Calculating Work Supplied by a Flywheel for Forming Machine Operations

[physea]

Homework Statement

A motor which drives a forming machine runs at an average angular velocity of 100−1. The machine forms steel shapes from flat sheet steel. The work done per operation is 7kJ. It takes one second to form each shape and four seconds to change over the next sheet of steel. If the pressing machine operates continuously, determine the work supplied by the flywheel.

Homework Equations

1/2*I*w^2

The Attempt at a Solution

I am not sure how this machine works. I assumed that the work done 7kj is the work supplied by the motor, which is then transferred to the flywheel (losslessly) and which then is transferred to sheets.

Apparently this is not correct and I am not sure what is the work supplied by the flywheel.

 

[jbriggs444]

Please share with us the result you obtained based on your original assumption, the work you did obtaining that result and the evidence which now leads you to doubt the assumption.

 

[Dr Dr news]

Do you mean the duty cycle of the electric motor is 1/100 and the duty cycle of the stamping machine is 1/5?

 

[haruspex]

determine the work supplied by the flywheel.

Is that the exact wording? The flywheel applies a certain quantity of work per cycle, or a certain rate of work on average.

I assumed that the work done 7kj is the work supplied by the motor, which is then transferred to the flywheel (losslessly) and which then is transferred to sheets.

Not exactly. Think more carefully about what is going on during the one second that the stamping takes.

 

[physea]

Is that the exact wording? The flywheel applies a certain quantity of work per cycle, or a certain rate of work on average.

Not exactly. Think more carefully about what is going on during the one second that the stamping takes.

Yes, that's the exact wording.
On that one second, the flywheel stamps the sheet. That's 1/5 of the total ot 7kj work. The flywheel transfers then 1/5 of the 7kj to the sheet during that second. The rest 4 seconds, the flywheel transfers 4/5 of the 7kj to change the sheet.

Please don't make it so horrendously tiring, if you have a different idea, post it, don't ask me questions. What I understand is what I posted in the first place.
I understand that the motor, transfers energy to the flywheel and the flywheel transfers energy to the metal sheets.
That energy is always 7kj, due to the energy conservation.

 

[haruspex]

The flywheel transfers then 1/5 of the 7kj to the sheet during that second. The rest 4 seconds, the flywheel transfers 4/5 of the 7kj to change the sheet

No, that's not the process.
During the four seconds the flywheel is doing nothing to the sheet. What is it doing?
But the subtle part is what is happening during the one second? Yes, the flywheel transfers its energy to the sheet, but what is the motor doing meanwhile. That has not stopped.

You perhaps do not understand how these homework forums work. Ask questions, provide hints, point out errors, is what we do. What we do not do is post solutions.

 

[physea]

You don't post solutions, but at the same time you go in circles. I have already answered the questions you are asking.

During the four seconds, the flywheel DOES something to the sheet. It CHANGES the sheet.
During the first one second, the motor transfers 1/5 of 7kj to the flywheel and the flywheel transfers 1/5 of 7kj to the sheet.

If you think that that's not the process, then you know something about 'these machines' that I do not know.

 

[haruspex]

During the four seconds, the flywheel DOES something to the sheet. It CHANGES the sheet.

The way I read the question, the flywheel is not involved in switching to a new sheet.

During the first one second, the motor transfers 1/5 of 7kj to the flywheel and the flywheel transfers 1/5 of 7kj to the sheet.

During the one second, the whole 7kJ has to be used on the sheet, not just 1/5 of it.

Now, I admit there are two ways to think of this. During the one second, is the motor adding another 7/5kJ to the flywheel, which the flywheel passes on, or is the motor providing 7/5kJ straight to the sheet, with the flywheel providing the rest? I believe the question setter is taking the second view.

 

[physea]

The way I read the question, the flywheel is not involved in switching to a new sheet.

During the one second, the whole 7kJ has to be used on the sheet, not just 1/5 of it.

