on a welding positioner? Even if you’re rotating pipe you’d balance the pipe.

I would not.
I was just trying to make the maths easy and I figured if I went for a worse case set up then it would all be ok.
Its eaa enough to find a positioner rated at 10kg, or 50kg or 100kg but none of them really explain what that rating means?

I know if you put 100kg of pipe perfectly central then the only force needed to turn it would be inertia and to overcome the friction along the drive train etc.

So if you wanted to use a rotary table and just took the hand wheel off and drive the shaft directly with a motor and that table had a 90:1 ratio it's not actually going to need a very big motor is it?

 

[Agroshield]

Its easy enough to find a positioner rated at 10kg, or 50kg or 100kg but none of them really explain what that rating means

Food for thought on ratings:

https://www.genstartech.com/welding-positioner-2

 

[Brad93]

I would not.
I was just trying to make the maths easy and I figured if I went for a worse case set up then it would all be ok.
Its eaa enough to find a positioner rated at 10kg, or 50kg or 100kg but none of them really explain what that rating means?

I know if you put 100kg of pipe perfectly central then the only force needed to turn it would be inertia and to overcome the friction along the drive train etc.

So if you wanted to use a rotary table and just took the hand wheel off and drive the shaft directly with a motor and that table had a 90:1 ratio it's not actually going to need a very big motor is it?

Tiny. Like my 0.3hp 3~ motor With a 300:1 reduction box. Would easily turn my weight.

 

[MoreWellie]

it's not actually going to need a very big motor is it?

true if horizontal

but you started this post with

In a "worse case" the table was mounted vertically (like a lathe chuck) as a rotator with a 100kg load at 1m away from the center,

which is so far away from being a normal welding positioner usage

 

[hotponyshoes]

I picked the 100kg at 1m as I thought it would make the maths easier to have round numbers!

Admittedly it's not a real-life problem but a 30kg object that was not round or was mounted off center could well end up as a load of 9.5kg at 142mm off center.
As long as I know the basic maths is right I dont mind using it for the more complicated sums,

In a real application I think it will be more a case of finding a motor that will cover the required speed range and making sure it easily has more than enough power rather than trying to save 20quid by going for the smallest one that will do the job