Calculate Rotational Torque: 8700 kgm2 @ 18 RPMs

[Pinon1977]

Hey guys and gals. I need someone to double check my math. I have a wheel with a MOI = 8700 kgm2. It is rotating at 18 RPMs. T=MOI*angular acceleration?
Or
8700 kgm2 * 1.83 rad/sec= 15979.6 Nm?

 

[sophiecentaur]

You are quoting the angular velocity but the formula calls for angular acceleration. What is the acceleration? (At constant velocity, the only torque needed is to overcome friction.) Do you have some more information?

 

[Pinon1977]

Ok, let me back up a little. I have a flywheel that has a MOI of 8700 kgm2. I need to know how much torque the flywheel is creating at the axel at 18rpms. I converted to 18 rpms to rad/sec by dividing by 9.8 = 1.83 rad/sec. I thought thw rorque would be derived by multiplying the MOI * 1.83? Is that not correct? Please advise.

 

[sophiecentaur]

From what you say, you can tell the Kinetic Energy of the flywheel at a given angular velocity. Just as linear KE is half m vsquared, angular KE is half I omega squared. Also, the angular momentum is I omega.
Torque only comes into it to overcome friction or to accelerate the flywheel (or slow it down).
What is your actual scenario and what is the function of the flywheel? The flywheel doesn't create torque as such. I guess you want to connect the spinning flywheel and make something happen?

 

[Pinon1977]

Yes my scynerio is a little unique. I have a flywheel that is powered by a "motor" of sorts. This giant flywheel turns at 18rpms while being powered by the "motor". I want to hook a generator up to the flywheel to produce power. I need to know how much torque i can expect at the axel axle of the flywheel so i can properly size my generator.

 

[CWatters]

Yes my scynerio is a little unique. I have a flywheel that is powered by a "motor" of sorts. This giant flywheel turns at 18rpms while being powered by the "motor". I want to hook a generator up to the flywheel to produce power. I need to know how much torque i can expect at the axel axle of the flywheel so i can properly size my generator.

How much more torque can the "motor" deliver?

or put it another way...

If your motor _only_ has enough torque to overcome friction and turn the flywheel at 18 rpm, then adding a generator will cause it to slow down.

 

[CWatters]

You might also ask yourself.. Why doesn't the flywheel go faster and faster? Why does it turn at a constant 18rpm? Is it because the "motor" is using all of it's available power to overcome friction? In which case there is no spare power left to turn a generator.

 

[sophiecentaur]

I want to hook a generator up to the flywheel to produce power

The agony isn't over yet, I'm afraid. The power supplied to the generator from the flywheel will depend on the Electrical Load. If there is no load then the only energy taken from the Wheel will be due to losses in the generator (friction / field windings etc.). So you would need to specify what load (in kW, perhaps) that you would want to feed and see how long you could feed that load with the total Energy that's in the flywheel.
There's an added complication that the wheel will immediately start to slow down and the volts will drop if there is no regulation. So you can probably only get, say, half of the energy out of the wheel before it's too slow to maintain volts.
My question is what is your load in kW? You can then work out how long the flywheel could feed that load if you can only use half of the stored energy. That's pretty easy as I told you how to work out the total KE of the full speed wheel.
Notice, the torque didn't come into that particular argument at all. The only time that torque would come into it would be if your instantaneous electrical load was extremely high and you were worried about mechanical strength of the coupling; I can't think of any other time where torque would need to be considered.
BTW what type is the motor that's supplying the original energy to spin the flywheel. If it's electrical, it would be relevant to calculate how long it would take to spin up the wheel.

