# Calculating torque on revolving sign

1. Oct 18, 2016

### Lourenssign

I have a triangular sign, each face is 9000mm long horizontal, 2616mm high vertically, the "turntable /bearing "diameter is 3450 mm, the longest length from the sign edge is 5100mm from the centre shaft, the deadweight of the structure is 3200kg, i want to turn at 0,6 RPM, what torque shoul my motor be​

2. Oct 18, 2016

### Staff: Mentor

Welcome to the PF.

The torque to maintain the sign turning depends mostly on the loaded bearing friction. If the wind interacts with the sign, then you will need additional available torque to overcome those forces. The torque to start up the sign and get it to rotational speed depends on the Moment of Inertia and how long it takes to spin it up (in addition to overcoming the bearing friction).

3. Oct 18, 2016

### Lourenssign

Thanks PF,
The windload /force is 1.1kpa? , im a total idiot when it comes to this, but my motor specialist wants this to recomend the right motor/gearbox
Rgds

4. Oct 18, 2016

### Lourenssign

How do i calculte the friction on the bearings

5. Oct 18, 2016

### Staff: Mentor

You probably need to measure it, or look in the datasheet to see if it is specified for various loads...

6. Oct 18, 2016

### Lourenssign

The load is spread over 6 custom wheel carriages, 2,wheels each with a 700kg load capacity on each wheel, they run on a rolled channel (ring), thus i cannot get the info from a spec sheet

7. Oct 18, 2016

### Staff: Mentor

Have you looked into similar size signs to see what size motor they use? That's a pretty heavy sign! Have you considered trying to build it with a lighter structure? Why does it have to be so heavy? Do you have any pictures?

8. Oct 19, 2016

### Mech_Engineer

Your limiting factor will likely be the torque required at startup to accelerate the sign to the required speed in a reasonable amount of time. I would first estimate how much torque is required to accelerate the sign, and then apply a safety factor.

It seems to me wind load wouldn't be a huge concern for motor torque as long as the sign's geometry is symmetric and it is rotating about its center. Wind coming from any particular direction should balance equally. Wind load will be more important for preventing the sign from toppling over in high winds, have you taken this into consideration?

Last edited by a moderator: Oct 19, 2016
9. Oct 19, 2016

### JBA

The bearing friction is not a factor of the wheels' size, or their contact surface on which they are running, that is another issue, the bearing friction is a function of the loading on each wheel and the design and type of bearing between the wheel and the shaft; so, the first question for that is "what type of bearing are you using?"and unless you are making your own bearings then the manufacturer should be able to supply the bearing load vs rolling or sliding friction information that you need.

With regard to the bearing loading from the maximum wind loading directly on the face of the sign should be included. This loading will somewhat reduced for the composite set of all wheel bearings because as the load is increased on the downwind wheels it will be correspondingly reduced on the upwind wheels.

10. Oct 26, 2016

### Lourenssign

Hi, The bearings are type 6006, deep groove bearings, the friction was given as 0.015, if that helps.
Thank you Mech-Engineer for the advise on wind load, is there a mathematical statement for it, as my motor supplier is very adamant it play a huge roll.
Thus we are no at a moto that needs to be 120KN torque, which might be over specked.
Can I attach a pdf file here?