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With a normal spring, you compress it using a linear force to store energy and then it decompresses and releases the energy, again in a form of linear force. Is there a mechanical mechanism that stores energy by rotating force and releases energy by rotating force? It doesn't have to be spring operated, but I think it's the only way to work, with springs.

I want it to work for many many rotations (to store energy) and to produce many many rotations! what's the best mechanism for this?

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    $\begingroup$ How about a flywheel? $\endgroup$ – John1024 Jul 20 '15 at 18:39
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    $\begingroup$ Rotational (torsion) spring? $\endgroup$ – grfrazee Jul 20 '15 at 18:41
  • $\begingroup$ excuse me, but I need the rotations to store energy to the mechanism and the rotations caused during the release of energy to be many, not just 1-2! $\endgroup$ – sloupioc Jul 20 '15 at 19:37
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    $\begingroup$ Can you give us a better idea of what you're working on? If you're looking for a rotational storage device that can torque for "many rotation" a flywheel is good, you could also consider a tape measure. oharespring.com/constant.htm or springcompany.com/products/… $\endgroup$ – willpower2727 Jul 20 '15 at 20:16
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    $\begingroup$ Look at any "windup" toy car or hobby clockwork motor. You input energy rotationally with the input "key" and the energy is released rotationally to drive wheels or similar. Almost all wind up mechanical clocks or watches also do this to drive the hands. $\endgroup$ – Russell McMahon Jul 21 '15 at 2:17
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Both a torsion bar and a "coil" spring do what you want. The torsion bar is intended for rotation of only a fraction of a circle, and generally has a high spring constant. The coil spring can be designed for a number of rotations, generally with a lower spring constant.

Look at any old windup watch or clock and most likely the energy storage mechanism is a coil spring. Some old clocks are powered by dropping weights, but these are usually not "wound" to add the energy.

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    $\begingroup$ Many roll-up garage doors also have a large coil-type torsion spring to counterbalance the weight of the door, instead of a linear spring plus pulley arrangement. $\endgroup$ – Dave Tweed Jul 21 '15 at 16:25
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The mainsprings of mechanical timepieces provide an excellent example of how you can accomplish what you want. They are a type of torsion spring made from a metal ribbon which can typically be wound 6 or 7 full turns. The images below show what some of these look like when unwound and wound.

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enter image description here

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For "many many rotations", a pneumatic motor can act as both a compressor and motor. Spinning the motor causes air to be forced through a tube, one-way valve, and storage tank. Opening the valve allows the compressed air in the tank (potential energy) to flow back through the tube and motor, spinning it in reverse. As the number of rotations increases, the pressure increases, acting to stop the axle for a given torque. This presents a rotational limit, based on the volume of compressed gas, size of motor, size of tank, etc. For fast rotary motion this could work, but for slow motion, the pneumatic motor may "leak" and store little or no energy.

For "many many many rotations", a permanent magnet motor/generator -> DC rectifier -> battery (or supercapacitor) may work to store considerably more energy. Similar to the pneumatic concept, slow motion will not produce much charge. But the density of stored potential energy is much higher in batteries than it is air tanks.

There are significant losses in both of these, so there won't be enough energy stored to return the axle to it's original starting position.

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A spring loaded piston actuating a rack, and turning a pinion gear. Smaller the pinion gear, relative to the length of the rack... the more rotations achieved in a given stroke. And also the greater the piston pressure required...

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