Calculate torque of an 80kg object rotating on its central axis

[Yafimski]

From what I understand they meant "bearing bore diameter [mm]" to mean the size of the ball bearing right?
I have a collection of them positioned at 45deg, but each one has a diameter of 8mm, yes.

The entire plate is 300mm in diameter if that's what you mean.

 

[JBA]

The correct d diameter is the diameter of the circle where your ball bearings run on the bottom of the plate.

 

[Yafimski]

In that case it comes to about 430Nmm which is 0.43Nm

But that still doesn't make sense because the motor I'm using now is more powerful than that...

 

[JBA]

Are your balls running dry or lubricated?

 

[JBA]

What is the gear ratio of your planetary gearbox?

 

[Yafimski]

Running dry.

The specs again:
NEMA 17 Planetary Geared Stepper Motor
Weight = 0.5Kg
Rated Voltage = 12V DC
Rated Current = 0.4A DC
Rotor Inertia = 35gcm2
Holding Torque before gearbox = 26Ncm

Gearbox:
Gear ratio = 26 (103/121)
Max. Permissible Torque = 3Nm
Moment Permissible Torque = 5Nm
Shaft maximum Axial load = 50N
Shaft maximum Radial load = 100N

 

[JBA]

At this point I have to admit that I am pretty much stumped.

I reviewed your application related to an SKF standard angular load ball bearing unit with a 200 mm bore diameter and the result is still about the same as your calculated value and well below you rated 5Nm torque capability with a 90kg load. They did give an alert that the minimum speed for good bearing oil lubricant distribution should be 12 rpm. I suggest you might try lubricating your bearings to see if that improves the situation; but, apart from that it would appear that you bearings should not be creating any torque friction for you; and I cannot think of any other factors, particularly at the very low 5kg load that appears to be your present limit. The only other thing I might suggest is that you run a static torque test on your NEMA 17 unit to see if it meets its published torque capability.

 

[CWatters]

Earlier you said the system exists. So why not put an 80Kg weight on it and just measure the starting torque a few times? Perhaps add a factor of 2 or 3 for safety/wear. To measure the torque add a lever arm, pulley, and rope with a pan. Add weights to the pan until it moves. Disconnect the motor and gearbox for the test.

 

[Yafimski]

Okay, thank you I will try to see what I can do.
Thanks for all the help.

 

[Nidum]

There is nothing to stop that assembly as drawn from running badly crooked . Most of the problems with getting it started in rotation probably come from getting it unstuck from a random resting position unfavourable for starting .

If you just want a turntable driven by a motor then there are far better ways of doing it . We can talk about that in detail if you wish but surely a central shaft with an arrangement of conventional bearings is what is really needed ?

A large diameter thrust bearing is unlikely to be necessary but if you must have one then just use a single ready made one ?

If you tell us what you are actually trying to do with this turntable we may between us on PF be able to suggest a nice simple solution .

 

[Yafimski]

Hi,
Thanks, it's just the the entire mechanism involves a few gears and things like that as well, and it's quite complex so I'd rather avoid getting into that since we haven't even figured out the simple stuff.

I think i'll try to buy a much bigger motor and see what it can do for now :)

Thanks.

 

[JBA]

At this point I am going to risk stepping way out of my realm of knowledge; but, based upon your supplied data the selected motor should be able to provide sufficient torque to easily rotate more than your 5 kg weight at your very low speed. So are you sure you have the correct driver and hookup for this motor?

 

[Yafimski]

Yes I'm positive. I haven't measured it, it was just an object I placed on top so it was around 5kg I think. I'll probably just order a bigger motor and test it.