 

[Pinon1977]

I appreciate all the information and all concerns have been taken into consideration already.
My motor is not a motor, per say, but more of a prime mover which is contained within the flywheel itself. The prime mover allows the flywheel to rotate at 18 rpms (constant) and it's self regulating (because at some point, the centrifugal force becomes to great for the prime mover to overcome).
So, the prime mover is the flywheel and, conversely, the flywheel is the prime mover. They work collectively in a symbiotic nature to create rotational movement.
So, we need to forget the typical train of thought here and just focus on what we do know.
I have an apparatus with a MOI of 8700 kgm2 rotating about an axle at 18rpms. Disregarding any load or force from any outside source (other than gravity), what would my available torque at the axle be? I thought it might be in the neighborhood of MOI * angular velocity or 15921 Nm or 11742 ftlbs

 

[Pinon1977]

You might also ask yourself.. Why doesn't the flywheel go faster and faster? Why does it turn at a constant 18rpm? Is it because the "motor" is using all of it's available power to overcome friction? In which case there is no spare power left to turn a generator.

 

[Pinon1977]

Centrifugal force is the limiting factor which governs the RPMs. 18 Is about as much as it can handle before the centrifugal force becomes too great to overcome. So it's self regulating.

 

[sophiecentaur]

So it's self regulating.

You'll have to explain that. Do you mean there's a centrifugal clutch / brake to limit the top speed?

They work collectively in a symbiotic nature to create rotational movement

You would need to explain this, too. You have asked an 'Engineering" question so you really need to be supplying "Engineering" facts if you want an answer to it.

So, we need to forget the typical train of thought here and just focus on what we do know.

Unfortunately, we (PF) don't know enough from what you have written. At the moment, the viability looks very questionable. Your system needs a Power Input and the flywheel can store Energy, later to be returned.
Torque is irrelevant, as I have already said, or at least it is unknowable.

So, we need to forget the typical train of thought here

I'm not sure what you mean by that but a 'typical' train of though delivers working systems. Anything else is unproven.

 

[CWatters]

So, we need to forget the typical train of thought here and just focus on what we do know.
I have an apparatus with a MOI of 8700 kgm2 rotating about an axle at 18rpms. Disregarding any load or force from any outside source (other than gravity), what would my available torque at the axle be? I thought it might be in the neighborhood of MOI * angular velocity or 15921 Nm or 11742 ftlbs

Unfortunately it doesn't work like that.

If we are assuming constant rpm then the nearest relevant equation would be ..

T_out = T_in - T_F

Where..

T_out is the torque available to turn the generator.
T_in is the torque generated by the motor/prime mover.
T_F is the torque lost to friction in the flywheel bearings (and any other losses such as air resistance).

There is no generator connected yet so no output torque eg T_out = 0

 

[Pinon1977]

Unfortunately it doesn't work like that.

If we are assuming constant rpm then the nearest relevant equation would be ..

T_out = T_in - T_F

Where..

T_out is the torque available to turn the generator.
T_in is the torque generated by the motor/prime mover.
T_F is the torque lost to friction in the flywheel bearings (and any other losses such as air resistance).

There is no generator connected yet so no output torque eg T_out = 0

Ok, now we may be on to something. The generator I would like to hook up to it is a 10kw unit and requires 55ftlbs of torque at 1800 rpms to generate 10kw.

But what I need to know is the output torque of my prime mover. That way I can know for sure which geneator I can turn.

 

[Pinon1977]

And yes, I know the available torque will be divided by the 100:1 step up I will need to get to 1800 rpms. I am using a bidirectional 100:1 gearbox running in reverse. So if my torque is 11000f ftlbs @ 18 rpms, the torque is reduced to 110ftlbs at 1800 rpms.

But is 11000 ftlbs a viable value based upon the MOI and RPMs?

 

[CWatters]

Ok so for the prime mover...

Power = torque * angular velocity

Which you can rearrange to give...

Torque = power/angular velocity

In SI units Torque would be in Nm, Power in Watts, and angular velocity in radians per second.

How much power can your prime mover generate?

 

[Pinon1977]

Ok so for the prime mover...

Power = torque * angular velocity

Which you can rearrange to give...

