flywheel FEA 1

[tjakeman]

This is a cross post with the mechanica forum, general help would be useful though:

Has anyone carried out FEA analysis of an automotive flywheel before?

I need to do so this week, using mechanica. I have quarter model with rotational symmetry constraints, centrifugal load etc. What I'm really lacking is information on a realistic load case. Heat input, shear on the clutch face etc etc.

Any help appreciated...

 

[GregLocock]

Before building a model it is wise to consider what you want out of it. So what do you want out of it? Weight reduction? Explanation of a problem? NVH properties?

The coefficient of friction of the plate is probably 0.3



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tjakeman]

Primary goal is inertia reduction. I have a benchmark flywheel which is proven to work, and I have mechanica at my disposal. At the moment I’m considering three very basic load cases to apply to the benchmark flywheel model and a light weight version, I plan to compare and check that stresses don’t go through the roof.

Simple torsion case, constrained holes at the crank, apply torque to the clutch face.

Temp case, apply heat flux to clutch face for given time, compare stress due to mass reduction.

Bursting case, compare stresses at max engine rpm.

Basically I have very little analysis experience as you can probably tell, I thought someone might give me a nudge in the right direction.

Thanks

Tom

 

[GregLocock]

OK, that sounds sensible. I don't know anything about the heat side. The other two load cases you can work out from the info you now have.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

When you look at an automatic transmission flex plate, it becomes quickly apparent that by far the greatest influence on flywheel weight is clutch operation and heat sink.

Regards
Pat
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[tbuelna]

tjakeman,

Do you want inertia reduction or weight reduction? The main purpose of a flywheel is to increase the engine's rotating inertia so that it suffers less variation in angular velocity between cylinder firing events (ie. so it runs "smoother"). The fact that the flywheel face provides a convenient location for a clutch friction surface, and the flywheel OD is perfect for locating a starter ring gear, are simply secondary functions of a typical flywheel.

The weight of the flywheel can usually be reduced without reducing its inertia, simply by optimizing the distribution of the required material mass.

Analyzing a flywheel is fairly straightforward:

-The crank attachment should be analyzed for taking the max sum of instantaneous cranking torques produced by all of the cylinders, through the bolted flange joint solely by friction.

-As others noted, friction clutch components are designed primarily by thermal capacity. The flywheel should have sufficient thermal mass and heat transfer away from the friction face such that the clutch friction face does not experience a temperature rise that will cause mechanical failure or de-tempering of a heat treated metal alloy. The heat input from the clutch is a function of the power that must be absorbed over the period of time that clutch slippage is required to synchronize the transmission and engine speeds.

-Unless you're using a low strength, cast material for your flywheel, or running very high rotational speeds, then burst strength is likely not an issue. But still, for rotating components a very conservative analysis FoS is definitely in order.

-If your flywheel incorporates a ring gear for the starter, be sure to check that your material has adequate surface compressive strength for the gear tooth contact loads produced by the starter pinion gear. These contact stresses can actually be quite high, even for a lightly loaded, low cycle starter gear mesh.

-Fianlly, be sure to check your flywheel for structural frequency modes. It should not have any that might couple with firing frequencies produced in the engine, or even possibly meshing frequencies produced in the transmission gears.

Hope that helps.
Terry

 

[tjakeman]

Thanks everyone for your input.

Terry, Inertia and mass reduction would be nice.

The application of this flywheel is a flat plane racing V8 engine. It supports a five plate racing clutch which in turn supports the ring gear. Machined in to the perimeter of the flywheel is a tooth pattern for the crank sensor. The engine shouldn’t see more than 11,000 rpm.

Currently the flywheel is 1kg. Smooth running isn’t really a concern, idle is set to around 4000 rpm.

I agree that the greatest loading applied to the flywheel will be heat from clutch operation. I intend to talk to the clutch supplier to see if they have any recommendations for the flywheels thermal capacity.

I think I can use: h = dQ / A*dT*dt to calculate the heat transfer coefficient, providing I can decide on an appropriate energy input (dQ)

“The heat input from the clutch is a function of the power that must be absorbed over the period of time that clutch slippage is required to synchronize the transmission and engine speeds”

This is definitely true, but how can I apply numbers to this?

Thanks again for your input, this is becoming quite an interesting project.

 

[GregLocock]

Get a plot of driveshaft speed and engine speed during an engagement. The sliding velocityis then some function of that. You could then take a stab at the torque being transferred vs time (closely related to th acceleration).

The work being done at the interface is then a function of the sliding velocity and the torque.

There may be more elegant methods, I'd use as many as I can think of.

Another example is stalling the engine from the redline.

The energy into the interface =the kinetic rotational energy of the engine beforehand. I don't think that is worst case though.

Without doing any maths the result should be of the order of 100 kJ, or at least, more than 10 kJ and less than 1000 kJ.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tbuelna]

tjakeman,

GregLocock pretty much laid it out for you. The power loss (and thus heat load input) can be established if you know the relative speeds and the friction torque being produced by the clutch. It is safe to assume that most of the heat load transfer will be into the metal flywheel mass, since clutch friction facing materials tend to have very low thermal conductivity through their thickness.

The heat load input will be transient- initially increasing rapidly as the clutch contacts and slips at high speed, then tapering off as the relative shaft speeds synchronize and the clutch locks up.

Some of the heat will be absorbed through conduction in the metal flywheel mass, and thus raise it's temperature. To calculate for this effect you will need (among other things) the specific heat value of your flywheel material.

Some of the heat load input into your flywheel will also be rejected into the ambient airflow around the spinning flywheel through convection. To calculate for this effect you will need to establish a convection heat transfer coefficient (based on the local surface and fluid temperatures, and areas).

Hope that was helpful.
Terry

 

[tjakeman]

Greg & Terry,

Thanks for your continued input. I am still struggling though, sorry.

I agree with the points made, just struggling to put some maths to it. I have a plot of engine speed, wheel speed, clutch hydraulic pressure etc. I know that the rated torque capacity of the clutch is 1142Nm and I know the torque of the engine. I'm missing a bit of maths that will tell me the wasted energy.

It doesn't seam right to assume that subtracting 1142 from the torque of the engine (at that speed, in that gear) will give the amount of torque generating heat energy. Or is that what I should be doing?

Please stick with me.

Tom

 

[tjakeman]

Ignore that third paragraph, brain not working very well at the moment.

 

[GregLocock]

Well, yet another approach would be to work out the load from a standing start. Say the engine is doing 2200 rpm at max torque, and the disc has a mean radius of 100 mm.

Then the sliding velocity is 2200/60*2*pi*.1 m/s.

The torque is 1142 Nm, so the force is 1142/.1 N. So the power at the interface is force*sliding velocity, so under this condition you'd get 260 kJ per second.

Now, that is an overestimate, since in reality the sliding velocity will drop away as the vehicle accelerates. On the other hand somebody could ride the clutch for longer than a second.


As I implied, you really need to think about the dynamics of the vehcile as well.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.