Flywheel structural questions

[skenn]

Help please...
I am designing a flywheel energy storage system, and need to know...
how to calculate the tension in a solid ring when it is spinning. Think of it as a segmented flywheel...what tension would a band round the outside have to deal with ?
assume 1 tonne, with effective diameter of 1m, spinning at 100rps.

Thanks
Shane

 

[ccfowler]

It seems that you may be way over your head based on your posting. Take care. The energy stored in a flywheel can be both very large and very dangerous. Flywheel failures tend to be quite ugly.

If you are serious about this, get a very competent, experienced person directly involved. Flywheel design is not for beginners. The outcome of inadequate design or manufacture or improper operation can very easily be maimed or dead people.

 

[skenn]

I AM well aware of the frightening amount of energy stored( 1GJ ?) in a/this flywheel, so
1) I intend to test to destruction, at least one...in a container, in the sea !.
2) The unit will be limited to less than half the failure speed, or 10k rpm..whichever is lower.
3) The unit, in use, will be buried, in a steel container.

I am in 2 minds about one aspect...whether to build it as a segmented mass, contained by steel and kevlar bands...so that, if it does break loose from it's bearings, it fragments, and dissipates it's energy radially, or
a single ring, so that, rather than being a "fragmentation grenade", the flywheel remains intact, with the risk of it climbing out of it's pit, and demolishing everything in it's path for a kilometer or so !

 

[ccfowler]

skenn,

Sorry if my response seemed a bit much. No offense was intended, but this is a bit of an intense matter for me. I have had direct experience with the after-effects of flywheel failures involving severe disabling injuries, fatalities, and extensive property damage. Unless you have had some experience with them, I'm sure that the potential for destruction is almost certainly beyond your wildest immaginations. Most likely, whatever you think will contain the failure probably will not. From experience, your kilometer estimate may be low by a factor of at least 2.

 

[skenn]

I stand, or sit, forwarned. We do have an experienced mechanical engineer, and an engineering comapny involved in this as consultants. They(in South Africa) will be dealing with that side of the project.
I'm sorry to hear of your direct involvement with failures. Hopefully, due in part to your report, we may avoid repeating the experience.

 

[sreid]

If my memory serves me, Roark's has formulas for flywheel stresses.

 

[skenn]

To sreid, Thanks for the pointer, I'll have a look.

To ccfowler,
Can you elaborate of the failure you experienced...how big was the wheel, how fast, what failed etc ?. How much TNT would contain/release 1GJ !.

 

[ccfowler]

There were several severe failures. Although none involved a flywheel that was intended for storage and recovery of energy, they were machine parts that were effectively flywheels storing substantial amounts of energy by virtue of their configuration and operation. I can't provide full details on any of them, but I can provide some useful anecdotal information.

The worst case involved a solid-on-shaft high speed gearwheel that failed at a speed in the range of 10k rpm to 20k rpm as it was being accelerated toward its intended operating speed. I believe that the salient problem involved a combination of a stress concentrating configuration (relatively sharp corner at the junction of the shaft and the wheel) and a rough passage through a critical speed.

All other failures involved shaft speeds below 4k rpm (some below 1k rpm) and also involved stress concentrating factors such as material imperfections (inclusions, sharp corners, stress corrosion cracking, etc.) at (or very near) the point where the brittle failure initiated. Also present in all cases were seemingly excellent operation, maintenance, and safety practices.

Also consistent in all cases, once the brittle failure commenced, the inherent imbalance and loss of symmetrical stress or load distributions, portions of irregular size and shape fractured apart. Most, but not all, parts tended to fly in reasonably predicted directions. Most property damage was suffered along the reasonably predictable paths.

Some of the worst injuries involved pieces not flying in the expected direction for reasons such as altered trajectories due to secondary fracturing of the already odd-shaped pieces (subsequent to the initial failure) or impacts against adjacent structures or other moving parts. (People tend to be concentrated in adjacent areas presumed to be relatively more safe.)

 

[ccfowler]

skenn,

Additional thoughts (philosophical in nature):

First, one of my engineering professors, for whom I have particularly high regard, provided some very important insight into prudent engineering design. His admonition was to first design for "graceful failure" because everything we design will eventually fail. It is an engineer's responsibility and challenge to design things so that when they fail nobody should be injured and property damage should be minimal. Only after first considering failure modes should efforts be expended toward optimizing the design for function and economic considerations.

Second, a much older engineer (nearing retirement at the time) whom I knew very early in my career (and for whom I developed much respect) suggested that codes should be printed in red ink to remind everyone that they were developed on the basis of information and experience arising from the blood, death, pain, and suffering of those who were injured by the failure of equipment or systems. I've since developed a strong opinion that all codes should include provisions requiring that designs that simply conform to the minimum requirements of the code be considered as being in specific violation of the spirit and intent of the code.

Despite my cautionary statements, I am inclined to think (and, indeed, hope) that flywheels will become important, practical energy storage devices. Because the energy stored in a flywheel can be so very difficult to confine with sufficient safety, I do fear that there will be serious failures that will impede the useful deployment of flywheel storage systems.

I do wish you very well in your project.

 

[skenn]

Philosophical, and pertinent. It was with this consideration, that one of my earlier posts compared the device to a fragmentation grenade, though even "anti-tank-mine" may be underestimating the damage wrought by such a device. Partly due to the responses in this forum, safe distribution /absorbtion of this energy has become a much more significant part of it's design. Certainly underground, and with flywheel horizontal/axis vertical.
Active condition monitoring is likely to be devised, including constant magnetometer type crack detection in the stress components.
My side of this project is the electrics/electronics, esp microcontrollers, and this is likely to bring in DSP's as well.

 

[lengould]

My only experience with flywheel failure was when a kid driving a Camaro missed a shift on the street in front of our house one night about 3AM. There was a crash, and we went into our livingroom to find a significant chunk (about 1/4 or less) of his engine's flywheel smoking on the carpet in the livingroom. It had hit the doorjamb of the brick house, torn the door completely off the house and sprayed parts of it inward. That after blowing through his transmission housing, the car's body, and across 100 ft of yard.

Be careful.

Pechez les vaches.

 

[nbucska]

About 40 yrs ago I have read about some Swiss experiments with FW driven busses -- they used about 1 ton FW made
of -- horribile dictu -- laminated wood.

I assume today some carbon-fiber composite would be the optimal.





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