I am trying to create a prototype for a gravity energy storage system, and I need to release potential energy stored in a heavy load of mass M, raised to a height *H.

I intend to use the mass traversing the length, to drive a piston, which pushes upon a hydraulic fluid to turn the P.E into K.E.

I am at a loss as to what mechanism I can use to efficiently (and simply) translate the descent into a vertical movement of the piston - since the piston will be contained in a sealed cylinder.

I have included a rather crude sketch below:



  1. The parallelogram represents the load bearing surface,
  2. The two dark circles represent the piston/plunger

My question is this: Is there a mechanism that allows a plunger/piston to be moved along an axis WITHIN a cylinder, by means of external force applied OUTSIDE the cylinder containing the piston?

I think what I'm looking for is some kind of telescopic mechanism.

  • 2
    $\begingroup$ Perhaps the sketch can be better. Are both ends of the cylinder sealed? How is the mass connected to the piston? Is there fluid on both sides of the piston? If not, why not just add a rod to the piston on the unsealed side? $\endgroup$
    – AJN
    Jan 8, 2022 at 16:11
  • $\begingroup$ I think you need to explain why a hydraulic cylinder (such as on an excavator or tipper truck) won't do the job. What's the point of sealing the cylinder? $\endgroup$
    – Transistor
    Jan 9, 2022 at 19:51
  • $\begingroup$ @AJN is there a good (preferably free) online sketching tool you can recommend? Regading your questions: 1. The top part of the cylinder is unsealed 2. Fluid is contained in the bottom part 3. Because it is the weight on the platform, that is supposed to drive the piston downward - I don't see how adding a rod on the unsealed side helps. $\endgroup$ Jan 13, 2022 at 10:11
  • $\begingroup$ Depending on the complexity of the diagram, perhaps you can use Powerpoint, LibreOffice Draw, or Inkscape. There are also CAD programs which are available for free download. None of these are online editors. Why don't you add a top view and a front view, and a side view apart from what is posted here. $\endgroup$
    – AJN
    Jan 13, 2022 at 12:32
  • $\begingroup$ Personal favorite online sketch tool - app.diagrams.net $\endgroup$
    – jrbe228
    Feb 20, 2022 at 16:35

2 Answers 2


Energy storage is a fairly common feature in hydraulic systems. Storing energy via nitrogen compression in accumulators is far more cost efficient than storing via mass/gravity. Personally I would just use something off the shelf. Here are a few popular brands:
I'm highlighting piston type accumulators since you specifically asked for that style. Other varieties such as diaphragm type or bladder type are usually cheaper per kJ of storage. Piston type accumulators are really for specialty applications such as extreme pressure (700+ bar), corrosive fluids that dissolve bladder materials, narrow installation dimensions, horizontal installation (bladder type service life is best with vertical installation), etc.

Note that with any type of energy storage, there are many safety considerations regarding how to dissipate energy in machine failure situations. Don't simply throw together an accumulator + pump + hoses. You need additional valves for pressure relief, accumulator dump, pump outlet check valve, etc. Further reading - https://www.machinerylubrication.com/Read/30331/hydraulic-accumulators-dangers


Yes there is, it is a magnetically coupled hydraulic cylinder. Here is how it works:

The piston inside the sealed cylinder is a very strong permanent magnet, sealed against the inside of the cylinder walls with an O-ring or two. the cylinder walls are thin stainless steel through which the magnetic field can penetrate. outside the cylinder is a second set of permanent magnets encircling the cylinder which strongly attract the piston magnet inside, and instead of O-rings this magnet assembly is mounted on a set of linear ball bearing slides. Any movement of the piston inside the cylinder will be translated to the slider outside by the magnetic attraction, and the slider can b easily connected to the machine element of your choice.

  • $\begingroup$ I did think of such an arrangement earlier, but had to discount it earlier on because the payload is going to be in the region of five to ten tons, and that would require a pretty big magnet to hold the weight up there - given that the device is supposed to be generating electricity, the amount of energy likely to be required to hold the weight in place will at best, completely offset that being generated by the descent of the piston. $\endgroup$ Jan 13, 2022 at 10:17
  • $\begingroup$ @HomunculusReticulli, don't you think that little 5 to 10 ton requirement should be mentioned in the question? $\endgroup$
    – Transistor
    Jan 13, 2022 at 20:51

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