Torque = power/angular velocity

In SI units Torque would be in Nm, Power in Watts, and angular velocity in radians per second.

How much power can your prime mover generate?

Well that's what I'm trying to figure out. Ha ha. Needless to say, this is a little bit of a unique situation. The mover creates a moment arm at the axle which equates to approximately 600 ftlbs of torque applied constantly. The mass of the PM and flywheel will rotate as a result of this leverage to 18 rpms. Then centripetal force takes over at 18 rpms and prevents the moment arm from doing it job until it slows down a bit. I have appllied a make-shift load at the axle to try and stall the apparatus, but to no avail.

 

[Pinon1977]

You'll have to explain that. Do you mean there's a centrifugal clutch / brake to limit the top speed?

You would need to explain this, too. You have asked an 'Engineering" question so you really need to be supplying "Engineering" facts if you want an answer to it.

Unfortunately, we (PF) don't know enough from what you have written. At the moment, the viability looks very questionable. Your system needs a Power Input and the flywheel can store Energy, later to be returned.
Torque is irrelevant, as I have already said, or at least it is unknowable.

I'm not sure what you mean by that but a 'typical' train of though delivers working systems. Anything else is unproven.

This is a new concept, I know. But it is a viable machine. I have already built it as well as been granted a patent for the device. It is a prime mover at its very core, bur I want to explore rhe different things it can power. Clearly electrical power generation is a hot market right now. That's why i am tryi g ti soec a generator for it. They are upwards of 5-7000$ for a good one and they are not returnable. I want to make certain i order the correct one.

 

[Nidum]

Reading through your other threads makes me think that you could just be working on a perpetual motion machine .

Before this discussion continues please tell us how this machine works . In particular tell us what is the primary source of power ie is it fuel or falling water or wind or donkeys or what ?

 

[Pinon1977]

I would like preface this thread by stating that this is in no way, shape, or form a perpetual motion machine. Not at all. While I cannot disclose all of the "ins and outs" of the driver, I can share with you guys a few things.

It is a mechanical advantage prime mover (or driver) that creates rotation of its overall mass via a series of offset mass movements within the driver. It's primary power source is gravity; however, movement is also done pnumatically via a Fibonacci engine which supplies the aforementioned air requirements.

Does that help? I just need to know how much power i can expect to have at the axle with 3980lbs disc shapes mass moving at a constant 18rpms.

 

[Pinon1977]

Reading through your other threads makes me think that you could just be working on a perpetual motion machine .

Before this discussion continues please tell us how this machine works . In particular tell us what is the primary source of power ie is it fuel or falling water or wind or donkeys or what ?

Just to make clear- I am not working on the machine anymore. It's already built and tested (simulated load with no way of measuring absolute load). Thanks.

 

[sophiecentaur]

Snake oil, I suspect.
He won't say what the energy source is so it must be powering itself.

 

[CWatters]

It's primary power source is gravity.

Unfortunately gravity cannot be a source of power (even a hydroelectric power station gets its power from the sun.)

 

[CWatters]

Well that's what I'm trying to figure out. Ha ha. Needless to say, this is a little bit of a unique situation. The mover creates a moment arm at the axle which equates to approximately 600 ftlbs of torque applied constantly. The mass of the PM and flywheel will rotate as a result of this leverage to 18 rpms. Then centripetal force takes over at 18 rpms and prevents the moment arm from doing it job until it slows down a bit. I have appllied a make-shift load at the axle to try and stall the apparatus, but to no avail.

One way to measure the power output would be to arrange for it to stir water in a tank. As a result the water should heat up. It's possible to calculate the power output from the mass of the water and the rate at which the temperature changes. Insulate the tank.

 

[CWatters]

I would like preface this thread by stating that this is in no way, shape, or form a perpetual motion machine. Not at all. While I cannot disclose all of the "ins and outs" of the driver, I can share with you guys a few things.

