How much torque needed to turn a 1.2m disc

[Designer@Life]

Hi
I a building an exhibit for a science museum and hoped I could get some help with the math that is puzzling us.

We need to turn three discs that are 1.2m, 0.8 and 0.6m in diameter at a speed of roughly 60rpm. Because it is a musuem they need a clutch and at the minute we are looking at a friction clutch for each disc.

What i need to know is how much torque would be needed to start each of the discs from still?

Hope someone can help

Lindsey

 

[Mech_Engineer]

It depends on how fast you want them to accelerate. An infinitesimal torque could theoretically accelerate the disc, albeit at a very slow rate.

The equations you're probably interested in can be found here: http://en.wikipedia.org/wiki/Angular_acceleration

 

[Unrest]

Hi
60rpm. Because it is a musuem they need a clutch and at the minute we are looking at a friction clutch for each disc.

Why do museums require clutches? If you want to limit the torque in case somebody puts their finger in it, you could perhaps use a low-torque motor with an electronic/thermal overload cutout to prevent overheating when it's stalled.

 

[Designer@Life]

The decision has been made that we need to have a clutch plate.
What I am struggling with is how to calculate the size of motor that we need.
It will be a motor with a worm gear that turns three pulleys, each one turning an alumium disk.

I need to know how much torque will be needed to get all three disks working

Thanks Lindsey

 

[cstoos]

Like Mech Engineer said, the size of the motor isn't really the biggest question. As long as the motor has enough torque to overcome the initial bearing friction it should be able to accelerate the discs to the desired speed (eventually).

Is the clutch to protect the gears and motor or is it more a patron safety thing? If people are able to come into contact with it I would advise a brake of some sorts, because a metal disc running with that size and speed is going to have quite a bit of inertia. A simple friction clutch won't do much to prevent injuries.

 

[Designer@Life]

Bit of both really, protect motor and gears mostly.

How do I calculate the start up torque needed? I know that it must be more than the resistance friction but am unsure how to calculate this.

It would be starting up on a morning (only once hopefully) and running all day, so that time to get up to speed doesn't not have to be instant but neither do we want it to take all day.

The other plan is that the user starts the discs spinning with they are ready and they are on for a certain length of time. Obviously if we follow this design, the start up time would want to be much shorter.

Lindsey

 

[cstoos]

Lindsey,

I think the reason you are having trouble getting a clear answer is that without a set time period you cannot calculate required torque.

Force = Mass x Acceleration
Torque = Force x Radius = Mass x Acceleration x Radius

Once the initial static friction is overcome, any motor will be able to get you to speed.
The static friction should be calculated depending on the type of setup but will be something along the lines of:

Torque Required = Coefficient of Friction x Mass x Gravity x Radius of bearing

 

[Unrest]

Force = Mass x Acceleration
Torque = Force x Radius = Mass x Acceleration x Radius

Not generally true. More like
Torque = Rotational inertia * Angular acceleration

Angular acceleration is in radians/s. eg:
0-60rpm in 10s
0-2pi rad/s in 10s
angular acceleration = 2pi/10 rad/s²

For Rotational inertia you can just sum the rotational inertias of all 3 disks so they're treated as one.

It gets a bit more complicated because many motors produce less torque at higher speeds. But if you just need a rough estimate, use the stall torque, which it will be producing when first started.

 

[Designer@Life]

Thanks for eveyones help so far

 

[W R-P]

Sorry I'm a bit late, I'm new here.Another option is to mount each disc on the shaft (I don't have a drawing so I'm assuming they are all on the same shaft) and wrap a string around the shaft.At the free end of the shaft mount some deadweights, increase the weight until the shaft starts to rotate. this will give you the starting load of the system. multiply this load by the shaft radius to determine the starting torque. eg. 10 kg *9.81 m/s2 *0.1 m =9.81 Nm as a starting torque.Please correct me if I'm wrong somewhere.