Friction between motor and flywheel

[Yarno]

TL;DR

For a model, I am making I need to know the friction between a motor and a flywheel. The electric motor is on the side of the flywheel and the idea is when this motor turns the flywheel will start to rotate due to the friction between them. I however can not find a good way to calculate this friction. I could assume that there is no slip and use the static friction formula but then I would also not know how to calculate the normal force between the motor and the flywheel. Some help would greatly

For a model, I am making I need to know the friction between a motor and a flywheel. The electric motor is on the side of the flywheel and the idea is when this motor turns the flywheel will start to rotate due to the friction between them. I however can not find a good way to calculate this friction. I could assume that there is no slip and use the static friction formula but then I would also not know how to calculate the normal force between the motor and the flywheel. Some help would greatly

 

[russ_watters]

How, exactly, are they interfacing?

 

[berkeman]

Welcome to PF.

That doesn't sound like a very good way to turn a flywheel, IMO. Why not use a belt drive or a standard geared drive? Or even direct drive?

Can you post more details about the project, including sizes, weights, power, speeds, etc.? And a sketch would also be helpful (click the "Attach files" link below the Edit window to upload PDF or JPEG files). Thanks.

 

[jack action]

That doesn't sound like a very good way to turn a flywheel, IMO.

It is a very good way if you assume no slip as mentioned in the OP. This is how all vehicle clutches work.

I could assume that there is no slip and use the static friction formula but then I would also not know how to calculate the normal force between the motor and the flywheel. Some help would greatly

The following video shows how such a clutch works. The normal force is determined by the diaphragm spring. The normal force required is the one that will provide the static friction you need to lock both rotating parts together.

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I also include the following video which is rather long but you see a real clutch in much more detail that you may also need in your design. Near the end of the video, another style of a clutch is presented. Instead of a diaphragm spring, it has normal springs and 3 levers ("fingers") that may illustrate better how a clutch works.

​

 

[berkeman]

It is a very good way if you assume no slip as mentioned in the OP. This is how all vehicle clutches work.

Oh, I didn't think of the clutch geometry -- I had a simpler (worse) geometry in my head when I read the OP. That's why a sketch would be a big help...

 

[Yarno]

Okay so here is a little bit more information about the design. We sort of have a floating flywheel with magnetic bearings. That is the reason why we can not just use an axle connected to a motor. The motor itself would when needed be pressed against the flywheel to make it rotate. The contact between the two would be rubber on rubber. The motor should also be able to disconnect when needed.

The difficulty that I have when finding the friction is finding the normal force between them because I assume that is just how hard you press the motor against the flywheel, right? Also if there was slip we would need the kinetic friction coefficient but the difficulty with that is that it is dependent on temperature and speed.

I would appreciate if someone could help with finding the normal force or the kinetic friction coefficient.

 

[berkeman]

We sort of have a floating flywheel with magnetic bearings.

Well if the flywheel is "floating" you can't just press against it at one point to add energy to it, right?

I'd be inclined to use 4 rubber wheels, spaced at noon, 2 o'clock, 6 o'clock and 8 o'clock around the circumference of the wheel. I would use a motor and belt drive to be able to drive the wheels at noon and 6 o'clock.

The wheels would normally not make contact with the floating flywheel, but when it is time to spin up the flywheel, the rubber wheels would all press in together, making contact at the same time. The motor would be turned on to spin up the flywheel, and then the 4 rubber wheels would be pulled away from the flywheel.

 

[jack action]

The difficulty that I have when finding the friction is finding the normal force between them because I assume that is just how hard you press the motor against the flywheel, right?

You need a free body diagram (FBD). Ignoring other resistance forces and simplifying everything, it will boil down to:
$$T=I\alpha$$
Where $I$ is the inertia for the system, flywheel and motor together.
Your torque $T$ is determined by the friction force $F_f$ and the radius $r$ where it is applied on your flywheel, so:
$$ F_f r = I\alpha$$
Thus, considering the friction coefficient $\mu$, the normal force $F_n$ is:
$$F_n = \frac{I\alpha}{\mu r}$$
This is good if both motor and flywheel are at the same speed.

But if you do the FBD for the motor side and one for the flywheel side, you should see what will be the deceleration on the motor side and the acceleration on the flywheel side as you increase the normal force. When does one catch up with the other? Because the motor could accelerate the flywheel, but the flywheel could stop the motor as well.

Don't forget that beyond friction, your motor has to be able to provide the friction force, and that will be determined by its available power and the rpm at which this happens.

I would appreciate if someone could help with finding the normal force or the kinetic friction coefficient.

Here you have 3 different static friction coefficients for rubber on rubber that vary from 0.62 to 1.15. I would expect to have an even greater range for the kinetic one, especially considering temperature variations.

Either you ask the manufacturer for this information or, even better, you test it yourself.

 

[Averagesupernova]

So it sounds like you want the flywheel to be free floating yet be able connect to it and spin it up. There's only one way I know of that this can be done and that is with Eddy current coupling.

 

[sophiecentaur]

So it sounds like you want the flywheel to be free floating yet be able connect to it and spin it up. There's only one way I know of that this can be done and that is with Eddy current coupling.

Air jets can be spaced around the flywheel if there are turbine slots.
But something relevant needs to be discussed and that is the application. Is the flywheel to be used for energy storage? In which case the coupling needs to be strong. If it's to be used as a gyroscope then good bearings and light coupling are appropriate.