# best energy to mass ratio for a spring

• posted

Hi,

Which type of spring (compression, torsion, hairspring etc) would give me the highest energy to mass ratio? i.e. which one can store the most energy compared to how much it weighs? Lets assume they are all made out of spring-steel.

Also, where on the web can I learn more about hairsprings? Like formulaes, different types available etc.

Thanks.

Best regards, Tobias Skytte

• posted

This may be an interesting problem for FEA.

If you aren't concerned with having large deflections the best shape might be a simple rod or wire under tension which places ALL the material under stress.

On the other hand the complicated stress profile of a coil spring or torsion rod MIGHT be more efficient, even if all the material wasn't uniformly stressed.

My first guess is they would all be more or less the same, if not identical.

A long thin line of nylon mono fishing line will wear the fish out just because the line stretches so much it is difficult to break.

And a short heavy line of the same material will absorb the same amount of energy if the mass is the same as the thin line.

Either way you need the same size reel for the same size and kind of fish. The diameter of the line is irrelevant.

Even switching to a non stretch kevlar braid might not change the overall toughness/weight ratio much since the increased strength/weight is cancelled out by the low stretch.

Is this some manifestation of conservation of energy? ;-)

Bret Cahill

snipped-for-privacy@itservices.co.mz (Tobias Skytte) >

• posted

Who's the FEA?

Ok, Do you know where I can learn more about springs in general? Formulaes, types, materials and so on. Stuff like: How much energy would it take to compress spring A, X mm. Or: How much energy would it take to turn hair-spring B, X revolutions? and so on.

heh. nope. Just need some very large springs in a project that would perform the best if it weighs the minimum possible with the largest possible springs (size of springs are also limited by other factors obviously). This actually brings me to another variation of my question: What materials would give the highest energy to mass ratio in a given spring?

Or even: energy to volume ratio?

Regards, Tobias Skytte

• posted

As far as materials go, the relative performance of materials can be determined fairly easily.

Energy is force x distance,

Energy stored by a uniformly stressed material ( elastic ) is 1/2 force X extension.

force is proportional to maximum stress, extension is proportional to stress / modulus, and mass is proportional to density.

thus a relative performance index figure =

1/2 ( max stress^2 ) / ( young's modulus x density )

This is the joules per Kg ( or whatever units ) your material can store

Any geometry which fails to achieve a uniform maximum stress when it's deflected is wasting the materials potential to store energy.

A leaf spring wastes most of the potential of the material near it's neutral axis, but I *think* the coil spring may achieve a perfect uniform stress....

• posted

Finite Element Analysis

No, that's why valve springs on cars are made from fancy materials.

Well, if you are into dead trees try Shigley, or

Again, relying on automotive technology, possibly air, as used in Formula 1. I think you may need to define your problem rather more accurately. Springs in general are fatigue limited, so you need to decide how many cycles, over what stress range, is important.

If you restrict yourself to solid springs, then some form of diamond whisker would probably be the best leaf spring, and a carbon uniaxial layup might be the lightest large leaf spring.

Cheers

Greg Locock

• posted

The materials aspect of this problem needs to be quantitatively solved because I have a lot of braggart blue water fishermen relatives on the lower Chesapeake who need to be put in their places.

For example, Spyderwire has little give but you can feel everything on the bottom of the channel. If you snag an oyster bed, just rip it loose.

Oysters make great bait.

Bret Cahill

• posted

. . .

It would be interesting to reduce the weight of an air or gas suspension system as much as possible -- ideal conditions, the best materials etc. -- and then compare it with solid springs.

Energy storage comparisons, always interesting, are getting critically important really fast.

Capacitor powered vehicles were fun until it turned out the best capacitors available today would need weigh 100 tons to equal 5 tons of lead acid batteries or 100 pounds of gas or 200 pounds of TNT. The TNT comparison is more appropriate because capacitors discharge even faster than TNT explodes. If the thing shorted out in, say, an accident it would blow a crater

30 feet in diameter.

I'd rather store 50 pounds of H2 in my garage.

A math guy -- it's sometimes useful to be on speaking terms with a math guy -- is very pessimistic about engineers designing a nice affordable sustainable democratic way off the limb of the oil age.

"Apparently we are going to party for a couple of centuries and then, when the oil is gone, just turn [back] to coal."

I said, "and that is assuming there isn't going to be any 'political problems' caused by the increasing cost of transportation and food."

Bret Cahill

• posted

Getting rid of the sulphur and CO2.

Exactly. They do all kinds of things to crude oil before they sell it. Why treat coal any differently?

Coal fired scooter. More miles per dollar than a bicycle which requires eating expensive potatos.

I certainly wouldn't want to plow a field with oxen.

It already is cheaper for large electric power plants. Slightly over half of U. S. power comes from coal.

I told a coal industry news person that France was planning to be coal free in 5 years and he laughed, "that's a pipe dream."

Bret Cahill

• posted

Well, roll on the coal economy. Lots of jobs for engineers at least.

You either

(a) make oil out of coal

or

(b) find a practical way of using coal for vehicles

Both have been solved many times.

Of course once we get rid of knee jerk NIMBYism we'll have PWRs near every city, and then electric cars, and electric trains.

The hardest single problem for ground transport is agricultural tractors, I think.

OK, this is very OT for this thread.

So far as increased cost, coal is about \$15 per tonne, oil is around \$200, it might even be cheaper to go back to coal, even allowing for inefficiencies.

Cheers

Greg Locock

• posted

Ok, great, any idea where I can find a list of materials that shows the J/kg ?

Also, has anybody worked it out for compressed gas/air cylinders ? Regards, Tobias Skytte

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