It is a mechanical advantage prime mover (or driver) that creates rotation of its overall mass via a series of offset mass movements within the driver. It's primary power source is gravity; however, movement is also done pnumatically via a Fibonacci engine which supplies the aforementioned air requirements.

Does that help? I just need to know how much power i can expect to have at the axle with 3980lbs disc shapes mass moving at a constant 18rpms.

How long does it take for the flywheel to go from stationary to 18rpm?

 

[Pinon1977]

Snake oil, I suspect.
He won't say what the energy source is so it must be powering itself.

I clearly stated it was pneumatically assisted and gravity driven. No smoke and mirrors here, I promise.

 

[Pinon1977]

Unfortunately gravity cannot be a source of power (even a hydroelectric power station gets its power from the sun.)

I'm not debating what gravity is or is not capable of doing; however, in my system it *is* one of the sources of power.

And you're right, before now, gravity was probably never perceived as a source for creating power. It's always been the enemy (except hydro).

 

[Pinon1977]

How long does it take for the flywheel to go from stationary to 18rpm?

It takes 3 to 3.5 seconds to go from 0 to 18 RPMs.

 

[Pinon1977]

I feel like we are losing our focus here. Regardless of what's making the driver rotate, I just need to know how to calculate the available torque at the axle at 18rpms.

 

[CWatters]

Ok so if it takes say 3 seconds to go from zero to 18 rpm you can estimate the torque and power as follows...

Torque = MOI * Change in angular velocity/Time

18 rpm = 0.1 rads/s

= 8700 * 0.1/3
= 290 Nm

However if it has stopped accelerating at 18rpm you won't get that torque at 18rpm. Better use a lower value say 9rpm (0.05rads/s) for safety..

Then..

Power = Torque * angular velocity
= 290 * 0.05
= 14W

I have assumed your figure for the MOI is correct.

 

[CWatters]

I feel like we are losing our focus here. Regardless of what's making the driver rotate, I just need to know how to calculate the available torque at the axle at 18rpms.

Problem is you are asking a question that's impossible to answer based on the info provided. If it stops accelerating at 18rpm then there is NO torque available. It's all being used to overcome losses somewhere. You even explained that centrifugal force somehow stops or reduces the power to stop it accelerating further.

That's why I suggested using a lower rpm in the calculation above.

 

[Pinon1977]

Ok so if it takes say 3 seconds to go from zero to 18 rpm you can estimate the torque and power as follows...

Torque = MOI * Change in angular velocity/Time

18 rpm = 0.1 rads/s

= 8700 * 0.1/3
= 290 Nm

However if it has stopped accelerating at 18rpm you won't get that torque at 18rpm. Better use a lower value say 9rpm (0.05rads/s) for safety..

Then..

Power = Torque * angular velocity
= 290 * 0.05
= 14W

I have assumed your figure for the MOI is correct.

While I do not know exactly what the torque is at the axel, I do know it's more than 290Nm. I used a 4000 ftlbs prony brake last week and it maxed it out in less than 2 rotations. So the torque is at least 4000 ft lbs at 18rpms.

What's wrong with this calculation:
8700kgm2 * (18 RPMS/9.8) OR
8700kgm2 * 1.8 rad/sec = 15660Nm

 

[Pinon1977]

14 watts? Really? Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right? I can generate 100 watts of power with my bicycle generator.

 

[Pinon1977]

This driver or (prime mover) I have designed is 10' in diameter with an overall weight of 3900lbs. The entire assembly creates a MOI of 8700kgm2. The driver is rotating at a constant speed of 18 rpms. That being said, you're telling me one can only expect 14 watts of power out of that beast?

 

[A.T.]

Common sense would tell you that a flywheel with a MOI of 8700 kgm2 spinning at 18 rpms is going to produce more than 14 watts of power. Right?

Depends for how long.

The driver is rotating at a constant speed of 18 rpms.

If it doesn't slow down, you will get out the power you put in, minus losses.