Now, I admit there are two ways to think of this. During the one second, is the motor adding another 7/5kJ to the flywheel, which the flywheel passes on, or is the motor providing 7/5kJ straight to the sheet, with the flywheel providing the rest? I believe the question setter is taking the second view.

How do you figure out that the flywheel is not involved in switching sheets? To be able to say that, it means that you know how these machines would work. I have no indication in the question that what you say happens.

Why during the one second the whole 7kj is used on the sheet? It says the (whole) 'operation' is 7kj. And he moves on describing the (whole) 'operation', by saying one second is to form the sheet and the rest four are to change the sheet.

In your last sentence, you say '7/5kJ is used on the sheet, while in your previous sentence you say the whole 7kJ is used on the sheet. You contradict yourself.

Additionally, in your first question you say that the flywheel is not involved in switching sheets, but in your last sentence you say that the flywheel provides the rest energy (i.e. to switch the sheets). Again you contradict yourself.

Is this exercise hideously worded and totally unexplained or I miss something?

 

[haruspex]

How do you figure out that the flywheel is not involved in switching sheets?

It might be, but switching sheets would take negligible energy compared to the stamping, so you can ignore that.

Why during the one second the whole 7kj is used on the sheet?

You need to understand why such a machine would have a flywheel. The stamping itself takes a considerable force. The motor would not be able to supply that, so instead it builds up momentum in a flywheel, and it is the inertia of the flywheel that ensures there is enough oomph.

In your last sentence, you say '7/5kJ is used on the sheet, while in your previous sentence you say the whole 7kJ is used on the sheet.

The motor provides 7kJ over 5 seconds, or 7/5kJ per second.
During the four seconds it spins up the flywheel, giving it 28/5kJ of KE.
During the remaining second of the cycle 7kJ are need for the stamping. That comes from the 28/5kJ in the flywheel and another 7/5kJ from the motor.

Now, I have never worked with such a machine. What I write here is based on a feel for mechanical processes: that the stamping takes a large force for a short time, while the motor provides steady power, and that a flywheel would be a suitable energy store to match the two up.

 

[physea]

It might be, but switching sheets would take negligible energy compared to the stamping, so you can ignore that.

You need to understand why such a machine would have a flywheel. The stamping itself takes a considerable force. The motor would not be able to supply that, so instead it builds up momentum in a flywheel, and it is the inertia of the flywheel that ensures there is enough oomph.

The motor provides 7kJ over 5 seconds, or 7/5kJ per second.
During the four seconds it spins up the flywheel, giving it 28/5kJ of KE.
During the remaining second of the cycle 7kJ are need for the stamping. That comes from the 28/5kJ in the flywheel and another 7/5kJ from the motor.

Now, I have never worked with such a machine. What I write here is based on a feel for mechanical processes: that the stamping takes a large force for a short time, while the motor provides steady power, and that a flywheel would be a suitable energy store to match the two up.

So, to understand, you are saying that the motor provides 7kj over 5 seconds. During the four seconds it charges the flywheel and during the fifth second the whole KE of the flywheel (ie 4*7/5) plus the final 7/5 of the motor, altogether go to stamp the sheet.

Where in your analysis, the energy to change the sheets comes from? And why the flywheel is not involved in the changing of the sheets?
This question is ridiculously phrased and vague.

 

[haruspex]

the motor provides 7kj over 5 seconds. During the four seconds it charges the flywheel and during the fifth second the whole KE of the flywheel (ie 4*7/5) plus the final 7/5 of the motor, altogether go to stamp the sheet.

Yes.

Where in your analysis, the energy to change the sheets comes from?

I agree it is unclear, but if the distribution of work between the forming and the advancing to the new sheet is considered unknown then there is insufficient information. It is a matter of feel for mechanical processes that deforming a metal sheet takes a lot more work then merely shifting it sideways a few tens of centimetres.